How can you check if AC_PROG_CXX has failed?

Question

The AC_PROG_CXX macro checks for possible C++ compilers and in the event of failure, happily just sets CXX to g++ even if it knows g++ doesn't really exist.

My question is, how can you understand if AC_PROG_CXX did actually find a proper compiler?

I am asking because I have a C++ library in my project that could be optionally compiled. I can't seem to figure out how on earth I'm supposed to know whether I can actually compile anything with CXX.

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Some of the macros like AC_PROG_CC_C99 give a variable (in this case ac_cv_prog_cc_c99) that can tell me if the feature exists or not. I tried ac_cv_prog_cxx and similar things, but it doesn't seem to exist.

Answer 1

You could try compiling a minimal program, e.g.,

AC_LANG_PUSH([C++])
AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
  [[#ifndef __cplusplus
    #error "broken C++"
    #endif]])],,
  [CXX=;])
AC_LANG_POP([C++])

If the compilation fails, we explicitly set CXX to an empty string. So depending on what you want to do:

if test "x$CXX" != x ; then
  ... we have a working C++ compiler - setup optional stuff ...
fi

Or as part of a conditional build in a Makefile.am file:

AM_CONDITIONAL([ENABLE_CXX_SRC], [test "x$CXX" != x])