Why constructor is called only once?

Question

Consider the below program:

class A
{
    public:
    A(int i)
    {
            cout<<"Called"<<endl;
    }
};

int main()
{
    vector<A> v(5,A(1));
    return 0;
}       

I am getting the output: http://ideone.com/81XO6

 Called

Why the constructor gets called only once even if we are constructing 5 objects?
How vector is internally handled by the compiler?

Answer 1

Your class has two constructors and you are watching only one of them. std::vector creates its elements by copy-constructing them from the original element you supplied. For that purpose, the copy-constructor of class A is called 5 times in your example.

The copy-constructor for A in your example is implicitly declared and defined by the compiler. If you so desire, you can declare and define it yourself. If you print something from it, you will see that it is called at least 5 times.

Answer 2

It gets called once since the line

vector<A> v(5,A(1)); 

will call the constructor and the line becomes vector v(5,X);

where X is the object constructed after calling the constructor.

After that the copy constructor is used.

Try adding

A(const &A);

To the class declaration to verify this.