Passing a CMake variable to C++ source code

Question

First of all, I already read How to read a CMake Variable in C++ source code, and I don't really know if it can help me.

Second, my question. Top level CMakeLists.txt says SET( MY_VAR /some/path ) and in top level source.cpp I need to achieve somehow a std::string containing /some/path/to/files naturally using MY_VAR value.

It says quite clearly how to pass a value from Cmake to C++ in that answer using macros. — user1937198, Mar 07 '14 at 20:34

score 16 · Answer 1 · edited Apr 06 '23 at 18:54

16

add_executable(myexe main.cpp)
if (MY_VAR)
    target_compile_definitions(myexe PRIVATE MY_VAR=${MY_VAR})
endif()

EDIT: Then the variable would be available in C++ code like that:

#ifdef MY_VAR
std::cout << MY_VAR << std::endl;
#endif

edited Apr 06 '23 at 18:54

baziorek

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answered Mar 09 '14 at 02:58

steveire

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So this way I can use MY_VAR in .cpp? – Javier Mar 10 '14 at 12:41
Yes. The MY_VAR=${MY_VAR} is of the form 'CPP_NAME=${CMAKE_NAME}'. The two are the same in my example though. – steveire Mar 10 '14 at 14:56
Can't do this in version 2.8.9. And I'm not able to upgrade. – Javier Mar 10 '14 at 15:28
It's still the right answer :) – steveire Mar 10 '14 at 17:19

lisyarus · Accepted Answer · 2019-12-13T13:47:01.073

13

You can use the configure_file(filename output_file) command. It replaces something like ${VARIABLE} or @VARIABLE@ in file filename with the actual value of cmake variable VARIABLE and writes the result to output_file.

So, you can write something like

// in C++ code
std::string my_var = "@MY_VAR@";

# in CMakeLists.txt
configure_file(filename.h.in filename.h)

Also, I think it is a good idea for the output file to be generated somewhere inside build directory (and its' containing directory added to include paths).

configure_file in cmake documentation

edited Dec 13 '19 at 13:47

answered Mar 07 '14 at 20:10

lisyarus

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Oh, $Deity. Now I *know* I want to stay well clear from CMake. Thanks! – vonbrand Mar 07 '14 at 20:26
This almost worked. I mean, it did, but it replaced forever @VARIABLE@ with the value of VARIABLE. What if I change the value of VARIABLE? There's no more @VARIABLE@ to be replaced in next compilation. – Javier Mar 07 '14 at 20:46
What I ment by 'forever' is that I opened source.cpp and I saw no `@VARIABLE@`, but `/some/path` now. – Javier Mar 07 '14 at 20:50
@Javier: if the variable's value changes, the file will be regenerated when cmake will be called (and it is called automatically if building with cmake-generated makefiles). – lisyarus Mar 07 '14 at 20:50
3

@Javier: this command should be used to generate a real source file from a some kind of template file, and not to call it like `configure_file(source.cpp source.cpp)`. – lisyarus Mar 07 '14 at 20:52

vonbrand · Answer 3 · 2014-03-07T20:07:40.577

-3

The only way to pass in values during compilation is by passing a macro. E.g., if CMake sets a macro called $(MY_VAR) (as make(1) does):

g++ -DMY_VAR=$(MY_VAR) .... xxx.cc

and xxx.cc uses MY_VAR as it wishes.

edited Mar 07 '14 at 20:07

answered Mar 07 '14 at 19:59

vonbrand

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I'm sorry, I'm a noob in CMake. Could you tell me what's the code to do that? – Javier Mar 07 '14 at 20:05
@Javier, oops. Fixed to use the macro (I assume CMake uses `make(1)` syntax here...) – vonbrand Mar 07 '14 at 20:08
1

That's not the only way, look at my answer. – lisyarus Mar 07 '14 at 20:10
@lisyarus, that is mangling the file to be compiled. I stand by my assertion. – vonbrand Mar 07 '14 at 20:28
@vonbrand: what do you mean saying "mangling the file to be compiled" ? – lisyarus Mar 07 '14 at 20:31
You aren't compiling the given file, it is being edited into the file to be compiled. – vonbrand Mar 07 '14 at 20:34
@vonbrand: if the value it got from cmake, I assume this value it some configuration variable and does not change between builds, and it's better to put it into the code once rather then ad it during every compilation. – lisyarus Mar 07 '14 at 20:37
Well, actually I would like to put it during every compilation, because the variable may be changed (not often though). – Javier Mar 07 '14 at 21:42
@Javier: that's not a problem: the file will be regenerated if the variable changes. – lisyarus Mar 07 '14 at 23:01

Passing a CMake variable to C++ source code

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