Using variables in the preprocessor directives

Question

which global variable can be used in the preprocessor directive file.cpp

int variable = 1;
#if variable >= 1
    int a = 0;
#else 
    int a = 1;
#endif

or

file.cpp

const int variable = 1;
#if variable >= 1
    int a = 0;
#else 
    int a = 1;
#endif

or file.cpp

#include "header.h"
// extern in variable; in the header.h
#if variable >= 1
    int a = 0;
#else 
    int a = 1;
#endif

What are the rules which governs using the variables in the proprocessor directive? If a variable which can be consant folded, can it be used in the #if/#elif#else directives?

Preprocessor directives and variables are completely different. You can't do this, at all. — Ry-
– Ry- ♦, Commented Jan 19, 2013 at 19:57
The preprocessor doesn't understand C++! It has its own interpretation. — David G
– David G, Commented Jan 19, 2013 at 20:28

Lily Ballard · Accepted Answer · 2013-01-19 20:04:30Z

12

Sorry, you can't do this at all. Variables are not visible to the preprocessor. The preprocessor is at its heart a text manipulator. The only values it can see are ones defined with #define, not variables.

answered Jan 19, 2013 at 20:04

Lily Ballard

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2 Comments

user1992416 Over a year ago

i used to think that const can be used as a replacement for #define for example, #define TEST 1 or const int TEST = 1, can we use these as #if TEST == 1 .... #endif

zwol Over a year ago

No. You can only use a const instead of a #define if you don't need to use the constant in #if. Often this is fine. Other times all you care about in #if is whether or not the constant has some value, so for instance you may see #define EPERM EPERM in <errno.h> and the numeric value is set with an enum. That lets you do #ifdef EPERM or #if defined EPERM but not #if EPERM == 23.

zwol · Accepted Answer · 2013-01-19 20:05:17Z

2

Only macros defined with #define will have their expected value in an #if. All other symbols (more precisely, all identifiers that remain on an #if line after macro expansion, except defined and, in C++, certain "alternative spellings" of arithmetic operators, such as and, or, bitand, bitor, and compl) are interpreted as having the value 0.

answered Jan 19, 2013 at 20:05

zwol

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