Use variable from CMAKE in C++

Question

I want to use a value declared in my CMakeLists.txt in my C++ code. I've tried to do like that :

ADD_DEFINITIONS( -D_MYVAR=1 )

and

#if -D_MYVAR == 1
    #define var "someone"
#else
    #define var "nobody"
#endif
int main(){
    std::cout << "hello" << var << std::endl;
    return 0;
}

But it doesn't work, and I don't understand why. Maybe I don't use ADD_DEFINITIONS correctly...

Ideally, I wish do something like that :

ADD_DEFINITIONS( -D_MYVAR=\"someone\" )

and

#define var D_MYVAR

int main(){
    std::cout << "hello" << var << std::endl;
    return 0;
}

Is it possible ?

Thanks !

possible duplicate of How to read a CMake Variable in C++ source code — André
– André, Commented Jul 2, 2014 at 7:18

user1795160 · Accepted Answer · 2014-06-30 10:50:22Z

11

add_definitions ( -DVARNAME=... )

is the correct way of using add_definitions.

To check for a constant then, use

#ifdef VARNAME
...
#endif

answered Jun 30, 2014 at 10:50

user1795160

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Comments

Community · Accepted Answer · 2017-05-23 12:25:23Z

6

Thanks to πάντα ῥεῖ

His solution works for my first question, and I've could do that :

CMakeLists.txt:

ADD_DEFINITIONS( -D_VAR=\"myValue\" )

main.cpp:

#include <iostream>

#ifdef _VAR
    #define TXT _VAR
#else
    #define TXT "nobody"
#endif

int main(){
    std::cout << "hello " << TXT << " !" << std::endl;
    return 0;
}

edited May 23, 2017 at 12:25

CommunityBot

11 silver badge

answered Jun 30, 2014 at 10:50

cooow

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Comments

Carlos Segarra · Accepted Answer · 2021-03-10 12:26:09Z

1

As of CMake 3.12 add_definitions has been superseeded by other alternatives.

For this question, you should use add_compile_definitions in the same way you were using add_definitions:

add_compile_definitions( MY_VAR )

(note that the -D is not needed anymore).

And then in your *.cpp file:

#ifdef MY_VAR
// do something if var defined
#endif

edited Mar 10, 2021 at 12:26

answered Mar 10, 2021 at 12:01

Carlos Segarra

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Comments

DevSolar · Accepted Answer · 2014-06-30 13:31:51Z

0

Actually there is a much more elegant way to do this, that does not require to go through the C/C++ preprocessor. (I hate #ifdef...)

In CMakeLists.txt:

set( myvar "somebody" )

# Replaces occurrences of @cmake_variable@ with the variable's contents.
# Input is ${CMAKE_SOURCE_DIR}/main.cpp.in,
# output is ${CMAKE_BINARY_DIR}/main.cpp
configure_file( main.cpp.in main.cpp @ONLY )

In main.cpp.in:

#include <iostream>
int main(){
    // myvar below gets replaced
    std::cout << "hello @myvar@" << std::endl;
    return 0;
}

Note, however, that a file so configured gets saved in ${CMAKE_BINARY_DIR}, so you have to prefix it as such when listing it in the source files (as those default to ${CMAKE_SOURCE_DIR}):

add_library( myproject foo.cpp bar.cpp ${CMAKE_BINARY_DIR}/main.cpp )

edited Jun 30, 2014 at 13:31

answered Jun 30, 2014 at 11:10

DevSolar

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3 Comments

Spyros Mourelatos Over a year ago

Why you hate ifdef ? This is equivalent or maybe worse as it ties your program even more with cmake and on the other hand a definition could exist on a cpp folder ,this doesn't

DevSolar Over a year ago

@SpyrosMourelatos: And for that you necro a six-year-old answer?

DevSolar Over a year ago

@SpyrosMourelatos: I didn't say that. I said that I (pesonally) dislike #ifdef. In most Unix-ish software, #ifdef has been over-/abused to the point where it gets really hard to see which parts are actually compiled and which aren't. And half the time those #ifdef's don't actually check what the author thought they would. Also, many of the things that are #ifdef'ed (platform dependencies) usually impact linker phase / library selection as well. So why not have and handle them at one central point (CMakeLists.txt), together with e.g. version numbers, author contact info...

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