How would you find out the no of instance of a class?

Answer 1

Use a static variable in the class, initialised with the value zero. Each time any constructor is invoked, increment the variable and assign it to an instance member variable. The only exception is the move constructor which should take on the identity of its argument.

class foo

{

private:

static size_t counter;

size_t identity;

public:

foo (): identity (++counter) {} // default constructor (new identity)

foo (const foo& rhs): identity (++counter) {} // copy constructor (new identity)

foo (foo&& rhs): identity (std::move(rhs.identity)) {} // move constructor (take identity from original)

foo& operator= (const foo& rhs) {} // copy assign (retain current identity)

foo& operator= (foo&& rhs) {} // move assign (retain current identity)

~foo() {}

const size_t get_identity() const { return identity; } // accessor to identity

};

// all static variables must be initialised outside of the class in which they are declared

size_t foo::counter = 0;