Question

I am still confused with the CTORs:

Question 1:
why line 15 call A:A(int) instead of A:A(double&)?

Question 2:
why line 18 did not call A:A(B&)?

#include <iostream>
using namespace std;

class B{};

class A{
public:
   A(int )    {cout<<"A::A(int)"<<endl;}
   A(double&){cout<<"A::A(double&)"<<endl;} // it will work if it is A(double), without the &
   A(B&){cout<<"A::A(B&)"<<endl;}
};

int main()
{
/*line 15*/   A obj((double)2.1);  // this will call A(int), why?
   B obj2;
   A obj3(obj2);
/*line 18*/   A obj4(B);          // this did not trigger any output why?
}

Answer 1

Line 15: A(double&) can only take lvalues, i.e. variables that can be assigned to. (double)2.1 is an rvalue. Use A(const double&) if you need to accept rvalues as reference too.

Line 18: B is a type, not a value. A obj4(B); only declares a function with name obj4 taking a B and returning an A.