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From: asc@concurrent.co.uk (Andy Chittenden)
Newsgroups: comp.lang.c++
Subject: Enumerated types in C++
Message-ID: <874@sl10c.concurrent.co.uk>
Date: 14 Apr 89 06:22:47 GMT
Reply-To: Andy Chittenden <asc@concurrent.co.uk>
Organization: Concurrent Computer Corp (ESDG), Slough, U.K.
Lines: 80

Enumerated types in C++ are not strictly typed like they are in other
languages.  A statement of the form:

	enum { FALSE = (0 != 0), TRUE = (0 == 0) } bool;

does not prevent values other than TRUE or FALSE to be assigned to the
variable bool.

In C++, the above statement is synonymous with:

	declaring an int called bool
	declaring two constants called FALSE and TRUE.

What this means is that strict type checking is not performed for
enumerated types so if we have another enumerated type:

	enum { RED, BLUE, GREEN } colour;

we are still allowed to say:

	colour = TRUE;

Solution
________

The above problem can be circumvented by using two C++ classes for each
enumerated type required. For example,

	class ColourEnumerator {
		enum { red, blue, green } colour;
	public:
		void RED();
		void BLUE();
		void GREEN();
	};

	void ColourEnumerator::RED() {
		colour = red;
	};

	..... and similarly for BLUE() and GREEN()

	typedef void (ColourEnumerator::*ColourEnumeratorMemberPtr)();
			// it would be nice if this typedef could be
			// declared within the Colour class (although
			// Cfront doesn't object, the generated C does
			// not compile)
	class Colour {
		ColourEnumerator colour;
	public:
		Colour();
		Colour& operator=(const Colour&);
		Colour& operator=(ColourEnumeratorMemberPtr);
				// I would like to make this a const
				// parameter also but C++ won't let me (any
				// ideas?)
	};

	Colour::Colour() {}
	Colour& Colour::operator=(const Colour& p_colour) {
		colour = p_colour.colour;
		return *this;
	}
	Colour& Colour::operator=(ColourEnumeratorMemberPtr& p_setting) {
		(colour.*p_setting)(); // actually set the colour
		return *this;
	}

Although the above looks complicated to set up, it enables very strong
type checking when actually using a Colour.  To set a Colour use:

	Colour x;
	x = ColourEnumerator::RED; // turns x RED

Rgds, Andy Chittenden 
Concurrent Computer Corporation
227 Bath Road
Slough
SL1 4AX
England