how to use if else macro while compilation

[devphylosoff]

I have cpp file with :

#if 1
#include"myfile.h"
#else
#include "libfile.h"
#endif

how it works ?

 

[Victor Bazarov]

I have cpp file with :

#if 1
#include"myfile.h"
#else
#include "libfile.h"
#endif

how it works ?

The preprocessor lets through everything between '#if 1' and '#else'
and drops everything between '#else' and '#endif'. The result of
the filtering is

#include"myfile.h"

which then preprocessed, and then the code from 'myfile.h' is itself
included and preprocessed.

V

 

[Neelesh Bodas]

I have cpp file with :

#if 1
#include"myfile.h"
#else
#include "libfile.h"
#endif

how it works ?

w.r.t. subject line: "macros" are considered in the pre-processing
stage, which comes before compilation.

The preprocessor evaluates the #if macro and expands the appropriate
clause(s).
In the above example, since 1 evaluates to true, the file "myfile.h"
will get included.

-N

 

[devphylosoff]

rights, #if 1 is always true, so if I need switch I should use
different condition.

for example
#if sth
....

and compile with -Dsth.
it's really no way to make #if 1 equals false using -D ?

 

[Michal Nazarewicz]

I have cpp file with :

#if 1
#include"myfile.h"
#else
#include "libfile.h"
#endif

how it works ?

Any textbook on C or C++ should explain that. If condition given in #if
statement is true the portion between #if and #else will be compiled
otherwise the part between #else and #endif. (There is also #ifdef,
#ifndef, #elsif.)

 

[Chris Thomasson]

[...]

Any textbook on C or C++ should explain that. If condition given in #if
statement is true the portion between #if and #else will be compiled
otherwise the part between #else and #endif. (There is also #ifdef,
#ifndef, #elsif.)

[...]

#elif

 

[Guest]

rights, #if 1 is always true, so if I need switch I should use
different condition.

for example
#if sth
...

and compile with -Dsth.
it's really no way to make #if 1 equals false using -D ?

Not sure, but perhaps if you define 1 to be 0. I'm not sure of the order
of expansion though.

 

[Victor Bazarov]

Compile with -Dsth=1. Just defining it is like defining it 0.
Unless you're using

#ifdef sth

or

#if defined( sth )

Not sure, but perhaps if you define 1 to be 0. I'm not sure of the
order of expansion though.

I believe it's better than #define black white Not possible,
however, macros are named preprocessor values, and '1' is not a name.

V

 

[Michal Nazarewicz]

#elif

Bah... Too much Perl.