gistfile1.h

#include <iostream>
#include <msgpack.hpp>

// source file
void doit_impl(void (*func)(void*, const char*, size_t), void* obj)
{
    for(int i=0; i < 10; ++i) {
        (*func)(obj, "test", 4);
    }
}

// header file
template <typename T>
static void trampoline(void* obj, const char* data, size_t size)
{
    reinterpret_cast<T*>(obj)->write(data, size);
}

template <typename T>
void doit(T* stream)
{
    void (*func)(void*, const char*, size_t) = &trampoline<T>;
    void* obj = reinterpret_cast<void*>(stream);
    doit_impl(func, obj);
}

// user program
int main(void)
{
    doit(&std::cout);  // any class that implements T::write(ptr, size)

    msgpack::sbuffer sbuf;
    doit(&sbuf);
}