variable assignment - Copy constructor and = operator overload in C++: is a common function possible?

Question

Since a copy constructor

MyClass(const MyClass&);

and an = operator overload

MyClass& operator = (const MyClass&);

have pretty much the same code, the same parameter, and only differ on the return, is it possible to have a common function for them both to use?

Answer 1

Yes. There are two common options. One - which is generally discouraged - is to call the operator= from the copy constructor explicitly:

MyClass(const MyClass& other)
{
    operator=(other);
}

However, providing a good operator= is a challenge when it comes to dealing with the old state and issues arising from self assignment. Also, all members and bases get default initialized first even if they are to be assigned to from other. This may not even be valid for all members and bases and even where it is valid it is semantically redundant and may be practically expensive.

An increasingly popular solution is to implement operator= using the copy constructor and a swap method.

MyClass& operator=(const MyClass& other)
{
    MyClass tmp(other);
    swap(tmp);
    return *this;
}

or even:

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

A swap function is typically simple to write as it just swaps the ownership of the internals and doesn't have to clean up existing state or allocate new resources.

Advantages of the copy and swap idiom is that it is automatically self-assignment safe and - providing that the swap operation is no-throw - is also strongly exception safe.

To be strongly exception safe, a 'hand' written assignment operator typically has to allocate a copy of the new resources before de-allocating the assignee's old resources so that if an exception occurs allocating the new resources, the old state can still be returned to. All this comes for free with copy-and-swap but is typically more complex, and hence error prone, to do from scratch.

The one thing to be careful of is to make sure that the swap method is a true swap, and not the default std::swap which uses the copy constructor and assignment operator itself.

Typically a memberwise swap is used. std::swap works and is 'no-throw' guaranteed with all basic types and pointer types. Most smart pointers can also be swapped with a no-throw guarantee.