如何在不使用复制构造函数的情况下初始化类成员

Question

I'm now writing C++ code to do some maths. However, it seems impossible to use normal constructors to initialize class members.

To get instantiated, the constructor of A is first called. By the time, p and q are just something like declaration so p and q don't get instantiated either. So I should be able to call constructor of B to instantiate them. Is that right? I think maybe my understanding of C++ classes is wrong somewhere so I post here to confirm.

class B{
    // default constructor and copy constructor
}

class A{
private:
    B p;
    B q;
public:
    explicit A(int _p, int _q);
}

// implementation
A::A(int _p, int _q){
    // some computing goes here so I can't apply 
    // A::A(int _p, int _q):p{_p}, q{_q}

    // p = B{_p}
    // q = B{_q} 
    // this works

    p{_p}; // Can't compile.
    q{_q}; // What's wrong with this?
}

Answer 1

No, the constructor's behavior actually looks like this:

A::A(/* parameters */)
    : /* member constructors called here, explicitly or by default, in order of declaration */
{
     /* your constructor code */
}

In other words, by the time you enter a constructor's body, all class members are already fully constructed, either from the initializer list if it's specified there, or default-constructed if it is not in the initializer list. Thus, your A constructor behaves as if you wrote A::A(int _p, int _q) : p{}, q{} { /* code */ } . q = B{_q}; does not call the copy constructor; it constructs a temporary B object and calls the copy or move assignment operator of q , but since the compiler generates those for you in many cases, it compiles. q{_q}; , as a statement, is simply not valid C++.

Answer 2

The problem is that you simply can't use initialization lists to perform assignments by the way you are trying . And, since you can't initialize the variable at first, you are actually looking for an assignment (notice that p and q have already been default initialized ). When you write:

p{_p};
q{_q};

It's just not valid syntax, since there is no operation defined over lvalue<initialization_list> after the variable has already been initialized (at anywhere else out of the constructor's initialization list/variable declaration).