Can someone explain why this doeasn't work? The default constructor of "B" cannot be-CodePudding

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?

CodePudding user response：

A doesn't have a default constructor so it cannot be default constructed.

A class object must be fully initialized before entering the body of the constructor which means that in B you need to initialize a (either with data member init or with constructor list initialization).

If for whatever reason you want to delay the initialization of a you have a few options. Depending on the semantics of it you could make it std::optional<A> a or std::unique_ptr<A>.