TITLE: what use are elaborated type names?


PROBLEM: matth@extro.ucc.su.OZ.AU (Matthew Hannigan)

This article inspires me to post an interesting little example
of a function hiding a constructor:

A little wrapper class for an int:

	#include <iostream.h>

	class g {
	public:
		int p;
		g() { p = 7777; };
		operator int() {return p;};
	};
	 
	main() { cerr << g() << endl; }

works fine; however putting a function:

	int g() { return 9999; };

before main() _silently_ overrides the constructor (tested under Sun's CC
4.0.1 and g++ 2.6.3).


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 27 June 95

It doesn't override the constructor, it hides the type name. Because the
type name isn't visible, you cannot refer directly to the constructor.

For example, you cannot declare
	g x;
but have to use the elaborated type name:
	class g x;

This result is due to a C compatibility wart that wasn't in early C++,
but was added in (I think) 1989. The problem is that C allows
	struct x { ... } x;
	struct y { ... };
	int y;

C++ originally did not allow these conflicts, since the same name in the
same scope would refer to different things. But the restriction broke
too much C code being ported to C++. An object, typedef, or function name
is thus allowed in the same scope as a class name, and hides the
unelaborated class name.