I actually have a simple question, but couldn't find an answer. Maybe you can point me to a duplicate. So, the question is: is it possible to tell cmake to instruct a compiler to automatically include some header at the beginning of every source file, so there would be no need to put #include foo.h? Thanks!
asked Sep 25, 2015 at 1:13
2 Answers
CMake doesn't have a feature for this specific use case, but as you've hinted, compilers such as GCC have the -include flag which acts as if there was an #include "foo.h" in the source file, and since CMake can pass arguments to compilers, you can do it via add_definitions.
This answer covers what the flag is for GCC, Clang and MSVC which should cover a lot of bases. So in CMake, detect what the compiler is and pass the appropriate flag.
Here's what the CMake code might look like:
if(MSVC)
add_definitions(/FI"foo.h")
else()
# GCC or Clang
add_definitions(-include foo.h)
endif()
Comments
In general, doing this is a bad idea. Code inspection tools (like IDEs, or doxygen) will be confused by it, not to mention other humans looking at the code. If not all source files actually require the definition, adding extra #includes will slow down compile time. If you actually do need the same header (and it's not a system header) in all your source files, it may be symptomatic of high coupling in your code. And for what benefit? Not having to add one line to your files?
However, it's necessary to note that compilers support this for a reason; there are a few weird edge cases (example 1, example 2) where it's a useful thing to do.
Just be aware that you're doing this for the right reasons.
answered Sep 25, 2015 at 1:22
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1This is actually a very bad thing, as you cannot see which headers a file uses. It also works against tools like doxygen which cannot generate a proper dependency tree. Commented Sep 25, 2015 at 1:24
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4@Olaf hey, I'm not suggesting people do this, but this is how it can be done if they really must. After all, why do compilers like GCC provide the
-includeflag at all? Commented Sep 25, 2015 at 1:26 -
1@congusbongus: "why do compilers like GCC provide the -include flag at all?" That is the question (sorry, I currently don't have a skull within reach;-). My point is just to make clear that is should not be done. Commented Sep 25, 2015 at 1:33
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1@Aikei: I cannot (and will not) hold a tutorial about build tools. But just from your comments, it is clear. I'd recommend you do research on your own about the general idea and how these tools interact with the actual building (i.e. the tools performing the build actions) tools. Please do not understand this as an offence, just a message to learn. The current problem you have is you are trying to avoid some typing/editing at the cost of maintenance and (possibly) documentation. Commented Sep 25, 2015 at 1:36
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1@Aikei: (Last posting) Read what I wrote about "interacting" then think about. Then read the last part and forget about what think you want to do. Commented Sep 25, 2015 at 1:44
as pointed out in the examples above, "force including" could be useful for pre-compiled headers.
this is actually what cmake's target_precompile_headers does. https://cmake.org/cmake/help/git-stage/command/target_precompile_headers.html
for this context-of-use:
force including the header is actually a nice feature, since it allows you to use precompiled headers with MSVC, clang and gcc without changing your source.
it is also a nice feature, because it allows you to switch on/off PCH with just a configuration change (e.g. in cmake). this allows you to test that your source builds also without the precompiled headers (e.g. for compilers/situations that don't support it). for instance clang-tidy gets confused about PCH (at least in combination with MSVC).
#includethe header in every file it uses any of its declarations?ssize_t, so I can redefine it tosize_twithout modifying the external source.