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Asked: 2026-05-25T03:43:17+00:00

There is a C library (which I cannot change) that supports a callback function

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There is a C library (which I cannot change) that supports a callback function of the type 

void (*callback)(void *appContext, int eventid)

I want to set a C++ function as the callback.

Specifically I have following questions?

  1. Do I need to declare the callback function under "extern C" block?

  2. Does a member function need to be static to be the callback function? Is it possible to use a non-static member function? If yes, how? And when is it recommended to use a non-static member function?

  3. Does it matter if the function is a template function or not?

  4. Does a non-class C style function have any advantages over a class member function?

I am trying these variants on a old VC++ compiler, which does not support the latest C++ standard. But the code needs to be platform independent and should work on most C++ compilers. I want to know what is recommended practice with callbacks?

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1 Answer

  1. Editorial Team

    Does callback function need to be declared under extern “C”?

    NO. extern “C” is necessary only when you are calling a C++ function directly, without the use of function pointers, from C. If function pointers are used, extern “C” is not required.

    Can I use non-static member functions as a callback?

    NO. Non-static member functions of class A have an implicit first parameter corresponding to this pointer.

    Can I use static member functions as a callback?

    YES, as long as signature matches with that of the callback.

    Does it matter if the function is a template function or not?

    NO, template function can be used as callbacks as long as the signature of the instantiated template matches with the callback.

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