CPP is not available for Ruby extension C file names

A very simple file is the example (http://www.ruby-cn.org/book/ProgrammingRuby/ext_ruby.html) on programming Ruby, and if the extension is CPP, the following error occurs:

Error c2664: 'rb _ define_method ': cannot convert parameter 3 from
'Unsigned long (unsigned long) 'to 'unsigned long (_ cdecl *)(...)'
None of the functions with this name in scope match the target type

You need to change the file to. C.

To use CPP, you can modify it as follows:

Rb_define_method (ctest, "initialize ",Ruby_method_func(T_init), 0 );
Rb_define_method (ctest, "add ",Ruby_method_func(T_add), 1 );