c++ - How to initialize class members without using the copy constructor -

i'm writing c++ code maths. however, seems impossible use normal constructors initialize class members.

to instantiated, constructor of first called. time, p , q declaration p , q don't instantiated either. should able call constructor of b instantiate them. right? think maybe understanding of c++ classes wrong somewhere post here confirm.

class b{     // default constructor , copy constructor }  class a{ private:     b p;     b q; public:     explicit a(int _p, int _q); }  // implementation a::a(int _p, int _q){     // computing goes here can't apply      // a::a(int _p, int _q):p{_p}, q{_q}      // p = b{_p}     // q = b{_q}      // works      p{_p}; // can't compile.     q{_q}; // what's wrong this? } 

no, constructor's behavior looks this:

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

in other words, time enter constructor's body, class members constructed, either initializer list if it's specified there, or default-constructed if not in initializer list. thus, a constructor behaves if wrote a::a(int _p, int _q) : p{}, q{} { /* code */ }. q = b{_q}; not call copy constructor; constructs temporary b object , calls copy or move assignment operator of q, since compiler generates in many cases, compiles. q{_q};, statement, not valid c++.