WHAT DOES A COPY CONSTRUCTOR DO?

[ayanda83]

Hi there, can somebody tell me what a copy constructor does? and when do u know you need one? thanking you in advance.

[viliml]

a copy constructor looks like this:

class foo
{
       public:
       int x;
       foo (foo a) {x=a.x;}
       }

So basicly, it costructs a new object out of an existing one, copying all of it's elements into itself.

[ayanda83]

thank you very much viliml.

[andywestken]

Note that while viliml's example is not incorrect, it is not a good idea to use pass by value for copy constuctors. The canonical form uses pass by const ref.

And constructors should use the constructor initializer list to initialize member variables.

class Point
{
private:
    int m_x;
    int m_y;
    // etc (e.g. color)

public:
    Point()
    : m_x(0), m_y(0) // etc 
    {
    }

    Point(int x, int y)
    : m_x(x), m_y(y) // etc 
    {
    }

    Point(const Point& that)
    : m_x(that.m_x), m_y(that.m_y) // etc 
    {
    }

    // etc
};