在另一个类构造函数中初始化类对象

Question

I have a class, it names A and A class has 3 another class in its private.

class A{
    public:
          A();
          A(int num);
          A(C& mC, P& mP, M& mM, int num);
    //there is a getter and setter for all member this only one example(please check are they right?)
          M getM()const{return pM;} 
          void setM(M classM){ pM = classM ;}
        private:
            C& pC;
            P& pP;
            M& pM;
            int digit= 0;
        };

I'm doing that in parameter constucture:

A::A(C& mC, P& mP, M& mM, int num):pC(mc),pP(mP),pM(mM)
{
// doing someting here
}

But I can't write a code for default and first parameter constructure, when I write something compiler saying to me that :

error: uninitialized reference member in 'class A&' [-fpermissive] A::A(){

and

note: 'A& A::pP' should be initialized A& pP;

someting like this, several errors and notes.

What should I do? How can I initialize classes in default and first parameter constructure?

Answer 1

Class A contains references to other object. Unlike pointers, references cannot be null. To make this work, you either need to:

• Use pointers instead of references, and initialize them to nullptr is no valid object is provided in the constructor
• Store those members by value . This involves a copy of the original arguments, and different in semantics - might not be what you need.