In the following code how can I initialize a class member without using dummy values?

Question

// Example program

#include <iostream>
#include <vector>
#include <array>

class A{
    public:
        A(int tempY){
            y = tempY;
        }
        int y = 0;
};

class B{
    public:
        B(A tempZ){
            z = tempZ.y;
        }
        A z;

};

int main()
{
    int x = 1;
    A objA(x);
    B objB(objA);
    std::cout << "y = " << objB.z << "!\n";
}

Build at: http://cpp.sh/3yj2

There is an error in class B because I haven't passed a constructor parameter to member z. I don't want to initialize it with a dummy value, is there a way to only use constructor parameters to build member z, and how do I tell z to use B's constructor parameters?

If I'm missing a fundamental aspect of C++ please let me know I'm just starting out.

This isn't RAII. And you need to initialize `z` in the initializer-list if it has no default constructor. This is how it would look: `B(A tempZ) : z(tempZ) { }`. You should also take a const lvalue-reference to the parameter. — David G, Nov 07 '14 at 18:46
@LightnessRacesinOrbit Because he's not managing an allocated resource's lifetime through `B`. — David G, Nov 07 '14 at 20:12
@LightnessRacesinOrbit Does he manage an allocated resource's lifetime through `B`? — David G, Nov 08 '14 at 17:15
@LightnessRacesinOrbit So does *not* managing a dynamically-allocated object's lifetime through the lifetime of some wrapping class like `B` constitute this *not* being RAII? — David G, Nov 08 '14 at 18:09
@0x499602D2: What are you talking about? Every resource that `B` encapsulates is acquired on construction and released on destruction. That is the very definition of the RAII principle. I don't see the problem here. — Lightness Races in Orbit, Nov 08 '14 at 18:10

score 2 · Accepted Answer · answered Nov 07 '14 at 18:47

2

You can use initializer lists like this

class B{
    public:
        B(A tempZ) : z(tempZ) {
        }
        A z;
};

This way z will be initialized with a new instance of A created by the copy constructor.

answered Nov 07 '14 at 18:47

Eric

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In the following code how can I initialize a class member without using dummy values?

1 Answers1