exception safe assignment operator c

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Practical Modern C++

Exception safe assignment.

Longer title: exception safe assignment operator of resource owning objects . Uff. Because the object owns a resource, how do we write an exception safe assignment operator which will have to free up the old and allocate the new resource. By exception safe I don’t mean that it will never throw, that’s not possible. Instead, I mean safe in the sense that it either succeeds OR in case of exceptions the state of assigned to object is exactly as it was prior to the assignment. Like this:

main() S s1, s2; s1 = s2;

If assignment operator s1 = s2 throws an exception, we want the state of s1 and s2 to be as it was in line #3 .

The trick is two fold: 1) a copy constructor is needed, and 2) noexcept swap function. Like this:

(const S& s) m_resource(new int) *m_resource = *s.m_resource; swap(S& s) noexcept std::swap(m_resource, s.m_resource);

Here the copy constructor allocates the new resource first, then copies its content; the swap function just swaps pointers to the resources, which is always a noexcept operation. Having implemented a copy constructor and swap function we can now implement every assignment operator to have a strong exception guarantee like this:

& operator = (const S& s) S temp(s); // May throw swap(temp); // Will never throw return *this;

Here’s how it works: we first make a temporary copy, which does the resource allocation. At this stage exceptions can be thrown, but we have not yet modified the assigned to object. Only after the resource allocation succeeds do we perform the noexcept swap. The destructor of your temporary object will take care of cleaning up the currently owned resource (that’s RAII at its best).

Complete listing ( assignment.cpp ):

namespace std; S S() : m_resource(new int) { cout << "S()" << endl; *m_resource = 1; } S(const S& s) : m_resource(new int) { cout << "S(const S&)" << endl; *m_resource = *s.m_resource; } S(S&&) = delete; S& operator = (const S& s) { cout << "operator = (const S&)" << endl; S temp(s); // May throw swap(temp); // Will never throw return *this; } S& operator = (S&&) = delete; ~S() { cout << "~S()" << endl; delete m_resource; } void swap(S& s) noexcept { std::swap(m_resource, s.m_resource); } int* m_resource; ; main() S s1, s2; s1 = s2;

S() S() operator = (const S&) S(const S&) ~S() ~S() ~S() Program output.

Exception-Safe Class Design, Part 1: Copy Assignment Difficulty: 7 / 10

Is it possible to make any C++ class strongly exception-safe, for example for its copy assignment operator? If so, how? What are the issues and consequences?

JG Questions

1. What are the three common levels of exception safety? Briefly explain each one and why it is important.

2. What is the canonical form of strongly exception-safe copy assignment?

Guru Questions

3. Consider the following class:

Assume that any T1 or T2 operation might throw. Without changing the structure of the class, is it possible to write a strongly exception-safe Widget::operator=( const Widget& )? Why or why not? Draw conclusions.

4. Describe and demonstrate a simple transformation that works on any class in order to make strongly exception-safe copy assignment possible and easy for that class. Where have we seen this transformation technique before in other contexts? Cite GotW issue number(s).

Review: Exception Safety Canonical Forms

The canonical Abrahams Guarantees are as follows.

1. Basic Guarantee: If an exception is thrown, no resources are leaked, and objects remain in a destructible and usable -- but not necessarily predictable -- state. This is the weakest usable level of exception safety, and is appropriate where client code can cope with failed operations that have already made changes to objects' state.

2. Strong Guarantee: If an exception is thrown, program state remains unchanged. This level always implies global commit-or-rollback semantics, including that no references or iterators into a container be invalidated if an operation fails.

In addition, certain functions must provide an even stricter guarantee in order to make the above exception safety levels possible:

3. Nothrow Guarantee: The function will not emit an exception under any circumstances. It turns out that it is sometimes impossible to implement the strong or even the basic guarantee unless certain functions are guaranteed not to throw (e.g., destructors, deallocation functions). As we will see below, an important feature of the standard auto_ptr is that no auto_ptr operation will throw.

It involves two steps: First, provide a nonthrowing Swap() function that swaps the guts (state) of two objects:

Second, implement operator=() using the "create a temporary and swap" idiom:

The Cargill Widget Example

This brings us to the Guru questions, starting with a new exception safety challenge proposed by Tom Cargill:

Short answer: In general, no, it can't be done without changing the structure of Widget.

In the Example 3 case, it's not possible to write a strongly exception-safe Widget::operator=() because there's no way that we can change the state of both of the t1_ and t2_ members atomically. Say that we attempt to change t1_, then attempt to change t2_. The problem is twofold:

1. If the attempt to change t1_ throws, t1_ must be unchanged. That is, to make Widget::operator=() strongly exception-safe relies fundamentally on the exception safety guarantees provided by T1, namely that T1::operator=() (or whatever mutating function we are using) either succeeds or does not change its target. This comes close to requiring the strong guarantee of T1::operator=(). (The same reasoning applies to T2::operator=().)

2. If the attempt to change t1_ succeeds, but the attempt to change t2_ throws, we've entered a "halfway" state and cannot in general roll back the change already made to t1_.

Therefore, the way Widget is currently structured, its operator=() cannot be made strongly exception-safe.

Our goal: To write a Widget::operator=() that is strongly exception-safe, without making any assumptions about the exception safety of any T1 or T2 operation. Can it be done? Or is all lost?

A Complete Solution: Using the Pimpl Idiom

The good news is that, even though Widget::operator=() can't be made strongly exception-safe without changing Widget's structure, the following simple transformation always works:

The way to solve the problem is hold the member objects by pointer instead of by value, preferably all behind a single pointer with a Pimpl transformation (described in GotW issues like 7 , 15 , 24 , 25 , and 28 ).

Here is the canonical Pimpl exception-safety transformation:

Now we can easily implement a nonthrowing Swap(), which means we can easily implement exception-safe copy assignment: First, provide the nonthrowing Swap() function that swaps the guts (state) of two objects (note that this function can provide the nothrow guarantee because no auto_ptr operations are permitted to throw exceptions):

Second, implement the canonical form of operator=() using the "create a temporary and swap" idiom:

A Potential Objection, and Why It's Unreasonable

Some may object: "Aha! Therefore this proves exception safety is unattainable in general, because you can't solve the general problem of making any arbitrary class strongly exception-safe without changing the class!"

Such a position is unreasonable and untenable. The Pimpl transformation, a minor structural change, IS the solution to the general problem. To say, "no, you can't do that, you have to be able to make an arbitrary class exception-safe without any changes," is unreasonable for the same reason that "you have to be able to make an arbitrary class meet New Requirement #47 without any changes" is unreasonable.

For example:

Unreasonable Statement #1: "Polymorphism doesn't work in C++ because you can't make an arbitrary class usable in place of a Base& without changing it (to derive from Base)."

Unreasonable Statement #2: "STL containers don't work in C++ because you can't make an arbitrary class usable in an STL container without changing it (to provide an assignment operator)."

Unreasonable Statement #3: "Exception safety doesn't work in C++ because you can't make an arbitrary class strongly exception-safe without changing it (to put the internals in a Pimpl class)."

Clearly all the above arguments are equally bankrupt, and the Pimpl transformation is indeed the general solution to strongly exception-safe objects.

So, what have we learned?

Conclusion 1: Exception Safety Affects a Class's Design

Exception safety is never "just an implementation detail." The Pimpl transformation is a minor structural change, but still a change. GotW #8 shows another example of how exception safety considerations can affect the design of a class's member functions.

Conclusion 2: You Can Always Make Your Code (Nearly) Strongly Exception-Safe

There's an important principle here:

Just because a class you use isn't in the least exception-safe is no reason that YOUR code that uses it can't be (nearly) strongly exception-safe.

Anybody can use a class that lacks a strongly exception-safe copy assignment operator and make that use exception-safe. The "hide the details behind a pointer" technique can be done equally well by either the Widget implementor or the Widget user... it's just that if it's done by the implementor it's always safe, and the user won't have to do this:

Conclusion 3: Use Pointers Judiciously

To quote Scott Meyers:

"When I give talks on EH, I teach people two things: "- POINTERS ARE YOUR ENEMIES, because they lead to the kinds of problems that auto_ptr is designed to eliminate.

To wit, bald pointers should normally be owned by manager objects that own the pointed-at resource and perform automatic cleanup. Then Scott continues:

"- POINTERS ARE YOUR FRIENDS, because operations on pointers can't throw. "Then I tell them to have a nice day :-)

Scott captures a fundamental dichotomy well. Fortunately, in practice you can and should get the best of both worlds:

- USE POINTERS BECAUSE THEY ARE YOUR FRIENDS, because operations on pointers can't throw.

- KEEP THEM FRIENDLY BY WRAPPING THEM IN MANAGER OBJECTS like auto_ptrs, because this guarantees cleanup. This doesn't compromise the nonthrowing advantages of pointers because auto_ptr operations never throw either (and you can always get at the real pointer inside an auto_ptr whenever you need to).

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CS102: Introduction to Computer Science II

Exception Handling in C++

This page might seem like it duplicates some of what we have just seen, but it is valuable because it gives a different perspective on the topic. Read chapter 1 on pages 15-60.

Exception safety

In Chapter 7 we'll take an in-depth look at the containers in the Standard C++ library, including the stack container. One thing you'll notice is that the declaration of the pop( ) member function looks like this:

You might think it strange that pop( ) doesn't return a value. Instead, it just removes the element at the top of the stack. To retrieve the top value, call top( ) before you call pop( ) . There is an important reason for this behavior, and it has to do with exception safety , a crucial consideration in library design. There are different levels of exception safety, but most importantly, and just as the name implies, exception safety is about correct semantics in the face of exceptions.

Suppose you are implementing a stack with a dynamic array (we'll call it data and the counter integer count ), and you try to write pop( ) so that it returns a value. The code for such a pop( ) might look something like this:

What happens if the copy constructor that is called for the return value in the last line throws an exception when the value is returned? The popped element is not returned because of the exception, and yet count has already been decremented, so the top element you wanted is lost forever! The problem is that this function attempts to do two things at once: (1) return a value, and (2) change the state of the stack. It is better to separate these two actions into two separate member functions, which is exactly what the standard stack class does. (In other words, follow the design practice of cohesion – every function should do one thing well .) Exception-safe code leaves objects in a consistent state and does not leak resources.

You also need to be careful writing custom assignment operators. In Chapter 12 of Volume 1, you saw that operator= should adhere to the following pattern:

Make sure you're not assigning to self. If you are, go to step 6. (This is strictly an optimization.)
Allocate new memory required by pointer data members.
Copy data from the old memory to the new.
Delete the old memory.
Update the object's state by assigning the new heap pointers to the pointer data members.
Return *this .

It's important to not change the state of your object until all the new pieces have been safely allocated and initialized. A good technique is to move steps 2 and 3 into a separate function, often called clone( ) . The following example does this for a class that has two pointer members, theString and theInts :

For convenience, HasPointers uses the MyData class as a handle to the two pointers. Whenever it's time to allocate more memory, whether during construction or assignment, the first clone function is ultimately called to do the job. If memory fails for the first call to the new operator, a bad_alloc exception is thrown automatically. If it happens on the second allocation (for theInts ), we must clean up the memory for theString – hence the first try block that catches a bad_alloc exception. The second try block isn't crucial here because we're just copying int s and pointers (so no exceptions will occur), but whenever you copy objects, their assignment operators can possibly cause an exception, so everything needs to be cleaned up. In both exception handlers, notice that we rethrow the exception. That's because we're just managing resources here; the user still needs to know that something went wrong, so we let the exception propagate up the dynamic chain. Software libraries that don't silently swallow exceptions are called exception neutral . Always strive to write libraries that are both exception safe and exception neutral.

If you inspect the previous code closely, you'll notice that none of the delete operations will throw an exception. This code depends on that fact. Recall that when you call delete on an object, the object's destructor is called. It turns out to be practically impossible to design exception-safe code without assuming that destructors don't throw exceptions. Don't let destructors throw exceptions. (We're going to remind you about this once more before this chapter is done).

C++ Exception-Safe Assignment Overloading

Introduction

Implementing C++ assignment overloading can be error-prone if the object involves data on the memory heap.

In this blog post, I would like to quickly discuss how to implement exception safe C++ assignment overloading and how to apply the copy-and-swap idiom to implement it more elegantly.

C++ Assignment Overloading

Exception-safe and self-assignment.

Make sure the resource management inside the assignment overloading has taken throwing exceptions into account to prevent the data loss from the object. In the following example, we strictly follow allocate, populate and deallocate. If we deallocate the cstring member variable before allocate the new buffer, and allocating new buffer throws exception, we lose the cstring data forever.

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<cstddef>
#include <cstring>
#include <iostream>

class String
{

public:
String(const char* s = "") : String(s, std::strlen(s) + 1) {}

String(const String& other) : String(other.cstring) {}

~String() { delete[] cstring; }

String& operator=(const String& other)
{
// Ignore self assignment.
// A little bit problematic here because of the data on the heap.
// Need to overload == and the comparison might take a long time.
// This step might just be skipped.
if (this == &other)
return *this;
std::size_t n{std::strlen(other.cstring) + 1};
// The order matters to make sure the assignment is exception-safe.
char* new_cstring = new char[n]; // allocate
std::memcpy(new_cstring, other.cstring, n); // populate
delete[] cstring; // deallocate
cstring = new_cstring;
return *this;
}

bool operator==(const String& other) const noexcept
{
if (std::strlen(cstring) != std::strlen(other.cstring))
{
return false;
}
for (std::size_t i = 0; i < std::strlen(cstring); ++i)
{
if (cstring[i] != other.cstring[i])
{
return false;
}
}
return true;
}

operator const char*() const noexcept { return cstring; }

private:
String(const char* s, std::size_t n) : cstring(new char[n])
{
std::memcpy(cstring, s, n);
}

char* cstring;
};

int main()
{
String str_1{"abcd"};
String str_2{"def"};
str_1 = str_2;
}

Valgrind verified that there is no memory leak during runtime.

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$ valgrind --leak-check=full ./string
==56318== Memcheck, a memory error detector
==56318== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==56318== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==56318== Command: ./string
==56318==
==56318==
==56318== HEAP SUMMARY:
==56318== in use at exit: 0 bytes in 0 blocks
==56318== total heap usage: 4 allocs, 4 frees, 72,717 bytes allocated
==56318==
==56318== All heap blocks were freed -- no leaks are possible
==56318==
==56318== For lists of detected and suppressed errors, rerun with: -s
==56318== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Copy-And-Swap Idiom

A more elegant way of implementing assignment overloading is to apply the “copy-and-swap” idiom. We create a public or private noexcept swap member function and use the member function for the temporary copy of the assignment source.

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<cstddef>
#include <cstring>
#include <iostream>

class String
{

public:
String(const char* s = "") : String(s, std::strlen(s) + 1) {}

String(const String& other) : String(other.cstring) {}

~String() { delete[] cstring; }

String& operator=(const String& other)
{
// Ignore self assignment.
// A little bit problematic here because of the data on the heap.
// Need to overload == and the comparison might take a long time.
// This step might just be skipped.
if (this == &other)
return *this;
// copy-and-swap idiom.
String temp_string{other};
swap(temp_string);
return *this;
}

// no exception is allowed for swap.
void swap(String& other) noexcept
{
char* temp_cstring = cstring;
cstring = other.cstring;
other.cstring = temp_cstring;
}

bool operator==(const String& other) const noexcept
{
if (std::strlen(cstring) != std::strlen(other.cstring))
{
return false;
}
for (std::size_t i = 0; i < std::strlen(cstring); ++i)
{
if (cstring[i] != other.cstring[i])
{
return false;
}
}
return true;
}

operator const char*() const noexcept { return cstring; }

private:
String(const char* s, std::size_t n) : cstring(new char[n])
{
std::memcpy(cstring, s, n);
}

char* cstring;
};

int main()
{
String str_1{"abcd"};
String str_2{"def"};
str_1 = str_2;
}

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$ valgrind --leak-check=full ./string
==56745== Memcheck, a memory error detector
==56745== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==56745== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==56745== Command: ./string
==56745==
==56745==
==56745== HEAP SUMMARY:
==56745== in use at exit: 0 bytes in 0 blocks
==56745== total heap usage: 4 allocs, 4 frees, 72,717 bytes allocated
==56745==
==56745== All heap blocks were freed -- no leaks are possible
==56745==
==56745== For lists of detected and suppressed errors, rerun with: -s
==56745== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

https://leimao.github.io/blog/CPP-Exception-Safe-Assignment-Overloading/

Licensed under

Like this article support the author with, advertisement.

1 Introduction
2.1 Exception-Safe and Self-Assignment
2.2 Copy-And-Swap Idiom

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Exception Safe Assignment

Longer title: exception safe assignment operator of resource owning objects . Uff. Because the object owns a resource, how do we write an exception safe assignment operator which will have to free up the old and allocate the new resource. By exception safe, I don’t mean that it will never throw, that’s not possible. Instead, I mean safe in the sense that it either succeeds OR in case of exceptions, the state of assigned to object is exactly as it was prior to the assignment. Like this:

If assignment operator...

...throws an exception, we want the state of...

...to be as it was in line #3 .

The trick is two fold:

a copy constructor is needed, and
noexcept swap function. Like this:

Here, the copy constructor allocates the new resource first, then copies its content; the swap function just swaps pointers to the resources, which is always a noexcept operation. Having implemented a copy constructor and swap function, we can now implement every assignment operator to have a strong exception guarantee like this:

Here’s how it works: we first make a temporary copy, which does the resource allocation. At this stage, exceptions can be thrown, but we have not yet modified the assigned to object. Only after the resource allocation succeeds do we perform the noexcept swap. The destructor of your temporary object will take care of cleaning up the currently owned resource (that’s RAII at its best).

Complete listing ( assignment.cpp ):

Program output:

This article, along with any associated source code and files, is licensed under The MIT License

Comments and Discussions

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8-Apr-19 22:39

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Windows Programming
UNIX/Linux Programming
General C++ Programming
Copy constructors, assignment operators,

Copy constructors, assignment operators, and exception safe assignment

MyClass& other );

MyClass* other );

MyClass { x; c; std::string s; };

MyClass& other ) : x( other.x ), c( other.c ), s( other.s ) {}

, );

=( MyClass& rhs ) { x = other.x; c = other.c; s = other.s; * ; }

< T > MyArray { size_t numElements; T* pElements; : size_t count() { numElements; } MyArray& =( MyArray& rhs ); };

<> MyArray<T>:: =( MyArray& rhs ) { ( != &rhs ) { [] pElements; pElements = T[ rhs.numElements ]; ( size_t i = 0; i < rhs.numElements; ++i ) pElements[ i ] = rhs.pElements[ i ]; numElements = rhs.numElements; } * ; }

<> MyArray<T>:: =( MyArray& rhs ) { MyArray tmp( rhs ); std::swap( numElements, tmp.numElements ); std::swap( pElements, tmp.pElements ); * ; }

< T > swap( T& one, T& two ) { T tmp( one ); one = two; two = tmp; }

<> MyArray<T>:: =( MyArray rhs ) { std::swap( numElements, tmp.numElements ); std::swap( pElements, tmp.pElements ); * ; }

HOWEVER, if you have a type T for which the default std::swap() may result
in either T's copy constructor or assignment operator throwing, you are
politely required to provide a swap() overload for your type that does not
throw. [Since swap() cannot return failure, and you are not allowed to throw,
your swap() overload must always succeed.] By requiring that swap does not
throw, the above operator= is thus exception safe: either the object is
completely copied successfully, or the left-hand side is left unchanged.

Write clean and maintainable C++

Levels of Exception Safety

Exceptions are part of C++. They are thrown by the standard library classes, and sometimes even if we are not really using the standard library. So, unless we are in a very restrictive environment like embedded programming and have exceptions disabled in the compiler, we need to be prepared and deal with the fact that exceptions simply can happen.

The four levels

Any piece of code we write has one of four levels of exception safety: No guarantee, the basic guarantee, the strong guarantee anf the nothrow guarantee. Let’s consider them one by one.

What does it mean if code has no guarantee regarding exceptions? It simply means that if an exception is thrown during the execution of that piece of code, anything can happen. With “anything” I mean anything bad, from leaked resources to dangling pointers to violated class invariants. Here’s a very simple example:

At first glance this may look good, since the object passes both pointers straight to the two `unique_ptr`s that take care of the memory release. But this code may leak memory, since when the second of the two `new`s fails, it will throw a `std::bad_alloc`. The exception will propagate out of the function while the memory allocated by the first `new` has not been given to a `unique_ptr` and therefore will never be freed.

Arguably, when the allocation of memory for something tiny like an `int` or `double` fails, we are in big trouble anyways, but the point is that this code may leak resources and is therefore not exception safe.

Generally, any code that has not been proven to be exception safe should has no guarantee and should be considered unsafe. Code without any exception guarantee is hard to work with – we can’t know for sure the state of the objects after an exception is thrown, which means that we possibly even can’t properly clean up and destroy them.

Don’t write code that has no exception guarantee.

Easier said than done? Not really, because the basic guarantee really is pretty basic. It says that if an exception is thrown during the execution of our code, no resources are leaked and we can be sure that our objects class invariants are not violated. Nothing more, nothing less.

It does especially mean that we don’t necessarily know the content or state or values of our objects, but we know we can use and destroy them, because the invariants are intact. That we can destroy them is probably the most important part of the basic guarantee, since a thrown exception will incur some stack unwinding and affected objects may get destroyed.

Design your classes to have proper class invariants that are always met, even in the presence of exceptions.

The strong guarantee adds to the basic guarantee, that if an operation fails with an exception, then it leaves the objects in the same state they had before. In general, for the strong guarantee we have to do all actions that could possibly throw without affecting any existing object, and then commit them with actions that are guaranteed to not throw an exception.

An example for the strong guarantee is the copy and swap idiom for assignment operators:

The steps are simple: first create a copy of the other object. This may throw an exception, but if it does, the function is terminated early and nothing has happened to `*this` or the other object yet. Then swap `*this` with the copy. For this to work, the swap operation may not throw any exceptions. Examples are the exchange of a few pointers and other built in types. The swap is the commit action, after that the assignment is complete. When the function is left with the return statement, the temporary object is destroyed, cleaning up the state previously owned by `*this`.

Providing the strong guarantee can be costly. For example, imagine if the `Strong` object in the example allocates large amounts of memory. Instead of reusing the already allocated memory, the temporary has to allocate new memory just to release the old one after the swap.

Provide the strong guarantee only if needed. Document operations that have the strong guarantee, use the basic guarantee as default.

The last missing level is the nothrow guarantee . It simply means that an operation can not throw an exception. As you have seen, nothrow operations are needed to provide the strong and basic guarantee. There are some operations that should never throw, no matter what:

destructors have to be nothrow, because they are called during stack unwinding. If an exception is active and a second exception is thrown during stack unwinding, the program will be terminated.
Any cleanup operations like closing files, releasing memory and anything else that might be called from a destructor should not throw.
swap operations. They are commonly expected not to throw. If you have an operation that exchanges the values of two objects but can’t provide the nothrow guarantee, don’t call it `swap` but something different like `exchange`.

Consider using the keyword `noexcept` to document operations that provide the nothrow guarantee.

Reasoning about exception safety can be hard, but thinking in the four levels no guarantee, basic guarantee, strong guarantee and nothrow guarantee makes it much easier. Have a short look at each function you write and make sure that it has at least the basic guarantee. If you use code you have not written, assume it has the basic guarantee as well unless it is documented otherwise.

16 Comments

In your particular example with list-initiliazation

DoubleOwnership object { new int(42), new double(3.14) };

it will work just fine, because there is a sequenced before relation in list-initialization.

(see here related discussion and quote from Standard http://stackoverflow.com/questions/14060264/order-of-evaluation-of-elements-in-list-initialization )

No, it won’t work just fine. Order of evaluation is not the issue here. The order of execution will basically be:

pre class=”lang:c++ decode:true”>int* tempPtr1 = new int(42); double* tempPtr2 = new double(3.14); //call constructor DoubleOwnership(tempPtr1, tempPtr2) //… etc The unique_ptr s will be constructed inside the DoubleOwnership constructor, i.e. after both new expressions have been evaluated . Now, if the new double(3.14) throws, e.g. because there is no more memory, there is no owner of the memory allocated by the first new . Therefore it creates a leak. See also Jens’ comment.

Oh, yeah, got it, sitting in the restroom! Thanks!

You’ve written a nice summary here.

BTW, your assignment operator can be tightened up a bit. Make the temporary copy by passing “other” by value:

Strong & operator=(Strong other) { swap(other); return *this; }

(I suppose you might still want to say “other.swap(*this);”, to emphasize that you’re not calling a global swap function.)

The above is not only a bit more readable, it might also be a bit more efficient, as it is easier for a compiler to optimize.

I don’t understand why in the example, memory will be leaked. Doesn’t the first unique_ptr be destroyed?

Maybe I can understand it if you suggest a way to make it exception safe (with the same interface and the same main function).

Memory will be leaked because both “new” calls are evaluated before calling “DoubleOwnership” constructor. In that case if second new fails unique_ptr will not be created and we end up with allocated memory and no handle to it.

does the use of make_unique help? How would you make it safe? It looks like there is no way with the current interface. (I would make the argument unique_ptr in the first place)

Sadly, make_unique won’t help directly with this function signature, since it forces you to separate memory allocation (outside the function) and taking ownership (inside the function). The best way to make this safe is indeed to change the signature to expect two unique_ptr . A workaround would be to first allocate the memory safely and then pass it to the function in a second, nonthrowing step:

<pre class="lang:c++ decode:true">int foo() {

auto pi = make_unique(42); auto pd = make_unique(3.14); DoubleOwnership object { pi.release(), pd.release() }; //… }

Could explain why

int foo() { DoubleOwnership object { make_unique(42), make_unique(3.14) }; //… }

wouldn’t solve the problem? I think this first creates a temporary std::unique_ptr, and if the second constructor throws, the unique_ptr’s destructor will be called to release the memory (see also Herb Sutter’s GotW 102: http://herbsutter.com/gotw/_102/ ). The C++ Core Guidlines authors also think solves the problem:

fun(shared_ptr(new Widget(a, b)), shared_ptr(new Widget(c, d))); // BAD: potential leak

This is exception-unsafe because the compiler may reorder the two expressions building the function’s two arguments. In particular, the compiler can interleave execution of the two expressions: Memory allocation (by calling operator new) could be done first for both objects, followed by attempts to call the two Widget constructors. If one of the constructor calls throws an exception, then the other object’s memory will never be released!

This subtle problem has a simple solution: Never perform more than one explicit resource allocation in a single expression statement. For example:

shared_ptr sp1(new Widget(a, b)); // Better, but messy fun(sp1, new Widget(c, d));

The best solution is to avoid explicit allocation entirely use factory functions that return owning objects:

fun(make_shared(a, b), make_shared(c, d)); // Best

( https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines.html#Rr-single-alloc )

The only problem I see is when DoubleOwnership’s constructor releases the pointer from one of the unique_ptrs, and then throws.

You first snippet would indeed be the best. However, it does not use the constructor taking two raw pointers. That is exactly the point I wanted to make: that constructor was difficult to use safely. I don’t see the problem you see with throwing after releasing: unique_ptr ‘s constructors and release operation are noexcept , so once the memory allocation has succeeded the DoubleOnwership construction will succeed as well.

Any cleanup operations like closing files, … that might be called from a destructor should not throw.

And so the error return code of the system is gone on closing generally. No, that’s the wrongest tip to say. If you need a nonthrowing close or destroy, write it and also a throwing one as the default version.

Usually there is no errors in closing something, especially since you normally neither expect nor are interested in whether shutting something went smoothly or not. So in most cases it is only natural to have a nonthrowing cleanup. In addition, if you can’t clean up something due to an error, what are you going to do? You will throw it away anyways. In addition, having two operations that do (mostly) the same thing, where one can throw and the other don’t, is a maintenance burden and a disaster waiting to happen. Someone will of course use the wrong version, causing an exception during stack unwinding. Debugging that kind of errors can be a real nightmare.

Usually there is no errors in closing something

Not? Most Windows-Functions for closing and destroying can return an error, on other systems too, I guess. Think of files on servers: Opening may be checked, writing is not checked because use of cache. So the only possible error return point is at closing the file. Real sample: Windows 95 did not check writing on floppy disc, it wrote all data at closing (unlike Windows NT 4, where ejecting and reinsert while writing was recoverable). std::uncaught_exceptions() may be useful for antis of 2 equal functions. Or catch(…) in destructor. Or making the non-throwing version protected or private. Or whatever. The point is: Most cleaning up operations must throw. Normally you have to cleanup manually, not per destructor — your second of the 3 points is incorrect formulated.

My point is exactly that you should normally not have to cleanup manually. Having to clean up is a responsibility that should be encapsuled in a RAII class . I am writing about very general guidelines here, so there naturally will be cases where those guidelines don’t fit. However, I don’t see how in the context of exceptions in modern C++, error codes in a 20 year old C library do make a good counter example.

Errors that occur during cleanup are rare, but they often indicate something catastrophic going on. The POSIX close() function is a great example showing two kinds of cleanup errors.

(1) Errors such as EIO, which may happen if, for example, a network error occurs when closing a file on an NFS mount. This error indicates your data isn’t saved and is a problem that should not be ignored—no different than a write() call that fails due to EIO.

(2) Errors such as EINVAL, which happen as a result of closing an invalid file descriptor. This indicates a dangling file descriptor in your program (i.e., a logic bug) or a corrupted file descriptor (i.e., who knows?). Again, neither error should be ignored. EINVAL indicates your program is in “undefined behavior” territory, and continuing as though nothing bad happened is a poor choice.

It’s exactly because cleanup errors are, as you say, “unusual,” that ignoring them is a bad idea. The common case is that no cleanup error occurs and your program runs okay. But when a cleanup error does occur, something catastrophic may be going on and it’s better to be alerted sooner rather than later. I prefer terminating. After all, a robust program recovers from a previous crash—even an unusual one. It’s the non-robust programs that can’t deal with crashing.

As far as I know, those POSIX functions are C functions which do not report errors by throwing exceptions. Cleanups that return error codes are acceptable – you can chose to ignore them or deal with the problem. However, cleanups that throw exceptions are a problem. You can hardly write RAII classes that use them, since if an exception is thrown from a destructor, be it during stack unwinding or during normal operation, terminate will be called since destructors are implicitly declared noexcept . And just crashing a program is a bad decision, because it means just ending the program without further notice and possibly losing more than just the data of that problematic file. Instead, raising all red flags you have and then gracefully shutting down the program is the better option. Note that I am not generally advocating against raising exceptions when a cleanup fails. I am advocating against the cleanup itself raising the exception. The difference is that during a normal function, you can check for cleanup success and in case of a problem throw (which may or may not lead to program shutdown), while in the destructor of the RAII class that manages the cleanup you don’t throw.

Exception Safety In C++ Linked List Copy Assignment Operators

Exception safety is a critical aspect of robust C++ programming, especially when dealing with complex data structures like linked lists. In this article, we delve into the intricacies of ensuring exception safety in the copy assignment operators of linked lists. We explore the basic concepts, guarantees, and challenges of exception handling, and provide insights into designing exception-safe copy assignment operators. The article also covers techniques for managing resources during copying, handling special cases, and testing for exception safety to ensure that your linked list implementation is reliable and robust.

Key Takeaways

Understanding the levels of exception safety guarantees is crucial for writing robust C++ code.
Copy assignment operators in linked lists must be designed with deep copy semantics to ensure data integrity.
The strong exception safety guarantee is essential to prevent resource leaks and maintain program invariants.
Special cases such as self-assignment and circular references require careful handling to avoid undefined behavior.
Regular testing and debugging are necessary to validate the exception safety of linked list copy assignment operators.

Understanding Exception Safety in C++

Basic concepts of exception safety.

Exception safety in C++ is a critical aspect of robust software design, ensuring that programs behave predictably in the face of errors. Exception safety refers to the guarantees a piece of code provides about its state and the state of the program when exceptions occur. These guarantees are essential for writing reliable applications that can handle unexpected events without leaking resources or leaving the program in an inconsistent state.

Basic guarantee : The program remains in a valid state, and no resources are leaked.
Strong guarantee : Operations are either completed successfully or rolled back entirely, leaving the program state unchanged.
Nothrow guarantee : The code does not throw any exceptions and ensures successful completion.

Exception safety is not just about preventing crashes; it’s about maintaining the integrity of the program’s state and resources during exceptional circumstances.

Table of Contents

Understanding the different levels of exception safety is the first step towards implementing robust error handling in your C++ programs. It is crucial to recognize the implications of each guarantee and to design your code accordingly to uphold these promises, especially in complex operations such as copying linked lists.

Exception Safety Guarantees

In the realm of C++ programming, exception safety guarantees are commitments made by a piece of code to maintain certain invariants and to clean up resources properly when exceptions occur. Exception safety is not an all-or-nothing proposition ; it exists on a spectrum with varying levels of guarantees. These guarantees can be broadly categorized into three levels:

Basic Guarantee : At this level, the code ensures that invariants are preserved and no resources are leaked. However, the program state may still change in an unpredictable manner.
Strong Guarantee : This level aims to make operations either completely successful or have no observable effects at all, effectively rolling back to the initial state in case of an exception.
No-throw Guarantee : The highest level of exception safety, where the code guarantees to never throw exceptions, thus ensuring the program state remains unchanged.

It is crucial for developers to understand the implications of each guarantee level and to design their code accordingly, especially when dealing with complex data structures like linked lists. The choice of guarantee level can significantly impact the robustness and maintainability of the software.

Each guarantee level requires a different approach to exception handling and resource management. For instance, achieving the strong guarantee often involves the use of copy-and-swap idiom or transactional techniques, while the no-throw guarantee may necessitate careful use of no-throw operations and statically guaranteed resource release patterns.

Challenges in Exception Handling

Exception handling in C++ is a critical aspect of writing robust software. Ensuring exception safety in complex systems like linked lists can be particularly challenging. The primary difficulty lies in guaranteeing that operations can be rolled back to a consistent state when exceptions are thrown.

Several challenges arise when dealing with exception handling in C++ linked lists:

Resource management : Ensuring that all resources are properly released or rolled back in the event of an exception.
State consistency : Maintaining the integrity of the linked list’s state during copy operations that may throw exceptions.
Error propagation : Correctly propagating exceptions up the call stack without losing error information or causing resource leaks.

Exception safety is not just about catching exceptions; it’s about maintaining the integrity and consistency of data structures in the face of errors.

Understanding the nuances of exception handling keywords such as throw , try , and catch is essential. These keywords form the backbone of C++ exception handling, allowing developers to signal problems ( throw ), attempt operations that may fail ( try ), and handle failures ( catch ).

Designing a Copy Assignment Operator for Linked Lists

Copy assignment operator basics.

In C++ linked lists, the copy assignment operator plays a crucial role in managing object copies. A copy assignment operator is a non-template non-static member function that defines how objects of the same class are copied from one to another. It is typically denoted by operator= and must be carefully implemented to handle resource management and exception safety.

When designing a copy assignment operator, several considerations must be taken into account:

Ensuring that all resources are copied correctly.
Preventing memory leaks by releasing any resources held by the target object.
Providing exception safety to guarantee that the target object remains in a valid state if an exception occurs during copying.

It is essential to distinguish between deep and shallow copying, as this affects how resources are managed and copied. Deep copying involves creating new instances of the resources, while shallow copying only copies pointers or references, leading to shared resources between objects.

Correctly handling the copy assignment operator is vital for the integrity of the linked list and the correctness of the program. Failure to do so can result in undefined behavior, memory leaks, or even program crashes.

Deep Copy vs. Shallow Copy

When designing a copy assignment operator for a linked list in C++, understanding the difference between a deep copy and a shallow copy is crucial. A shallow copy duplicates only the member field values , which means that the copied object’s pointers still reference the same memory locations as the original object’s pointers. This can lead to issues such as multiple objects trying to free the same memory during destruction, leading to undefined behavior.

In contrast, a deep copy involves duplicating everything the original object has, including allocating new memory for the objects that the original pointers refer to. This ensures that each object manages its own memory and can be safely altered or deleted without affecting the other.

It is essential to consider the implications of each copying method on the exception safety of your linked list implementation. A deep copy can provide a stronger guarantee of exception safety by isolating each object’s state, but it also requires careful management of resource allocation and deallocation to prevent memory leaks or dangling pointers.

The choice between deep and shallow copying affects not only the correctness of the program but also its performance. Here’s a comparison:

Shallow Copy : Fast, but risky in the presence of dynamic memory.
Deep Copy : Safe and isolated, but potentially slower due to memory operations.

Managing Resources During Copy

When designing a copy assignment operator for a linked list, it is crucial to manage resources meticulously to prevent memory leaks and ensure data integrity. Proper handling of heap-allocated nodes is at the heart of this process. Each node must be copied correctly, and the original list’s structure must be preserved in the copy.

Ensure each node is individually copied and correctly linked.
Handle exceptions that may occur during node allocation.
Clean up any allocated resources in case of failure to maintain a consistent state.

Careful resource management during the copy operation is essential to maintain the integrity of the linked list and prevent resource leaks.

The allocation and deallocation of objects in the heap play a significant role in managing resources. A common approach is to use smart pointers that automatically manage the lifetime of heap-allocated objects, reducing the risk of memory leaks.

Ensuring Strong Exception Safety

Implementing the strong guarantee.

In the context of a linked list’s copy assignment operator , implementing the strong guarantee means ensuring that either the operation completes successfully or it has no effect at all, leaving the original state intact. This is crucial for maintaining data integrity in the face of exceptions.

To achieve this, one must carefully manage resources and handle exceptions in a way that either fully commits the changes or rolls them back without side effects. A common approach involves the use of copy-and-swap idiom, where the state is first copied to a temporary object and then swapped with the target object only if the copy was successful.

The strong guarantee is akin to an all-or-nothing approach, where the operation must be atomic — it must either complete entirely or not at all.

Here are some steps to consider when aiming for strong exception safety:

Create a copy of the data to be assigned.
Use exception-safe memory allocation techniques.
Implement transactional behavior to revert changes on failure.
Employ the copy-and-swap idiom for final assignment.

Exception-Safe Techniques and Patterns

In the realm of C++ programming, ensuring exception safety is a critical aspect of robust software design. Exception handling mechanisms such as try , catch , and throw are fundamental in managing runtime anomalies. A variety of techniques and patterns have been developed to facilitate exception-safe code, particularly in resource management scenarios like those encountered with linked lists.

One such pattern is the use of RAII (Resource Acquisition Is Initialization), which ensures that resources are properly released when exceptions occur. Another common practice is the use of smart pointers, such as std::unique_ptr and std::shared_ptr , which automatically handle resource deallocation.

Careful design of copy assignment operators can prevent resource leaks and ensure that the state of the program remains consistent even when exceptions are thrown.

The following list outlines some key exception-safe techniques:

Employ RAII to manage resource lifecycles
Utilize smart pointers for automatic resource management
Implement copy-and-swap idiom for strong exception safety
Leverage the nothrow guarantee of certain standard library functions
Design classes to be exception-neutral by separating resource management from logic

Handling Allocation Failures

In the context of a linked list’s copy assignment operator, handling allocation failures is critical to maintaining exception safety. Proper management of resources during copying is essential to prevent memory leaks and ensure the integrity of the list. When an allocation failure occurs, the operator must clean up any partially constructed state without affecting the existing data structure.

To address allocation failures effectively, consider the following steps:

Verify the availability of sufficient memory before attempting to allocate new nodes.
Use RAII (Resource Acquisition Is Initialization) wrappers to manage memory automatically.
Implement rollback mechanisms to undo partial changes in case of an exception.
Employ smart pointers, like std::unique_ptr , to handle temporary allocations safely.

It is imperative to design the copy assignment operator with a fail-safe approach, where the original state of the list is preserved until the new copy is successfully created and ready to replace it.

Dealing with Special Cases

Self-assignment considerations.

When implementing a copy assignment operator for a linked list, it is crucial to handle the case of self-assignment safely. Self-assignment occurs when an object is assigned to itself , and if not managed correctly, it can lead to resource leaks or corruption of the list’s data structure. To prevent these issues, the operator must first check if the this pointer is the same as the address of the passed object.

It is a good practice to implement an early exit strategy if self-assignment is detected. This not only ensures safety but also improves performance by avoiding unnecessary work.

Here are the steps to handle self-assignment in a copy assignment operator:

Check for self-assignment by comparing the object’s address with the this pointer.
If they are the same, return the object itself to end the function early.
If not, proceed with the copy operation, ensuring that any dynamic memory is safely managed to prevent leaks or corruption.

Copy-and-Swap Idiom

The copy-and-swap idiom is a robust technique for implementing exception-safe copy assignment operators. It leverages the copy constructor and a swap function to ensure strong exception safety . The process involves creating a temporary object with the copy constructor, which should already handle exceptions properly, and then swapping the contents of the temporary object with the current instance.

The benefits of this approach include:

Simplified exception handling, as the heavy lifting is done by the copy constructor.
Automatic reversion to the original state if an exception occurs during copying, preventing any changes to the original object.
Improved code maintainability and clarity.

By decoupling the copying and swapping operations, developers can isolate potential exception-throwing code, making it easier to reason about the exception safety of the copy assignment operator.

When implementing the copy-and-swap idiom, it’s crucial to ensure that the swap function itself does not throw exceptions. This often means it should only perform operations that are ‘noexcept’ guaranteed, such as swapping pointers or primitive data types. The idiom is particularly useful in managing resources during copy operations, as it inherently provides a strong exception guarantee, ensuring that either the operation succeeds completely or has no effect at all.

Handling Circular References

Circular references in linked lists pose a unique challenge for copy assignment operators, as they can lead to infinite loops or memory corruption if not handled properly. Proper management of circular references is crucial to ensure exception safety and maintain the integrity of the data structure. To address circular references, developers can employ several strategies:

Detecting circularity : Before initiating the copy process, check for the presence of circular references within the original list.
Breaking the cycle : Temporarily break the circular reference to create a linear structure that can be safely copied.
Restoring the structure : After the copy, re-establish the circular reference in the new list.

It is essential to ensure that these steps are performed atomically to maintain strong exception safety. Any failure during the process must leave the list in a valid state, either by rolling back changes or by completing the intended operation without leaking resources.

Careful implementation of these steps can prevent common pitfalls associated with circular references, such as dangling pointers and memory leaks, which are particularly problematic in languages like C++ where manual memory management is prevalent.

Testing and Debugging Exception Safety

Unit testing for exception safety.

Unit testing is a critical component in ensuring that the copy assignment operator for linked lists adheres to the desired exception safety guarantees. Tests should be designed to cover all code paths , including those that may throw exceptions due to resource constraints or logic errors.

Create tests that simulate out-of-memory conditions.
Verify that objects remain in a valid state after an exception is thrown.
Check that no memory leaks occur when exceptions are handled.

It is essential to include tests that specifically target the exception handling mechanisms. These tests help to confirm that the linked list remains consistent and that resources are properly managed under exceptional circumstances.

Remember to test edge cases, such as self-assignment, and use a variety of input data to ensure comprehensive coverage. The goal is to catch potential issues before they become problematic in production code.

Debugging Common Issues

When debugging exception safety issues in C++ linked list copy assignment operators, it’s crucial to identify the common pitfalls that can lead to exceptions. Careful inspection of resource management is often necessary to ensure that memory leaks and dangling pointers are avoided. Here are some typical issues to look out for:

Incorrect handling of self-assignment
Failure to release or properly reallocate resources before throwing an exception
Inadequate rollback mechanisms for partial state changes

It is essential to verify that the copy assignment operator adheres to the basic exception safety guarantee, which ensures that invariants are maintained and no resources are leaked even when exceptions are thrown.

Additionally, developers should be familiar with tools and techniques that can aid in debugging, such as static analysis tools, which can detect potential issues at compile time. The use of the [ C++ Core Guidelines ](https://github.com/isocpp/CppCoreGuidelines) can also provide valuable insights into best practices for writing robust code.

Best Practices and Tools

Ensuring exception safety in C++ requires a combination of best practices and the right tools. Always initialize objects to prevent undefined behavior during exceptions. Use RAII (Resource Acquisition Is Initialization) to manage resources such as memory and file handles, ensuring they are released properly in case of an exception.

When it comes to tools, static and dynamic analysis tools can help detect exception safety issues. For instance, Valgrind and AddressSanitizer can uncover memory leaks and access errors. Additionally, unit testing frameworks like Google Test can be instrumental in verifying exception safety.

Consistent code reviews and adherence to coding standards are crucial for maintaining exception safety.

Below is a list of tools that can aid in ensuring exception safety:

Boost.SmartPtr : A library providing smart pointers for automatic memory management.
Cppcheck : A static analysis tool for C++ code quality.
Clang-Tidy : A clang-based C++ linter tool.
Visual Studio : Offers built-in code analysis tools.
GCC : With the -fexceptions flag, it can help in tracking exception handling paths.

In this article, we have explored the intricacies of ensuring exception safety in C++ linked list copy assignment operators. We delved into the challenges of managing dynamic memory and handling potential exceptions to maintain the integrity of the data structure. By examining various strategies, such as the copy-and-swap idiom and the strong guarantee, we’ve highlighted how to write robust code that is resilient to exceptions. It’s crucial for developers to understand these concepts to prevent resource leaks, dangling pointers, and other issues that can arise from improper handling of exceptions. Ultimately, mastering exception safety in C++ requires a deep understanding of both the language’s features and the principles of safe resource management.

Frequently Asked Questions

What are the basic concepts of exception safety in c++.

Exception safety in C++ refers to the guarantees provided by a piece of code about its behavior in the presence of exceptions. It involves writing code that is robust against unexpected events such as resource allocation failures or logic errors that lead to exceptions being thrown.

What is the difference between deep copy and shallow copy in the context of a linked list?

A shallow copy duplicates the pointers to the nodes of the original list, leading to two lists sharing the same nodes. A deep copy, on the other hand, creates a completely new list with new nodes containing the same data as the original, ensuring that the two lists are independent.

How can I ensure strong exception safety in a linked list copy assignment operator?

To ensure strong exception safety, you can use the copy-and-swap idiom, where you create a temporary copy of the list and then swap its contents with the original list. This way, if an exception is thrown during the copy, the original list remains unchanged.

What special considerations are there for self-assignment in a linked list copy assignment operator?

Self-assignment must be checked for and handled properly to avoid issues such as resource deallocation that would invalidate the list. This can typically be done by checking if the source and destination objects are the same before proceeding with the assignment.

How do you test for exception safety in linked list operations?

To test for exception safety, you can write unit tests that simulate scenarios where exceptions are likely to be thrown, such as during memory allocation. The tests should verify that the list remains in a valid state and that no resources are leaked.

What are some common issues and best practices for debugging exception safety problems in C++?

Common issues include memory leaks, data corruption, and resource mismanagement. Best practices for debugging include using tools like static analyzers and memory checkers, writing exception-safe code with RAII patterns, and thorough testing to ensure that code behaves correctly in the face of exceptions.

Exceptional C++: 47 Engineering Puzzles, Programming Problems, and Solutions by Herb Sutter

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Chapter 9. Writing Exception-Safe Code—Part 2

Difficulty: 8

Now that we have the default constructor and the destructor under our belts, we might be tempted to think that all the other functions will be about the same. Well, writing exception-safe and exception-neutral copy and assignment code presents its own challenges, as we shall now see.

Consider again Cargill's Stack template:

Now write the Stack copy constructor and copy assignment operator so that both are demonstrably exception-safe (work ...

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Exception safe move operator

I generally (try to) write exception safe copy assignment operators using the copy-swap idiom, and I was wondering if I should be concerned about exceptions when writing the move assignement operators. Here is an example of a copy assignement operator:

But what about the move assignement ? I mean, if an exception is thrown somewhere else in the code during THIS move operation, I will lose the state of both objects right ? So I would have to create a local copy first and then delete everything but the newly created CLArray ...

Please note that data_ is a std::vector, and thanks for the answers !

2 Answers 2

Indeed, it can be difficult or impossible to provide exception guarantees if a move constructor might throw.

I would suggest doing as the standard library does: document that certain operations only have exception guarantees (or, in some cases, are only permitted) if move-construction of T doesn't throw. Ensuring the guarantee by copying the object destroys the benefit of move-assignment for all types, not just the (very rare) ones that might throw.

Thanks for the answer, I totally get why creating a copy would be stupid for a move assignement. – Athanase Commented Aug 29, 2013 at 9:55

Anyway you should add a swap member function and leverage the (copy / move) constructors in the (copy / move) assignment operators. (And put the operations that can't throw after those that might.)

Example (here inline in the class for brevity):

See C9 Lectures: Stephan T. Lavavej - Standard Template Library (STL), 9 of n (rvalue references) (video and STL's remarks and code in the comments).

You may also want to read Dave Abrahams's articles Your Next Assignment… and Exceptionally Moving! .

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Assignment Operators in C Example
Assignment Operators in C++
Assignment Operators in C++
What is assignment operator in C with example?

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Exception safe assignment
Exception safe assignment Longer title: exception safe assignment operator of resource owning objects. Uff. Because the object owns a resource, how do we write an exception safe assignment operator which will have to free up the old and allocate the new resource. By exception safe I don't mean that it will never throw, that's not possible.
c++ assignment operator exception safety
1. A possible reason for wanting assignment operators to not throw is that the nothrow-ness of the constructors and assignment operators "leaks" up to a container that contains those objects. There is a type trait is_nothrow_move_assignable which is apparently desirable to have. - M.M.
Copy constructors, assignment operators,
Copy constructors, assignment operators, and exception safe assignment Score: 4.3/5 (3169 votes)
PDF Appendix E
Everything will work just as you expect it to, unless your expectations are incorrect. - Hyman Rosen Exception safety — exception-safe implementation techniques — representing resources — assignment — p u s h _ b a c k () — constructors and invariants — standard container guarantees — insertion and removal of elements — guarantees and tradeoffs — s w a p ...
More C++ Idioms/Copy-and-swap
In C++11, such an assignment operator is known as a unifying assignment operator because it eliminates the need to write two different assignment operators: copy-assignment and move-assignment. As long as a class has a move-constructor, a C++11 compiler will always use it to optimize creation of a copy from another temporary (rvalue).
Exception-Safe Class Design, Part 1: Copy Assignment
GotW #59: Exception-Safe Class Design, Part 1: Copy Assignment. This is the original GotW problem and solution substantially as posted to Usenet. See the book More Exceptional C++ (Addison-Wesley, 2002) for the most current solution to this GotW issue. The solutions in the book have been revised and expanded since their initial appearance in GotW.
using swap to make assignment operator exception safe
Hello everyone, The following swap technique is used to make assignment operator exception safe (means even if there is exception, the current object instance's state is invariant). It used a temporary object "temp" in this sample, and assignment is made on a to temp ar first.
Exception Handling in C++: Exception safety
Exception-safe code leaves objects in a consistent state and does not leak resources. You also need to be careful writing custom assignment operators. In Chapter 12 of Volume 1, you saw that operator= should adhere to the following pattern: Make sure you're not assigning to self. If you are, go to step 6.
C++ Exception-Safe Assignment Overloading
Exception-Safe and Self-Assignment. Make sure the resource management inside the assignment overloading has taken throwing exceptions into account to prevent the data loss from the object. In the following example, we strictly follow allocate, populate and deallocate. If we deallocate the cstringmember variable before allocate the new buffer ...
Exception Safe Assignment
Exception safe assignment operator of resource owning objects Longer title: exception safe assignment operator of resource owning objects. Uff. Because the object owns a resource, how do we write an exception safe assignment operator which will have to free up the old and allocate the new resource.
Copy constructors, assignment operators,
Copy constructors, assignment operators, and exception safe assignment Jan 27, 2010 at 6:15am jsmith (5804)
22. Exception-Safe Class Design, Part 1: Copy Assignment
Exception-Safe Class Design, Part 1: Copy Assignment Difficulty: 7 Is it possible to make any C++ class strongly exception-safe, for example, for its copy assignment operator? If so, how? What are the issues and consequences? To illustrate, this Item explains and then solves the Cargill Widget Example.
PDF Exception Safety: Concepts and Techniques
This paper, based on Appendix E: Standard-Library Exception Safety of The C++ Pro-gramming Language (Special Edition) [1], presents a few fundamental concepts useful for discussion of exception safety effective techniques for crafting exception-safe and efficient containers some general rules for exception-safe programming.
Levels of Exception Safety
Reasoning about exception safety can be hard, but thinking in the four levels no guarantee, basic guarantee, strong guarantee and nothrow guarantee makes it much easier. Have a short look at each function you write and make sure that it has at least the basic guarantee.
Exception Safety In C++ Linked List Copy Assignment Operators
Exception safety is a critical aspect of robust C++ programming, especially when dealing with complex data structures like linked lists. In this article, we delve into the intricacies of ensuring exception safety in the copy assignment operators of linked lists.
Chapter 9. Writing Exception-Safe Code—Part 2
Now write the Stack copy constructor and copy assignment operator so that both are demonstrably exception-safe (work ... Get Exceptional C++: 47 Engineering Puzzles, Programming Problems, and Solutions now with the O'Reilly learning platform.
c++
8. Using the 4 level exception safety system: The compiler generated assignment operator has the basic exception guarantee if each of the members of the object provides the basic exception guarantee or "better", and if the objects invariants do not have intra-member dependencies. It has the no throw guarantee if each member's assignment also ...
OOP54-CPP. Gracefully handle self-copy assignment
The copy assignment operator uses std::move() rather than swap() to achieve safe self-assignment and a strong exception guarantee. The move assignment operator uses a move (via the method parameter) and swap.
c++
I generally (try to) write exception safe copy assignment operators using the copy-swap idiom, and I was wondering if I should be concerned about exceptions when writing the move assignement operators.