Thread: Precompiler problem

Hi, if define A as

Code:

#define A 100

and write

Code:

#if (A == 0)
   line1;
#else
   line2;
#endif

line1 is grayed out as is should be and line2 is active as it should be. But if I redefine A as

Code:

#define A 100.0

then line1 becomes active and line2 is grayed out. Why? A is not equal to 0! How can I test in compile time if a defined constant is zero or not, even if it is not an integer? Thanks in advance.

You can't.

Originally Posted by ISO/IEC 9899 6.10.1

The expression that controls conditional inclusion shall be an integer constant expression ...

This comes from C, but since C++ inherited the preprocessor from C, I think it is sufficient enough. (I actually skimmed the C++ standard after writing this the first time and came up with nothing but mentions of the preprocessor. So I'm referring to C.)

> "The expression that controls conditional inclusion"

I don't want to conditionally include a file, I just want to select between two snippets of code. But I take it all preprocessor conditions only can handle integer expressions, and that they can't do any arithmetic operations containing float values?

Originally Posted by iMalc

Right, well it says "conditional inclusion". It doesn't say "conditional inclusion of files". So that means it even applies to conditionally including anything, even a single line of code which is exactly what you are doing.

Ah, okay, that makes sense. Well, thanks for the answers.