C++ member function pointer to global function pointer

Question

I have to solve, at least for me, a tricky problem in C++. There is a dll which i can not modify. It gets a function pointer as argument. If I pass a pointer to a global function everything works fine. Unfortunatelly there is a list of same class objects to pass to the dll . In C# I solved this by using delegates. How can this be done in C++? Using std::function does not work. With that there are coding convention errors during runtime. Further using MSVC2010 would be optimal. I wrote a sample which describes the problem:

#include <stdio.h>


// global function which works
void __stdcall task_global(float x, float y) { printf("Called global function with: %f %f\n", x, y); }
typedef void(__stdcall *f_pointer)(float, float);



// try of using a member function
class BaseTask {
public:
    virtual void __stdcall task(float x, float y) = 0;
};


class ListeningTask :public BaseTask {
public:
    void __stdcall task(float x, float y) { printf("Called this member function with: %f %f\n", x, y); }
};

typedef void (BaseTask::*member_f_pointer)(float, float);



// the dll to use
class RegisterTask {
public:
    // no posibility to access or modify!
    void __stdcall subscribe(f_pointer fp) { fp(1.0f, 2.0f); }
    // just for demonstration how to use a member function pointer
    void __stdcall subscribeMemberDemo(member_f_pointer mfp) {  /*how to use mfp?*/};
};



int main() {

    RegisterTask register_task{};

    // use global function
    f_pointer pointer_to_global_task = task_global;
    register_task.subscribe(pointer_to_global_task);


    /*---------------------------------------------------------------*/
    // use member function?
    std::list<ListeningTask> listening_task_list;
    for(int i = 0; i < 10; i++) {
        listening_task_list.push_back(ListeningTask lt);
        member_f_pointer pointer_to_member_task = &listening_task_list.back().task; //error C2276: '&': illegal operation on bound member function expression
        register_task.subscribeMemberDemo(pointer_to_member_task);

        // the tricky and important one to solve
        // how to pass the member function to this subscribe(f_pointer)?
        register_task.subscribe(pointer_to_member_task);
    }

    getchar();
    return 0;
}

The important question is how to pass a member function pointer to the RegisterTask::subscribe(f_pointer) ?

The parenthetic question is how to pass a member function to the RegisterTask::subscribeMemberDemo(member_f_pointer) ?

I hope someone can help me to solve this? I am working on this since days.

Edit: I modified the question to emphasize the problem with the list of ListenerTask . How to pass a member function pointer is now clear to me through the answers of @pokey909 and @AndyG. Both of them provide a pointer to one object or rather a list of objects. If the callback is called the one ListenerTask or all std::list<*ListenerTask> are called at once. But how to let only one ListenerTask of the list to be called. Passing more than one callback to the dll. It ( RegisterTask ) can do that, because the following example with global functions works.

void __stdcall task_global_1(float x, float y) { printf("Called global function 1 with: %f %f\n", x, y); }
void __stdcall task_global_2(float x, float y) { printf("Called global function 2 with: %f %f\n", x, y); }
void __stdcall task_global_3(float x, float y) { printf("Called global function 3 with: %f %f\n", x, y); }

typedef void(__stdcall *f_pointer)(float, float);

int main() {
    // give the functions to the dll.
    f_pointer pointer_to_global_task_1 = task_global_1;
    register_task.subscribe(pointer_to_global_task_1);

    f_pointer pointer_to_global_task_2 = task_global_2;
    register_task.subscribe(pointer_to_global_task_2);

    f_pointer pointer_to_global_task_3 = task_global_3;
    register_task.subscribe(pointer_to_global_task_3);
}

There are three global function pointers. They are all given to the dll. Now, if the dll has a task for task_global_2 it notifies this only! How to achive this distinction with member function pointer?

Note: I got the source of the dll . Hope this helps. Unfortunately modifying, building is not possible. Here is the callback definition :

type TCallback = procedure( x : single; y : single; ); stdcall;

procedure subscribe(aCallback: TCallback ); StdCall;
begin
  TaskSocket.addTask( aCallback );
end;

procedure TSocket.addTask( aCallback : TCallback);
var newTask : TTask;
begin
  newTask := TTask.Create(aCallback);
  TaskList.addItem(newTask);
end;

Answer 1

You can use a freestanding function that calls a wrapper which binds your instance to it. Here is rough example

#include <iostream>
#include <string>
#include <functional>

// global function which works
std::function<void(float, float)> memberCb;
void task_global(float x, float y) { memberCb(x, y); }
typedef void(*f_pointer)(float, float);


// try of using a member function
class BaseTask {
public:
    virtual void  task(float x, float y) = 0;
};


class ListeningTask :public BaseTask {
public:
    void  task(float x, float y) { printf("Called this member function with: %f %f\n", x, y); }
};

typedef void (BaseTask::*member_f_pointer)(float, float);

void callbackWrapper(BaseTask* t, float x, float y) { t->task(x, y); }

// the dll to use
class RegisterTask {
public:
    // no posibility to access or modify!
    void  subscribe(f_pointer fp) { 
        fp(1.0f, 2.0f); 
    }
    // just for demonstration how to use a member function pointer
    void  subscribeMemberDemo(member_f_pointer mfp) {  /*???*/ };
};



int main() {

    RegisterTask register_task{};

    ListeningTask listening_task{};
    memberCb = std::bind(&callbackWrapper, &listening_task, std::placeholders::_1, std::placeholders::_2);
    register_task.subscribe(task_global);

    return 0;
}

Note

Based on the comment, I'm not sure if all of this works in MSVC2010, since I don't have this compiler version. But rudimentary C++11 support should be in there.

Edit

I'm not sure if thats what you are after, but would this solve your problem?

void callbackWrapper(const std::list<BaseTask*> &taskList, float x, float y) {
    for (auto t : taskList)
        t->task(x, y); 
}

int main() {

    RegisterTask register_task{};

    std::list<BaseTask*> taskList;
    for (int i = 0; i < 4; ++i)
        taskList.push_back(new ListeningTask);
    memberCb = std::bind(&callbackWrapper, taskList, std::placeholders::_1, std::placeholders::_2);
    register_task.subscribe(task_global);

    return 0;
}

Edit 2 Ok I think I got what you want. The best I can come up with without splattering your code with global functions manually is with template magic. Note however, that it is not as flexible as you might want because you have to bind those methods at compile time. If you need to add them at runtime, you can probably use the same trick but without templates. Simply put all the std::function objects in a vector and wrap that up in a singleton or something similar.

#include <iostream>
#include <string>
#include <functional>
#include <list>


/* Simulated DLL */
typedef void(*f_pointer)(float, float);
class RegisterTask {
public:
    void  subscribe(f_pointer fp) {
        fp(1.0f, 2.0f);
    }
};



/* Static function generator to ease the pain to define all of them manually */
template<unsigned int T>
std::function<void(float, float)> &getWrapper() {
    static std::function<void(float, float)> fnc;
    return fnc;
}

/* Same here */
template<unsigned int T>
void task_global(float x, float y) { getWrapper<T>()(x, y); }



class BaseTask {
public:
    virtual void  task(float x, float y) = 0;
};
class ListeningTask :public BaseTask {
public:
    ListeningTask(int taskNum) : m_taskNum(taskNum) {}
    void  task(float x, float y) { printf("Called this member of task %d function with: %f %f\n", getTaskNum(), x, y); }
    int getTaskNum() const { return m_taskNum; }
private:
    int m_taskNum;
};


/* Context injector */
void callbackWrapper(BaseTask* t, float x, float y) {
    t->task(x, y);
}

/* Convenience function to bind an instance to a task */
template<unsigned int T>
void bindTask(ListeningTask* t) {
    getWrapper<T>() = std::bind(&callbackWrapper, t, std::placeholders::_1, std::placeholders::_2);
}

int main() {

    RegisterTask register_task{};

    auto task0 = new ListeningTask(1337);
    auto task1 = new ListeningTask(1984);
    auto task2 = new ListeningTask(42);

    bindTask<0>(task0);
    register_task.subscribe(task_global<0>);

    bindTask<1>(task1);
    register_task.subscribe(task_global<1>);

    bindTask<2>(task2);
    register_task.subscribe(task_global<2>);

    return 0;
}

Run Code demo

Answer 2

pokey909's answer is totally great, but if you don't even have access to std::function and std::bind , we can hack our way around it.

The gist of the approach is that we are going to define a template class with an implicit conversion to the desired function type. The downside is that each new additional wrapper requires a new type declaration.

// assumes two function arguments
template<class Ret, class Mem, class Arg1, class Arg2, int>
struct MemBind
{
    typedef Ret(Mem::*mem_fn_type)(Arg1, Arg2);
    static void Set(mem_fn_type _fn, Mem* _instance)
    {
        fn = _fn;
        instance = _instance;
    }

    static Ret DoTheThing(Arg1 first, Arg2 second)
    {
        return ((*instance).*fn)(first, second);
    }

    typedef Ret(*fn_type)(Arg1, Arg2); 
    operator fn_type ()
    {
        return DoTheThing;
    }

    static mem_fn_type fn;
    static Mem* instance;
};

Given some struct Foo with our desired callback:

struct Foo
{
    void Bar(float a, float b)
    {
        std::cout << "Foo::Bar(float, float) " << a << " , " << b << std::endl;
    }
};

We have to define our static members:

typedef MemBind<void, Foo, float, float, 0> MemBindZero;
template<> Foo* MemBindZero::instance = nullptr;
template<>  void(Foo::*MemBindZero::fn)(float, float)  = nullptr;

We can have a caller that takes in a function pointer:

void Caller(void(*_fn)(float, float))
{
    _fn(42.0, 1337.0);
}

The key here is that MemBind has an implicit conversion to the desired function type. The 0 in the typedef for MemBindZero allows us to re-use the same types for the other arguments, but increment the counter to 1 when used. I think you could probably replace it with a __COUNTER__ macro or something like that, but it would be nonstandard so I did it manually.

Now the next bit is to create an instance of MemBindZero , then set the static members, and finally pass our instance into Caller :

 Foo f;
 MemBindZero bound;
 bound.Set(&Foo::Bar, &f);
 Caller(bound);

Demo

---

In the demo I wrapped the static member initialization into a more convenient macro:

#define MEMBIND(RET, CLASS, ARG1, ARG2, COUNT, ALIAS) \
typedef MemBind<RET, CLASS, ARG1, ARG2, COUNT> ALIAS; \
template<> CLASS * ALIAS::instance = nullptr; \
template<>  RET(CLASS::*ALIAS::fn)(ARG1, ARG2)  = nullptr;

So that I could call it like so:

MEMBIND(void, Foo, float, float, 0, MemBindZero)
MEMBIND(void, OtherFoo, float, float, 1, MemBindOne)