_invar.h

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/*
 *
 *
 *	_INVAR.H
 *	
 *	Purpose:
 *		Template class designed to call parameterized object's Invariant().
 *
 *	Overview (see also, usage):
 *		1)	declare and define a public const function BOOL Invariant( void ) in your class, with #ifdef DEBUG.
 *		2)	in the source: #define DEBUG_CLASSNAME to be the name of the class you're debugging.
 *		3)	followed by #include "_invar.h"
 *		4)	For every method you wish to check Invariants,
 *			insert the _TEST_INVARIANT_ macro once, usually at the beginning of a routine.
 *			OPTIONAL: You may optionally use the _TEST_INVARIANT_ON (x) to call x's Invariant directly.
 *
 *	Notes:
 *		Invariants are designed to be called at the beginning and upon exit of a routine, 
 *		testing the consistent properties of an object which remain invariant--always the same.
 *
 *		Functions may temporarily make an object inconsistent during their execution.
 *		A generalized invariant test should not be called during these inconsistent times;
 *		if there is a need for a function, which checks invariants, to be called during
 *		an inconsistent object state, a solution will need to be designed--the current design
 *		does not facilitate this.
 *
 *		Because it is entirely possible for an Invariant() function to recurse on itself
 *		causing a stack overflow, the template explicitly prevents this from happening.
 *		The template also prevents invariant-checking during the processing of Assert(),
 *		preventing another type of recursion. Assert() recursion is avoided by checking
 *		a global flag, fInAssert.
 *
 *		Currently Invariant() returns a BOOL, as I think this allows for it to be called
 *		from the QuickWatch window under VC++2.0. TRUE indicates that the invariant executed
 *		normally.
 *
 *	Usage:
 *		-the _invariant.h header should only be included in source files. An error will occur
 *			if included in another header file. This is to prevent multiple #define DEBUG_CLASSNAME.
 *		-Typical #include into a source file looks like this:
			#define DEBUG_CLASSNAME ClassName
			#include "_invar.h"
 *		-Typical definition of a class' Invariant() method looks like this:
			#ifdef DEBUG
				public:
				BOOL Invariant( void ) const;
				protected:
			#endif	// DEBUG
 *		-Typical declaration of Invariant() looks like this:
			#ifdef DEBUG

			BOOL
			ClassName::Invariant( void ) const
			{
				static LONG	numTests = 0;
				numTests++;				// how many times we've been called.

				// do mega-assert checking here.

				return TRUE;
			}

			#endif	// DEBUG
 *
 *
 *		
 *	
 *	Author:
 *		Jon Matousek (jonmat) 5/04/1995
 *
 *		Any problems? Please let me know.
 */

#ifndef _INVARIANT_H

#define _INVARIANT_H

#ifndef DEBUG_CLASSNAME
prior to including _invariant.h file, you must define DEBUG_CLASSNAME
to be the name of the class for which you are making Invariant() calls.
#endif


#ifdef DEBUG

template < class T >
class InvariantDebug
{
	public:
	InvariantDebug	( const T & t) : _t(t)
 	{
		static volatile BOOL fRecurse = FALSE;

		extern BOOL fInAssert;
		if ( fInAssert ) return;		/* Don't allow invar-checking if already throwing an assert. */
		if ( fRecurse )	return;		/* Don't allow recursion.*/
		
		fRecurse = TRUE;

		_t.Invariant();

		fRecurse = FALSE;
	}

	~InvariantDebug	()
	{
		static volatile BOOL fRecurse = FALSE;

		extern BOOL fInAssert;
		if ( fInAssert ) return;		/* Don't allow invar-checking if already throwing an assert. */
		if ( fRecurse )	return;		/* Don't allow recursion.*/
		
		fRecurse = TRUE;

		_t.Invariant();

		fRecurse = FALSE;
	}

	private:
	 const T &_t;
};

typedef InvariantDebug<DEBUG_CLASSNAME> DoInvariant;

#define _TEST_INVARIANT_ DoInvariant __invariant_tester( *this );
#define _TEST_INVARIANT_ON(x) \
					{\
						static volatile BOOL fRecurse = FALSE;\
						extern BOOL fInAssert;\
						if ( FALSE == fInAssert && FALSE == fRecurse )\
						{\
							fRecurse = TRUE;\
							(x).Invariant();\
							fRecurse = FALSE;\
						}\
					}

#else	// DEBUG

#define _TEST_INVARIANT_
#define _TEST_INVARIANT_ON(x)

#endif	// DEBUG


	// code that should be at the start and end of all Invariant() methods.

#else	// INVARIANT_H

	This file should only be included once per source file. jonmat

#endif	// INVARIANT_H