What is a copy constructor in C++?

Yes, copy constructors are certainly an essential part of standard C++. Read more about them (and other constructors) here (C++ FAQ).

If you have a C++ book that doesn't teach about copy constructors, throw it away. It's a bad book.


A copy constructor has the following form:

class example 
{
    example(const example&) 
    {
        // this is the copy constructor
    }
}

The following example shows where it is called.

void foo(example x);

int main(void)
{
    example x1; //normal ctor
    example x2 = x1; // copy ctor
    example x3(x2); // copy ctor

    foo(x1); // calls the copy ctor to copy the argument for foo

}

See Copy constructor on Wikipedia.

The basic idea is copy constructors instantiate new instances by copying existing ones:

class Foo {
  public:
    Foo();                // default constructor
    Foo(const Foo& foo);  // copy constructor

  // ...
};

Given an instance foo, invoke the copy constructor with

Foo bar(foo);

or

Foo bar = foo;

The Standard Template Library's containers require objects to be copyable and assignable, so if you want to use std::vector<YourClass>, be sure to have define an appropriate copy constructor and operator= if the compiler-generated defaults don't make sense.

Tags:

C++

Constructor

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