pass variable value to make using cmake - c++

Question

I'm trying to do something like
cmake -DUSE_FILES_FOR_INPUT=ON ..
and then get make to compile my files with -DUSE_FILES_FOR_INPUT=ON. But I don't get the =ON value. It seems that it should be written to a flags.make file, but it isn't:
$ grep USE_FILES_FOR_INPUT source/CMakeFiles/myprogram.dir/flags.make
CXX_DEFINES = -DUSE_FILES_FOR_INPUT
If I manually change that file with -DUSE_FILES_FOR_INPUT=ON then everything works fine.
How can I get cmake to add the value in flags.make?
A different approach to what I need to do: the variable I am using has type BOOL, so it would be good enough to get
CXX_DEFINES = -DUSE_FILES_FOR_INPUT when I do cmake -DUSE_FILES_FOR_INPUT=ON ..
and
CXX_DEFINES = when I do cmake -DUSE_FILES_FOR_INPUT=OFF ..
Is this possible?

Answer 1

Here is a small demonstration,
// demo.cpp
#include <iostream>
using namespace std;
int main()
{
#ifdef USE_FILES_FOR_INPUT
cout << "Using files.." << endl;
#else
cout << "Not using files.." << endl;
#endif
return 0;
}
The CMakeLists.txt,
cmake_minimum_required(VERSION 3.0)
add_executable(demo demo.cpp)
if (USE_FILES_FOR_INPUT)
add_definitions(-DUSE_FILES_FOR_INPUT)
endif()
And a sample output,
baris$ cmake . && make && ./demo
<...>
Not using files..
baris$ cmake -DUSE_FILES_FOR_INPUT=ON . && make && ./demo
<...>
Using files..

Answer 2

You should look at the configure_file documentation. They show an example using the special cmake construct #cmakedefine. This may be a little more work than Baris describes, but I prefer using configure_file and #cmakedefine to get cmake settings into C++ code.
Here is the complete example:
Consider a source tree containing a foo.h.in file:
#cmakedefine FOO_ENABLE
#cmakedefine FOO_STRING "#FOO_STRING#"
An adjacent CMakeLists.txt may use configure_file to configure the header:
option(FOO_ENABLE "Enable Foo" ON)
if(FOO_ENABLE)
set(FOO_STRING "foo")
endif()
configure_file(foo.h.in foo.h #ONLY)
This creates a foo.h in the build directory corresponding to this source directory. If the FOO_ENABLE option is on, the configured file will contain:
#define FOO_ENABLE
#define FOO_STRING "foo"
Otherwise it will contain:
/* #undef FOO_ENABLE */
/* #undef FOO_STRING */
One may then use the include_directories() command to specify the output directory as an include directory:
include_directories(${CMAKE_CURRENT_BINARY_DIR})
so that sources may include the header as #include <foo.h>.