Question

I’m having some trouble with the class casting in C++.
To learn, I wanted to create a class which just make operations, like sums, but it seems to crash everytime I launch it.

Here is my simple classes:

#include <iostream>


class CCalculation {
public:
CCalculation() {};
virtual int calculate() = 0;
};


class CCalc_CONST : public CCalculation {
    int x;
public:
    CCalc_CONST(int a) : x(a) {};
    int calculate() { return x; };
};    


class CCalc_ADD : public CCalculation {
    CCalculation *x;
    CCalculation *y;
public:
    CCalc_ADD(CCalculation *a, CCalculation *b) {
        this->x = a;
        this->y = b;
    };
    int calculate() {
        std::cout << "Calculation...\n";
        return x->calculate() + y->calculate();
    };
};

And my test:

    CCalculation *a = &CCalc_CONST(4);
    CCalculation *b = &CCalc_CONST(1);
    CCalculation *c = &CCalc_ADD(a,b);

    std::cout << "res: " << c->calculate() << "\n";

It seems to crash everytime (I got no compiler error or warning).
The only way to run it I found is when I’m printing a->calculate and b->calculate at the CCalc_ADD construction. I have absolutely no clue why i need to call the calculate function to make it work.

Can someone please explains to me how to actually do it ?

Answer 1

You’re not using new. Test like this:

CCalculation *a = new CCalc_CONST(4);
CCalculation *b = new CCalc_CONST(1);
CCalculation *c = new CCalc_ADD(a,b);

You can weed out things like address-of on temporaries by compiling with -Wall, which enables all compiler warnings. The compiler is your friend and is there to help. Love the compiler.