Question

I want to use the thunk function template to pass it around to pthread_create and stuff like that.

I would expect the compiler to instantiate the function with the given parameters, all the info is there, and then use the typedef as a function ptr to pass to those functions.

#include <string>
class ServerImpl{
    public:
        ServerImpl(std::string host, int port);
        void run();

};


template<typename T,void (T::*mem_fn)()>
void *thunk(void *obj) {
    (static_cast<T*>(obj)->*mem_fn)();
    return 0;
}

typedef void *(*Function) (void);
Function fun = (Function)&thunk<ServerImpl,&ServerImpl::run>;
fun();

JUST CHANGED IT

See, with this I get a linker error, which is what it brought me to the previous one. Omit the fact that I should be passing an object.

Server.o: In function `ServerImpl::ServerImpl(std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int)':
Server.cpp:(.text+0x11f): undefined reference to `void* thunk<ServerImpl, &(ServerImpl::run())>(void*)'

Answer 1

thunk is a function template. How can you use typedef on it? It doesn’t make sense. It is not a type. You can apply typedef only on types.

You should do this:

 ServerImpl *arg = new ServerImpl(); //why do I use new?
 pthread_create(&thread_id, NULL, &thunk<ServerImpl,&ServerImpl::run>, arg);

Your arguments to pthread_create are wrong. It takes 4 arguments, not 2.

Why did I use new?

I created an instance of ServerImpl using new, because the instance has to exist even if the function in which you created the thread have returned. If I don’t create the instance using new, and instead use local variable, then the code invoke undefined behavior, if the function returned but the thread continue running.

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Reply to your edit:

Function fun = (Function)&thunk<ServerImpl,&ServerImpl::run>;
fun();

This is wrong. Because thunk takes one argument. So you should be doing this instead:

Function fun = (Function)&thunk<ServerImpl,&ServerImpl::run>;
fun(new ServerImpl); //pass an argument

The member function run is invoked on this argument which you passed.