Question

I use a library that have a callback function where one of the parameters is of type void *. (I suppose to let to send value of any type.)

I need to pass a string (std::string or a char[] is the same).

How can I do this?

Answer 1

If you’re sure the object is alive (and can be modified) during the lifetime of the function, you can do a cast on a string pointer, turning it back into a reference in the callback:

#include <iostream>
#include <string>

void Callback(void *data) {
    std::string &s = *(static_cast<std::string*>(data));
    std::cout << s;
}

int main() {
    std::string s("Hello, Callback!");
    Callback( static_cast<void*>(&s) );
    return 0;
}

Output is Hello, Callback!