[CMake] Proper way to export a library

[Matthew Woehlke]

On 2013-10-31 05:26, Cyrille Faucheux wrote:
> Can you tell me a bit more about "implicit compile flags [...] for imported
> tagets"?

See documentation on target_compile_definitions.

> On the library I'm currently working on, with Visual Studio, I have to
> declare some compile definition in order to get the proper
> "__declspec(dllimport)" or "__declspec(dllexport)" defined (or not defined
> when compiling/using a static version of this library).

This sounds like you're doing it wrong.

When building with CMake, the preferred way is to unconditionally define 
an ABI export symbol via a header which exists for that purpose (e.g. 
config.h, my_package_exports.h, etc.). The value of the definition 
however is conditional on a symbol that is only defined when building 
the library, to choose between import and export as appropriate.

Even better, CMake will define <target>_EXPORTS for you when building a 
library, so you shouldn't need to manually specify any definitions at all.

IOW you libraries headers would somewhere contain:

#if defined(_WIN32)
#  define MYPROJECT_ABI_EXPORT __declspec(dllexport)
#  define MYPROJECT_ABI_IMPORT __declspec(dllimport)
#elif __GNUC__ >= 4
#  define MYPROJECT_ABI_EXPORT __attribute__ ((visibility("default")))
#  define MYPROJECT_ABI_IMPORT __attribute__ ((visibility("default")))
#else
#  define MYPROJECT_ABI_EXPORT
#  define MYPROJECT_ABI_IMPORT
#endif

...and:

#ifdef mylibrary_EXPORTS
#  define MYLIBRARY_EXPORT MYPROJECT_ABI_EXPORT
#else
#  define MYLIBRARY_EXPORT MYPROJECT_ABI_IMPORT
#endif


That said...

> Does "implicit compile flags" means that those compile definition could be
> automatically declared by the imported target?

...this was my understanding of how target_compile_definitions(PUBLIC) 
is supposed to work. (Note: I haven't actually used this feature myself 
yet.)

-- 
Matthew