New in version 3.10.
This is a CMake Environment Variable. Its initial value is taken from the calling process environment.
Preferred executable for compiling CXX language files. Will only be used by CMake on the first configuration to determine CXX compiler, after which the value for CXX is stored in the cache as CMAKE_CXX_COMPILER. For any configuration run (including the first), the environment variable will be ignored if the CMAKE_CXX_COMPILER variable is defined.
Note
Options that are required to make the compiler work correctly can be included; they can not be changed.
$ export CXX="custom-compiler --arg1 --arg2"
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https://cmake.org/cmake/help/v3.19/envvar/CXX.html