Secondly, the resultant DLL will be linkable from a VC++ program, but the C++ functions in it will not be accessible from code compiled with VC++.
Of course, the usual c++ name mangling incompatibilities.
And there comes the .def file to help.
You can define your needed aliases (mangled names for the target linker) there also.
Then you only have the other more serious C++ problems remaining: (From the section "Why I didn't become a C++ programmer")
different global initialization (can be overcome)
standard library issues esp. the runtimes are mutually exclusive,
also newlib is different to the msvc libraries in certain aspects.incompatible run-time issues (mostly memory management) across the g++/vc objects
implicit new/delete esp. with c++, with incompatible issues
e.g. MSVC operator new(size_t) behaves like new(size_t, nothrow_t&) => 0 instead of calling bad_alloc()
The C++ libs prefer to malloc their own objects, which is a serious C++ design problem, since the internal memory management is being left to the implementor. Other run-time issues (RTTI, exceptions, ... also of course). So libraries which leave some aspects (e.g. memory management) to their runtime, locking out competing products on the binary level.
Standardization was very implementor friendly then. And it was not Microsoft.
The C API is designed to malloc it in the caller. Certain gcc libs wanted to be cleverer that times (the notorious x... funcs for the lazy). Thanksfully it's better now.)
alignment (a typical library problem. can be easily overcome in the caller.)
vtable structure and handling, inheritance (can be overcome, since g++ understands the msvc object layout now)
exceptions (esp. across the library border)
All this makes it e.g. impossible to extend c++ objects at run-time, as any other "better" language can do. Then came COM.
FAQs: "Why can't g++ code link with code from other C++ compilers?" http://docs.freebsd.org/info/g++FAQ/g++FAQ.info.problems_linking_with_other_libraries.html
"Can I mix objects compiled with msvc++ and gcc?" http://www.cygwin.com/faq/faq_toc.html#TOC117 (which only mentions the mangling problems, not the other)
"Can I link with both MSVCRT*.DLL and cygwin1.dll?" http://www.cygwin.com/faq/faq_3.html#SEC95
"How do I link against a `.lib' file?" http://www.cygwin.com/faq/faq_3.html#SEC100
"How can an MSVC program call a MinGW DLL, and vice versa?" http://www.mingw.org/mingwfaq.shtml#faq-msvcdll -- Reini Urban http://xarch.tu-graz.ac.at/home/rurban/
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