Issue #17578 has been reported by cfis (Charlie Savage).

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Bug #17578: mkmf experimental C++ Support
https://bugs.ruby-lang.org/issues/17578

* Author: cfis (Charlie Savage)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x64-mingw32]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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I've been working on the Rice gem (https://github.com/jasonroelofs/rice) that wraps C++ code for use in Ruby.

I noticed that some c++ support was added to mkmf for Ruby 2.7. However, if I try to use it find a header it fails to work.  For example:

``` cpp
find_header('rice.hpp')
```

The reason is the conftest uses gcc -E instead of g++ -E. To fix that requires overlading the cpp_command to support C++.

This the fix I have put in that works:


``` cpp
MakeMakefile['C++'].module_eval do
  def cpp_command(outfile, opt="")
    conf = cc_config(opt)
    if $universal and (arch_flag = conf['ARCH_FLAG']) and !arch_flag.empty?
      conf['ARCH_FLAG'] = arch_flag.gsub(/(?:\G|\s)-arch\s+\S+/, '')
    end
    RbConfig::expand("$(CXX) -E #$INCFLAGS #$CPPFLAGS #$CFLAGS #{opt} #{CONFTEST_CXX} #{outfile}",
                     conf)
  end
end

```

The two changes over the default method are:

$(CC) -> $(CXX) -E
#{CONFTEST_c} -> #{CONFTEST_cxx}

Could this change be merged in? I can provide a patch file if you would like.

Last, it wasn't obvious to me how to activate the C++ support in mkfm. I ended up doing this:

``` ruby
include MakeMakefile['C++']
```

Is that correct?




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