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From: turk@Apple.COM (Ken "Turk" Turkowski)
Newsgroups: comp.lang.c++
Subject: Re: Variable sized objects
Message-ID: <5891@internal.Apple.COM>
Date: 20 Dec 89 06:27:47 GMT
References: <1262@amethyst.math.arizona.edu> <1881@odin.SGI.COM>
Organization: Advanced Technology Graphics, Apple Computer, Cupertino, CA, USA
Lines: 61

In article <1881@odin.SGI.COM> shap@delrey.sgi.com (Jonathan Shapiro) writes:
>First, this object cannot be built on a stack, because it's length
>isn't known to the compiler.  The semantic implications of a heap-only
>object aren't clear.

This is one of the defects of C that C++ has inherited.  Consider:

	int MatrixInvert(double *M, rows, cols)
	{
		double LU[rows * cols];
	}

Here, we need a temporary (auto) array that is used only within this procedure.
Unfortunately, C won't let you do this, although Fortran will (is this a step
forward???).  Alloca (auto allocation, from the stack) was devised to overcome
this, but not all systems support it, so one is left with declaring a maximum
size, hoping that it will never need to be larger:

	#define MAXROWS 10
	#define MAXCOLS 10
	int MatrixInvert(double *M, rows, cols)
	{
	#ifdef ALLOCA
		LU = alloca(sizeof(double) * rows * cols);
	#else !ALLOCA
		double LU[MAXROWS * MAXCOLS];
	#endif ALLOCA
		...
	}

Now the analogy in C++ is:
	class Matrix {
		int _rows, _cols;
		double *_m;

		Matrix(int rows, int cols) {
			_rows = rows;
			_cols = cols;
			_m = new double[rows * cols];
		}

		~Matrix() { delete _m; }

		int Invert() {
			Matrix LU(Rows(), Cols());
			...
		}
	}

In the C++ version, the new operator is called, and the corresponding
delete operator is called automatically upon returning.

My question is:  is there a way to determine whether the matrix is auto
or "new"ed?  If it is possible to determine if it is auto, then
alloca() could be used for allocating memory, without the tremendous
overhead of new/malloc().
-- 
Ken Turkowski @ Apple Computer, Inc., Cupertino, CA
Internet: turk@apple.com
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