Question

Is this a legal way of calling a base class constructor.

The Base Class is as follows

class base_class
{
    public:
    base_class(int x, int y);    
    protected:
    int a;
    int b;
};

base_class::base_class(int x,int y)
{
    a=x;
    b=y;
}

The Derived class is a follows

class derived_class: public base_class
{
    public:
    derived_class(int x,int y,int z);
    protected:
    int c;
};
derived_class:: derived_class(int x,int y,int z):base_class(x,y)  /*Edited and included the scope resolution operator*/
{
    c=z;
}

Is this way of defining a derived class constructor legal in C++, if yes how is the base class constructor called?

Answer 1

This is the correct (and only) way to initialise your base class(es) if you’re invoking a non-default constructor — if you add the second colon to the scope-resolution operator, of course.

It is more usual to initialise all of your members in the “initializer list” (the section between the colon and the function body), like so:

derived_class::derived_class(int x,int y,int z):
    base_class(x, y),
    c(z)
{
}

…where c(z) initialises your int c to the value of z.