How do determine default C++ compiler?

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Detecting the compiler (as it's not so much configured, as it is detected (or at least verified)) is actually not something catkin does, but is a CMake affair. As such, you can use any of the traditional ways (setting CC and/or CXX env flags before invoking catkin_make, etc) to influence it.

I don't have a direct link to anything that completely explains how CMake does it, but some related pages:

• cmake-toolchains
• CMake Cross Compiling

After the compiler has been detected, CMake sets a nr of (internal) variables that store the bits and pieces of information it needs to work with that compiler. Some of those are documented on the CMake Useful Variables page.

Note that much (if not all) CMake documentation has migrated from the wiki to the generated docs at cmake.org/documentation.