Copy Constructor - Definition

Definition

Copying of objects is achieved by the use of a copy constructor and an assignment operator. A copy constructor has as its first parameter a (possibly const or volatile) reference to its own class type. It can have more arguments, but the rest must have default values associated with them. The following would be valid copy constructors for class X:

X(const X& copy_from_me); X( X& copy_from_me); X( volatile X& copy_from_me); X(const volatile X& copy_from_me); X( X& copy_from_me, int = 0); X(const X& copy_from_me, double = 1.0, int = 42); ...

The first one should be used unless there is a good reason to use one of the others. One of the differences between the first and the second is that temporaries can be copied with the first. For example:

X a = X; // valid given X(const X& copy_from_me) but not valid given X(X& copy_from_me) // because the second wants a non-const X& // to create a, the compiler first creates a temporary by invoking the default constructor // of X, then uses the copy constructor to initialize as a copy of that temporary. // For some compilers both versions actually work but this behaviour should not be relied // upon because it's non-standard.

Another difference between them is the obvious:

const X a; X b = a; // valid given X(const X& copy_from_me) but not valid given X(X& copy_from_me) // because the second wants a non-const X&

The X& form of the copy constructor is used when it is necessary to modify the copied object. This is very rare but it can be seen used in the standard library's std::auto_ptr. A reference must be provided:

X a; X b = a; // valid if any of the copy constructors are defined // since a reference is being passed.

The following are invalid copy constructors (Reason - copy_from_me is not passed as reference) :

X(X copy_from_me); X(const X copy_from_me);

because the call to those constructors would require a copy as well, which would result in an infinitely recursive call.

The following cases may result in a call to a copy constructor:

1. When an object is returned by value
2. When an object is passed (to a function) by value as an argument
3. When an object is thrown
4. When an object is caught
5. When an object is placed in a brace-enclosed initializer list

These cases are collectively called copy-initialization and are equivalent to: T x = a;

It is however, not guaranteed that a copy constructor will be called in these cases, because the C++ Standard allows the compiler to optimize the copy away in certain cases, one example being the return value optimization (sometimes referred to as RVO).