Using variables in the preprocessor directives

问题

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?

回答1:

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.

回答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.

来源：https://stackoverflow.com/questions/14418258/using-variables-in-the-preprocessor-directives