[CMake] including paths - basic

[Philip Lowman]

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Mark Wyszomierski wrote:
> Hi Mike,
> 
> When I try CMAKE_BUILD_TYPE MATCHES Debug, it seems to never execute
> the contents of that IF statement (as if I am never CMake'ing a DEBUG
> build?).
> 
> I don't know if CMAKE_BUILD_TYPE is the same thing as what's in the
> CMAKE_CONFIGURATION_TYPES field. I really need something like:
> 
> IF (CMAKE_CONFIGURATION_TYPE MATCHES Debug)
>     // include some library....
> ELSE
>     // ...
> ENDIF
> 
> I'm not sure if that's the same thing?

One common misconception (about CMake) is that it's always capable of
doing what you think it should be doing.  :)  Part of the problem lies
in the fact that CMake tries to support both "single configuration type"
and "multiple configuration type" build tools simultaneously.

The Makefile generator only supports one configuration type at a time
(Debug, Release, etc.) which works out very nice as variables like
CMAKE_BUILD_TYPE can be read at CMake time and used to change the
configuration of the build.  Take the following code for example...

project(FOO)
if(CMAKE_BUILD_TYPE MATCHES Debug)
    INCLUDE_DIRECTORIES(bar)
else(CMAKE_BUILD_TYPE MATCHES Debug)
    INCLUDE_DIRECTORIES(top)
endif(CMAKE_BUILD_TYPE MATCHES Debug)
ADD_EXECUTABLE(foo foo.cc)

Should do exactly what you want to do... When you set the
CMAKE_BUILD_TYPE cache variable to Debug your Makefiles will add "bar"
to the include path and vice versa if you don't define it.

When you use a "multiple configuration types" generator like Visual
Studio, however, the CMAKE_BUILD_TYPE variable will no longer exist.
There is a hidden variable as you've discovered called
CMAKE_CONFIGURATION_TYPES but this only describes the types of
configurations which will show up in Visual Studio.  There is no
scripted support for dynamically changing project files to do different
things using the "configuration type" of choice.

You can introduce your own CACHE variables that change execution of
CMake to do whatever you want but it would not work through the multiple
build configuration feature of Visual Studio.  You would basically have
to use CMake to generate one solution files and then another set.

There is limited support via the use of SET_TARGET_PROPERTIES to modify
per configuration options that will work in Visual Studio but it's
limited.  Take the following which would change your generated ".exe" or
".lib" file for the target:

SET_TARGET_PROPERTIES(foo PROPERTIES DEBUG_OUTPUT_NAME foodebug)
SET_TARGET_PROPERTIES(foo PROPERTIES RELEASE_OUTPUT_NAME foorelease)

So the short answer is, the code people post to the mailing list may or
may not do what you want it to do... it all depends on what generator
you are using.  Sorry for any confusion, all of this probably should be
documented somewhere in a "CMake gotchas" section.

- --
Philip Lowman
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