What is the Effect of Declaring 'extern "C"' in the Header to a C++ Shared Library?

Question

Based on this question I understand the purpose of the construct in linking C libraries with C++ code. Now suppose the following:

I have a '.so' shared library compiled with a C++ compiler. The header has a 'typedef stuct' and a number of function declarations. If the header includes the extern "C" declaration...

#ifdef __cplusplus
extern "C"
{
#endif

  // typedef struct ...;
  // function decls

#ifdef __cplusplus
}
#endif

... what is the effect? Specifically I'm wondering if there are any detrimental side effects of that declaration since the shared library is compiled as C++, not C.

Is there any reason to have the extern "C" declaration in this case?

Answer 1

This is important so that the compiler doesn't name mangle. C++ uses name mangling to differentiate functions with operator overloads.

Run "/usr/bin/nm" against a binary to see what C++ does with your function names: _ZSt8_DestroyIN9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEEiEvT_S7_SaIT0_E

extern "C" prevents that name mangling.

IIRC, that makes it possible for program to dynamically link in symbols at run time. It's common for "plugin" type architectures.