Re: c++98/c++03 constructor overloading

From:

"Johannes Schaub (litb)" <schaub-johannes@web.de>

Newsgroups:

comp.lang.c++

Date:

Tue, 07 Dec 2010 13:31 +0100

Message-ID:

<idl986$3gg$00$1@news.t-online.com>

Chris Vine wrote:

Hi,

With gcc-4.2 and earlier, and with subsequent versions of gcc when
using the -std=c++98 option, the following code fails to compile:

--------------------------

#include <iostream>
#include <ostream>

enum ByRef {by_ref};

template <class T>
class MyClass {
  T t;
public:
  MyClass(const T& arg): t(arg) { // (1)
    std::cout << "Without ByRef tag" << std::endl;
  }
  MyClass(T arg, ByRef): t(arg) { // (2)
    std::cout << "With ByRef tag" << std::endl;
  }
};

int main() {
  int i = 0;
  MyClass<int&> m2(i, by_ref);
}

--------------------------

gcc ignores the ByRef tag, and picks constructor 1, and then fails to
compile because of the resulting reference to a reference.

No, It's an error right way when you implicitly instantiate MyClass<int&>.
Try

sizeof(MyClass<int&>);

On a literal C++03 compiler, this *must* result in a compile error because
the declaration of the destructor is invalid. You don't even get to the
point where a constructor could be chosen by overload resolution.

Without the -std=c++98 option, gcc-4.3 onwards pick the right overload
(constructor 2), as does Comeau in strict c++03 mode.

More recent compilers implement this according to various C++ DR
resolutions.