C++ constructor issues

Question

I wonder if anybody can help with what seems to me like strange behaviour in c++ (gcc latest version). Below is some code that compiles successfully where I would expect a compile time error due to the lack of an appropriate constructor. Can anybody explain what is going on?

#include <iostream>
using namespace std;

struct ClassA {
  ClassA() {cout << "hello" << endl;} 
  void speak() {cout << "I am class A" << endl;}
  ~ClassA() {cout << "goodbye" << endl;} 
};

struct ClassB {
    // just an empty struct
};

int main() {
    ClassA myClassA(ClassB()); // trying to construct class A from an rvalue reference to class B is ok?
    return 0;
}

}

If I try to call a function of class A, I do get a compile time error:

int main() {
    ClassA myClassA(ClassB());
    myClassA.speak();
    return 0;
}

results in:

error: request for member ‘speak’ in ‘myClassA', which is of non-class type ‘ClassA(ClassB (*)())’

Thanks!

Answer 1

You're never declaring an object at all. Instead, you've declared a function, and so there's no constructor needed at all. (Note that ClassB() denotes a function type!)

If you want to construct an object from a temporary, try one of these syntaxes:

ClassA x1 { ClassB{} };   // C++11 only

ClassA x2((ClassB()));    // parenthesized expression is never a type declaration

Answer 2

ClassA myClassA(ClassB()); declares a function called myClassA that returns a ClassA and takes one argument which is a pointer to a function that returns a ClassB and takes no arguments. This is the most vexing parse.

Answer 3

This is a situation called the Most Vexing Parse.

First and formost, ClassB does have a constructor. When you don't create one of your own the compiler provides one for you. And on the line where you passed an instance of ClassB into what seems like a valid copy construction, what actually happens is that the line is evaluated as the declaration of a function that returns an instance of ClassA and takes an anonymous instance of ClassB.