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Subject: Eliminating #ifdef: if const

Here's an idea I had for a non-preprocessor replacement for #if and  #ifdef.

	if const (<expr>) { <stuff> }

`if const' is a compile time version of `if'.  <expr> is a constant
expression which is convertible to bool.  If it yields true, <stuff>
is processed as normal.  Otherwise <stuff> is parsed into tokens, but
ignored except for counting { and } nesting.  <stuff> is anything that
can appear inside a namespace block.  `if const' does not introduce a
new scope.

The following is also legal, with the obvious meaning:

	if const (<expr>) { <stuff> } else const { <other stuff> }

There are also a number of compile-time pseudo-functions (I hesitate
to call them pragmas):

	bool #os_version(const char* str)

		os version (uname -r) at or later than str

	bool #os_name(const char *str)

		os name (uname -s) is str

	bool #compiler_name(const char *)
	bool #compiler_version(const char*)

		Same tests for the compiler

	bool #header_exists(const char* path)

		Would `#include <path>' succeed?

	bool #have_test(const char* varname)

		Would #varname(str) work (this would be useful to
		allow testing for implementation defined tests.

Using these you could do things like this:

	// Are we running IRIX 5.3
	const bool using_irix_53 = #os_name("IRIX") &&
				   #os_version("5.3") && !#os_version("5.4")

	if const (using_irix_53) {
		// Do some stuff needed just for IRIX 5.3
	}

	if const (#compiler("g++")) {
		// Do some g++ stuff
	}

	if const (#header_exists("sys/frobnitz9000.h")) {
		#include "sys/frobnitz9000.h"
		// Do stuff for the Frobnitz 9000
	}

	if const (#have_test("extension")) {
		const bool have_foo_extension = #extension_exists("foo");
	}
	else const {
		const bool have_foo_extension = false;
	}

	if const (have_foo_extension) {
		// Stuff using the `foo' extension
	}

With this I could personally stop using the preprocessor for anything
except #include.  If `include' (as described in D&E) is also
implemented, I could stop using the preprocessor altogether, which
would be nice.

I realise this is too late for the upcoming version of the standard,
but I think it would be a useful thing for a subsequent version.

Comments?

Mark Russell
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