Calling Base Class Constructors From Derived Class

[muhammadjaziem]

Hey Guys,

I'm having some difficulties in understanding the topic which I stated above.Can anyone here provide some good examples and references which will help me in future.

Thanks.

[J4ke]

There is no easy way to do this, as classes can be derived from multiple other classes. There is no way to tell with a single command in what super class you want to call the constructor.

If you make a function that initializes everything, I believe that you can use ((BaseClass) instance_of_derived_class).init();. I am not positive on this one though.

[firedraco]

There is no easy way to do this, as classes can be derived from multiple other classes. There is no way to tell with a single command in what super class you want to call the constructor.

If you couldn't do this, how would you make a class work like this:

class Base {
public:
    Base(int x) {}
};

class Derv : public Base {
public:
    Derv(int x) {} //how to call Base(int x) here?
}

To call it, you must use an initializer list.

[muhammadjaziem]

#include <iostream>
using namespace std;

class MyPoint
{
int x,y;

public:
double get1()
{
cout<<"Enter a value for x"<<endl;
cin>>x
}

double get2()
{
cout<<"Enter a value for y"<<endl;
cin>>y;
}

double difference()
{
double get;
return get=x-y;
}


};

class ThreeDPoint:public MyPoint
{
int z;

ThreeDPoint(int a):MyPoint(x,y)
{
a=0;
z=a;



}

}
## in the example how I want to make the value x and y =0 by calling the base class constructor

[tcs]

class MyPoint
{
private:
    int x,y;

public:
    MyPoint(int initialX = 0, int initialY = 0) : // Take 0 by default
        x(initialX),
        y(initialY)
    {}
    // ...
};

class ThreeDPoint : public MyPoint
{
private:
    int z;

public:
    ThreeDPoint(int a) :
        MyPoint(0, 0),
        z(0)
    {
    }
    // ...
};

[vlad from moscow]

There are two ways. Either you explicitly call a base constructor in mem-initializer list or you use a using declaration of the base constructor name in the derived class.

EDIT: Here is an example but you should find the compiler that will compile the code because not all compilers support inheriting constructors.:)

#include <iostream>
 
struct Base
{
    Base( int i ) : x( i ) {}
    int x;
};
 
struct Derived : Base
{
    using Base::Base;
    Derived() : Base( 0 ) {}
};
 
int main()
{
    Derived d1;
    
    std::cout << "d1.x = " << d1.x << std::endl;
    
    Derived d2( 10 );
    
    std::cout << "d2.x = " << d2.x << std::endl;
    
    return 0;
}