[cmake-developers] Adding an option for relative compiler invocations

[Brad King]

On 01/22/2015 04:46 AM, Michael Enßlin wrote:
> (1.2) Using compile-time string manipulations to sanitize the filename.
>       Due to limitations of C++, this requires template metaprogramming,
>       leading to unreasonable complexity and compile times.

See below.

> Over the last several decades, at least on the POSIX platform, it has
> become common practice to invoke compilers with relative file paths

This is only true for in-source builds.  CMake recommends out-of-source,
and then full paths are much more reliable.  Even if one used relative
paths then your messages would still have a bunch of "../" in them for
an out-of-source build.  Therefore I'll assume you're using in-source
builds.

Side note: To make relative paths behave right with __FILE__ you would
also need all include paths (-I) to be relative.  Otherwise headers will
still get full paths.  This would require much more work than just
trying to get compile lines to refer to source files with a relative
path.

So, assuming you have an in-source build then all sources and headers
must sit under a single prefix (the top of the source tree).  With that
you can do something like:

 string(LENGTH "${CMAKE_SOURCE_DIR}/" SRC_DIR_LEN)
 add_definitions(-DSRC_DIR_LEN=${SRC_DIR_LEN})

and then in the source code do:

 #define MY__FILE__ (__FILE__+SRC_DIR_LEN)

That will give you a compile-time constant __FILE__ with a relative path
and no runtime overhead.  Use MY__FILE__ in your log macros.

-Brad