Use variable from CMAKE in C++ - 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 !

Answer 1

add_definitions ( -DVARNAME=... )
is the correct way of using add_definitions.
To check for a constant then, use
#ifdef VARNAME
...
#endif

Answer 2

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;
}

Answer 3

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 )

Answer 4

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