Question

Can one of you explain why the following piece of code does not compile?

#include <iostream>

using namespace std;

class Foo
{
public:
  Foo() { cout << "Foo::Foo()" << endl << endl; }
  Foo& operator=(const Foo&) { cout << "Foo::operator=(const Foo&)" << endl << endl; }
private:
  Foo(const Foo& b) { *this = b; cout << "Foo::Foo(const Foo&)" << endl << endl; }
};

int main()
{
  Foo foo;

  foo = Foo();
}

The error I receive:

$ g++ -o copy_ctor_assign copy_ctor_assign.cc && ./copy_ctor_assign
copy_ctor_assign.cc: In function 'int main()':
copy_ctor_assign.cc:10: error: 'Foo::Foo(const Foo&)' is private
copy_ctor_assign.cc:17: error: within this context

Note: when I remove the private: keyword the code compiles but the copy ctor is never called. So why does it err when it’s private?

Not sure if it’s important but I’m using:

$ g++ --version
g++ (GCC) 4.1.2 20080704 (Red Hat 4.1.2-44)
Copyright (C) 2006 Free Software Foundation, Inc.

Answer 1

You are initializing a reference from temporary.
The standard states:
The temporary should be initialized (8.5.3 par 5)”using the rules for a non-reference copy initialization (8.5)”.

The copy construction is removed for the temporary (permitted by the standard. 12.8 par 5).
However, the standard clearly states (12.2 par 1):
“Even when the creation of the temporary object is avoided (12.8), all the semantic restrictions must be respected as if the temporary object was created. [Example: even if the copy constructor is not called, all the semantic restrictions, such as accessibility (clause 11), shall be satisfied. ]”

(also, when looking for the right quote, found this duplicate 🙂

Edit:
adding relevant location from the standard