How to use a std::function as a C style callback

In most cases you can't.

But when you store a C style callback in your std::function, you can use the target() member function.

Long answer: sort of. You can write a C function to pass to the API that calls your std::function:

// --- some C code I can not change ---
typedef void(*fun)(int);
void register_callback(fun f) {
    f(42); // a test
}
// ------------------------------------

#include <functional>
#include <iostream>

void foo(const char* ptr, int v, float x) {
    std::cout << ptr << " " << v << " " << x << std::endl;
}

namespace {
std::function<void(int)> callback;
extern "C" void wrapper(int i) {
    callback(i);
}
}

int main() {
    callback = std::bind(&foo, "test", std::placeholders::_1, 3.f);
    register_callback(wrapper); // <-- How to do this?
}