Can someone explain why this doeasn't work? The default constructor of "B" cannot be referenced -- it is a deleted function

Question

I'm currently making c++ project but this error is bothering me for long time and i cannot figure out why this doesn't work. I was searching about this error but still i don't understand it.

Thanks in advance.

#include <iostream>
using namespace std;


class A
{
public:
    int a = 0;
    A(int _a) : a(a) {}
};

class B
{
public:
    A a;
    void test()
    {
        A a1(6);
        a = a1;
    }
};
    
int main()
{
    B b1;
    b1.test();

    
    return 0;

}

I tried to initialized value in constructor in class and this worked but what if i don't want to do this?

Answer 1

First, your member initialization list in A's converting constructor is wrong. a(a) should be a(_a) instead.

Second, A has a user-defined constructor, so its compiler-generated default constructor is implicitly delete'd.

Thus, B::a can't be default-constructed, so B's compiler-generated default constructor is also implicitly delete'd.

Thus, b1 in main() can't be default-constructed, either.

If you want A to be default-constructible, you need to either:

• add a default constructor explicitly:

class A
{
public:
    int a = 0;
    A() {}; // or: A() = default; // <-- HERE
    A(int _a) : a(_a) {}
};

• Change your converting constructor so it can also act as a default constructor:

class A
{
public:
    int a = 0;
    A(int _a = 0) : a(_a) {} // <-- HERE
};

Otherwise, you will have to add a default constructor to B to construct B::a with a value:

class B
{
public:
    A a;
    B() : a(0) {} // <-- HERE
    void test()
    {
        A a1(6);
        a = a1;
    }
};