How can I pass a class member function as a callback?

I'm using an API that requires me to pass a function pointer as a callback. I'm trying to use this API from my class but I'm getting compilation errors.

Here is what I did from my constructor:

m_cRedundencyManager->Init(this->RedundencyManagerCallBack);

This doesn't compile - I get the following error:

> Error 8 error C3867: 'CLoggersInfra::RedundencyManagerCallBack': function call missing argument list; use '&CLoggersInfra::RedundencyManagerCallBack' to create a pointer to member

I tried the suggestion to use &CLoggersInfra::RedundencyManagerCallBack - didn't work for me.

Any suggestions/explanation for this??

I'm using VS2008.

Thanks!!

This is a simple question but the answer is surprisingly complex. The short answer is you can do what you're trying to do with std::bind1st or boost::bind. The longer answer is below.

The compiler is correct to suggest you use &CLoggersInfra::RedundencyManagerCallBack. First, if RedundencyManagerCallBack is a member function, the function itself doesn't belong to any particular instance of the class CLoggersInfra. It belongs to the class itself. If you've ever called a static class function before, you may have noticed you use the same SomeClass::SomeMemberFunction syntax. Since the function itself is 'static' in the sense that it belongs to the class rather than a particular instance, you use the same syntax. The '&' is necessary because technically speaking you don't pass functions directly -- functions are not real objects in C++. Instead you're technically passing the memory address for the function, that is, a pointer to where the function's instructions begin in memory. The consequence is the same though, you're effectively 'passing a function' as a parameter.

But that's only half the problem in this instance. As I said, RedundencyManagerCallBack the function doesn't 'belong' to any particular instance. But it sounds like you want to pass it as a callback with a particular instance in mind. To understand how to do this you need to understand what member functions really are: regular not-defined-in-any-class functions with an extra hidden parameter.

For example:

class A {
public:
    A() : data(0) {}
    void foo(int addToData) { this->data += addToData; }

    int data;
};

...

A an_a_object;
an_a_object.foo(5);
A::foo(&an_a_object, 5); // This is the same as the line above!
std::cout << an_a_object.data; // Prints 10!

How many parameters does A::foo take? Normally we would say 1. But under the hood, foo really takes 2. Looking at A::foo's definition, it needs a specific instance of A in order for the 'this' pointer to be meaningful (the compiler needs to know what 'this' is). The way you usually specify what you want 'this' to be is through the syntax MyObject.MyMemberFunction(). But this is just syntactic sugar for passing the address of MyObject as the first parameter to MyMemberFunction. Similarly, when we declare member functions inside class definitions we don't put 'this' in the parameter list, but this is just a gift from the language designers to save typing. Instead you have to specify that a member function is static to opt out of it automatically getting the extra 'this' parameter. If the C++ compiler translated the above example to C code (the original C++ compiler actually worked that way), it would probably write something like this:

struct A {
    int data;
};

void a_init(A* to_init)
{
    to_init->data = 0;
}

void a_foo(A* this, int addToData)
{ 
    this->data += addToData;
}

...

A an_a_object;
a_init(0); // Before constructor call was implicit
a_foo(&an_a_object, 5); // Used to be an_a_object.foo(5);

Returning to your example, there is now an obvious problem. 'Init' wants a pointer to a function that takes one parameter. But &CLoggersInfra::RedundencyManagerCallBack is a pointer to a function that takes two parameters, it's normal parameter and the secret 'this' parameter. That's why you're still getting a compiler error (as a side note: If you've ever used Python, this kind of confusion is why a 'self' parameter is required for all member functions).

The verbose way to handle this is to create a special object that holds a pointer to the instance you want and has a member function called something like 'run' or 'execute' (or overloads the '()' operator) that takes the parameters for the member function, and simply calls the member function with those parameters on the stored instance. But this would require you to change 'Init' to take your special object rather than a raw function pointer, and it sounds like Init is someone else's code. And making a special class for every time this problem comes up will lead to code bloat.

So now, finally, the good solution, boost::bind and boost::function, the documentation for each you can find here:

boost::bind docs, boost::function docs

boost::bind will let you take a function, and a parameter to that function, and make a new function where that parameter is 'locked' in place. So if I have a function that adds two integers, I can use boost::bind to make a new function where one of the parameters is locked to say 5. This new function will only take one integer parameter, and will always add 5 specifically to it. Using this technique, you can 'lock in' the hidden 'this' parameter to be a particular class instance, and generate a new function that only takes one parameter, just like you want (note that the hidden parameter is always the first parameter, and the normal parameters come in order after it). Look at the boost::bind docs for examples, they even specifically discuss using it for member functions. Technically there is a standard function called [std::bind1st][3] that you could use as well, but boost::bind is more general.

Of course, there's just one more catch. boost::bind will make a nice boost::function for you, but this is still technically not a raw function pointer like Init probably wants. Thankfully, boost provides a way to convert boost::function's to raw pointers, as documented on StackOverflow here. How it implements this is beyond the scope of this answer, though it's interesting too.

Don't worry if this seems ludicrously hard -- your question intersects several of C++'s darker corners, and boost::bind is incredibly useful once you learn it.

C++11 update: Instead of boost::bind you can now use a lambda function that captures 'this'. This is basically having the compiler generate the same thing for you.

That doesn't work because a member function pointer cannot be handled like a normal function pointer, because it expects a "this" object argument.

Instead you can pass a static member function as follows, which are like normal non-member functions in this regard:

m_cRedundencyManager->Init(&CLoggersInfra::Callback, this);

The function can be defined as follows

static void Callback(int other_arg, void * this_pointer) {
    CLoggersInfra * self = static_cast<CLoggersInfra*>(this_pointer);
    self->RedundencyManagerCallBack(other_arg);
}

This answer is a reply to a comment above and does not work with VisualStudio 2008 but should be preferred with more recent compilers.

---

Meanwhile you don't have to use a void pointer anymore and there is also no need for boost since std::bind and std::function are available. One advantage (in comparison to void pointers) is type safety since the return type and the arguments are explicitly stated using std::function:

// std::function<return_type(list of argument_type(s))>
void Init(std::function<void(void)> f);

Then you can create the function pointer with std::bind and pass it to Init:

auto cLoggersInfraInstance = CLoggersInfra();
auto callback = std::bind(&CLoggersInfra::RedundencyManagerCallBack, cLoggersInfraInstance);
Init(callback);

Complete example for using std::bind with member, static members and non member functions:

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

class RedundencyManager // incl. Typo ;-)
{
public:
    // std::function<return_type(list of argument_type(s))>
    std::string Init(std::function<std::string(void)> f) 
    {
        return f();
    }
};

class CLoggersInfra
{
private:
    std::string member = "Hello from non static member callback!";
    
public:
    static std::string RedundencyManagerCallBack()
    {
        return "Hello from static member callback!";
    }
    
    std::string NonStaticRedundencyManagerCallBack()
    {
        return member;
    }
};

std::string NonMemberCallBack()
{
    return "Hello from non member function!";
}

int main()
{
    auto instance = RedundencyManager();
    
    auto callback1 = std::bind(&NonMemberCallBack);
    std::cout << instance.Init(callback1) << "\n";
    
    // Similar to non member function.
    auto callback2 = std::bind(&CLoggersInfra::RedundencyManagerCallBack);
    std::cout << instance.Init(callback2) << "\n";
    
    // Class instance is passed to std::bind as second argument.
    // (heed that I call the constructor of CLoggersInfra)
    auto callback3 = std::bind(&CLoggersInfra::NonStaticRedundencyManagerCallBack,
                               CLoggersInfra()); 
    std::cout << instance.Init(callback3) << "\n";
}

Possible output:

Hello from non member function!
Hello from static member callback!
Hello from non static member callback!

Furthermore using std::placeholders you can dynamically pass arguments to the callback (e.g. this enables the usage of return f("MyString"); in Init if f has a string parameter).

What argument does Init take? What is the new error message?

Method pointers in C++ are a bit difficult to use. Besides the method pointer itself, you also need to provide an instance pointer (in your case this). Maybe Init expects it as a separate argument?

Is m_cRedundencyManager able to use member functions? Most callbacks are set up to use regular functions or static member functions. Take a look at http://www.parashift.com/c++-faq-lite/pointers-to-members.html">this page at C++ FAQ Lite for more information.

Update: The function declaration you provided shows that m_cRedundencyManager is expecting a function of the form: void yourCallbackFunction(int, void *). Member functions are therefore unacceptable as callbacks in this case. A static member function may work, but if that is unacceptable in your case, the following code would also work. Note that it uses an evil cast from void *.

// in your CLoggersInfra constructor:
m_cRedundencyManager->Init(myRedundencyManagerCallBackHandler, this);

// in your CLoggersInfra header:
void myRedundencyManagerCallBackHandler(int i, void * CLoggersInfraPtr);

// in your CLoggersInfra source file:
void myRedundencyManagerCallBackHandler(int i, void * CLoggersInfraPtr)
{
((CLoggersInfra *)CLoggersInfraPtr)->RedundencyManagerCallBack(i);
}

A pointer to a class member function is not the same as a pointer to a function. A class member takes an implicit extra argument (the this pointer), and uses a different calling convention.

If your API expects a nonmember callback function, that's what you have to pass to it.

Necromancing.
I think the answers to date are a little unclear.

Let's make an example:

Supposed you have an array of pixels (array of ARGB int8_t values)

// A RGB image
int8_t* pixels = new int8_t[1024*768*4];

Now you want to generate a PNG. To do so, you call the function toJpeg

bool ok = toJpeg(writeByte, pixels, width, height);

where writeByte is a callback-function

void writeByte(unsigned char oneByte)
{
    fputc(oneByte, output);
}

The problem here: FILE* output has to be a global variable.
Very bad if you're in a multithreaded environment (e.g. a http-server).

So you need some way to make output a non-global variable, while retaining the callback signature.

The immediate solution that springs into mind is a closure, which we can emulate using a class with a member function.

class BadIdea {
private:
    FILE* m_stream;
public:
    BadIdea(FILE* stream)  {
        this->m_stream = stream;
    }

    void writeByte(unsigned char oneByte){
            fputc(oneByte, this->m_stream);
    }
    
};

And then do

FILE *fp = fopen(filename, "wb");
BadIdea* foobar = new BadIdea(fp);

bool ok = TooJpeg::writeJpeg(foobar->writeByte, image, width, height);
delete foobar;
fflush(fp);
fclose(fp);

However, contrary to expectations, this does not work.

The reason is, C++ member functions are kinda implemented like C# extension functions.

So you have

class/struct BadIdea
{
    FILE* m_stream;
}

and

static class BadIdeaExtensions
{
    public static writeByte(this BadIdea instance, unsigned char oneByte)
    {
         fputc(oneByte, instance->m_stream);
    }

}

So when you want to call writeByte, you need pass not only the address of writeByte, but also the address of the BadIdea-instance.

So when you have a typedef for the writeByte procedure, and it looks like this

typedef void (*WRITE_ONE_BYTE)(unsigned char);

And you have a writeJpeg signature that looks like this

bool writeJpeg(WRITE_ONE_BYTE output, uint8_t* pixels, uint32_t 
 width, uint32_t height))
	{ ... }

it's fundamentally impossible to pass a two-address member function to a one-address function pointer (without modifying writeJpeg), and there's no way around it.

The next best thing that you can do in C++, is using a lambda-function:

FILE *fp = fopen(filename, "wb");
auto lambda = [fp](unsigned char oneByte) { fputc(oneByte, fp);  };
bool ok = TooJpeg::writeJpeg(lambda, image, width, height);

However, because lambda is doing nothing different, than passing an instance to a hidden class (such as the "BadIdea"-class), you need to modify the signature of writeJpeg.

The advantage of lambda over a manual class, is that you just need to change one typedef

typedef void (*WRITE_ONE_BYTE)(unsigned char);

to

using WRITE_ONE_BYTE = std::function<void(unsigned char)>; 

And then you can leave everything else untouched.

You could also use std::bind

auto f = std::bind(&BadIdea::writeByte, &foobar);

But this, behind the scene, just creates a lambda function, which then also needs the change in typedef.

So no, there is no way to pass a member function to a method that requires a static function-pointer.

But lambdas are the easy way around, provided that you have control over the source.
Otherwise, you're out of luck.
There's nothing you can do with C++.

Note:
std::function requires #include <functional>

However, since C++ allows you to use C as well, you can do this with libffcall in plain C, if you don't mind linking a dependency.

Download libffcall from GNU (at least on ubuntu, don't use the distro-provided package - it is broken), unzip.

./configure
make
make install

gcc main.c -l:libffcall.a -o ma

main.c:

#include <callback.h>

// this is the closure function to be allocated 
void function (void* data, va_alist alist)
{
     int abc = va_arg_int(alist);

     printf("data: %08p\n", data); // hex 0x14 = 20
     printf("abc: %d\n", abc);

	 // va_start_type(alist[, return_type]);
	 // arg = va_arg_type(alist[, arg_type]);
	 // va_return_type(alist[[, return_type], return_value]);

    // va_start_int(alist);
    // int r = 666;
    // va_return_int(alist, r);
}



int main(int argc, char* argv[])
{
 	int in1 = 10;

 	void * data = (void*) 20;
    void(*incrementer1)(int abc) = (void(*)()) alloc_callback(&function, data);
	// void(*incrementer1)() can have unlimited arguments, e.g. incrementer1(123,456);
    // void(*incrementer1)(int abc) starts to throw errors...
	incrementer1(123);
    // free_callback(callback);
	return EXIT_SUCCESS;
}

And if you use CMake, add the linker library after add_executable

add_library(libffcall STATIC IMPORTED)
set_target_properties(libffcall PROPERTIES
        IMPORTED_LOCATION /usr/local/lib/libffcall.a)
target_link_libraries(BitmapLion libffcall)

or you could just dynamically link libffcall

target_link_libraries(BitmapLion ffcall)

Note:
You might want to include the libffcall headers and libraries, or create a cmake project with the contents of libffcall.

A simple solution "workaround" still is to create a class of virtual functions "interface" and inherit it in the caller class. Then pass it as a parameter "could be in the constructor" of the other class that you want to call your caller class back.

DEFINE Interface:

class CallBack 
{
   virtual callMeBack () {};
};

This is the class that you want to call you back:

class AnotherClass ()
{
     public void RegisterMe(CallBack *callback)
     {
         m_callback = callback;
     }

     public void DoSomething ()
     {
        // DO STUFF
        // .....
        // then call
        if (m_callback) m_callback->callMeBack();
     }
     private CallBack *m_callback = NULL;
};

And this is the class that will be called back.

class Caller : public CallBack
{
    void DoSomthing ()
    {
    }
    
    void callMeBack()
    {
       std::cout << "I got your message" << std::endl;
    }
};

I can see that the init has the following override:

Init(CALLBACK_FUNC_EX callback_func, void * callback_parm)

where CALLBACK_FUNC_EX is

typedef void (*CALLBACK_FUNC_EX)(int, void *);

This question and answer from the C++ FAQ Lite covers your question and the considerations involved in the answer quite nicely I think. Short snippet from the web page I linked:

> Don’t. > > Because a member function is meaningless without an object to invoke > it on, you can’t do this directly (if The X Window System was > rewritten in C++, it would probably pass references to objects around, > not just pointers to functions; naturally the objects would embody the > required function and probably a whole lot more).

The type of pointer to non-static member function is different from pointer to ordinary function.
Type is void(*)(int) if it’s an ordinary or static member function.
Type is void(CLoggersInfra::*)(int) if it’s a non-static member function.
So you cannot pass a pointer to a non-static member function if it is expecting an ordinary function pointer.

Furthermore, a non-static member function has an implicit/hidden parameter to the object. The this pointer is implicitly passed as an argument to the member function call. So the member functions can be invoked only by providing an object.

If the API Init cannot be changed, a wrapper function (ordinary function or a class static member function) that invokes the member can be used. In the worst case, the object would be a global for the wrapper function to access.

CLoggersInfra* pLoggerInfra;

RedundencyManagerCallBackWrapper(int val)
{
    pLoggerInfra->RedundencyManagerCallBack(val);
}

m_cRedundencyManager->Init(RedundencyManagerCallBackWrapper);

If the API Init can be changed, there are many alternatives - Object non-static member function pointer, Function Object, std::function or Interface Function.

See the post on callbacks for the different variations with C++ working examples.

It is still difficult to connect C style callback functions with C++ class instances. I want to kind of rephrase the original question:

• Some library you are using requires a C style function to be called back from that library. Changing the library API is out of the question since it is not your API.

• You want the callback to be handled in your own C++ code in member methods

As you did not mention (exactly) what callback you want to handle I will give an example using GLFW callbacks for key input. (On a side note: I know GLFW offers some other mechanism to attach user data to their API, but that is not the topic here.)

I don't know any solution to this problem that doesn't include usage of some kind of static object. Let's look at our options:

Simple approach: Use C style global objects

As we always think in classes and instances we sometimes forget that in C++ we still have the whole arsenal of C at our hands. So sometimes this very simple solution does not come to mind.

Let's assume we have a class Presentation that should handle keyboard input. This could look like this:

struct KeyInput {
    int pressedKey;
} KeyInputGlobal;

void globalKeyHandler(GLFWwindow* window, int key, int scancode, int action, int mods) {
    KeyInputGlobal.pressedKey = key;
}

int Presentation::getCurrentKey()
{
    return KeyInputGlobal.pressedKey;
}

void Presentation::initGLFW()
{
    glfwInit();
    glfwSetKeyCallback(window, globalKeyHandler);
}

We have a global Object KeyInputGlobal that should receive the key pressed. The function globalKeyHandler has exactly the C style API signature needed by the GLFW library to be able to call our code. It is activated on our member method initGLFW. If anywhere in our code we are interested in the currently pressed key we can just call the other member method Presentation::getCurrentKey

What is wrong with this approach?

Maybe it is all fine. Depends entirely on your use case. Maybe you are totally fine to just read the last pressed key somwhere in your application code. You don't care to have missed key pressed events. The simple approach is all you need.

To generalize: If you are able to fully process the callback in C style code, calculate some result and store it in a global object to be read later from other parts of your code, then it may indeed make sense to use this simple approach. On the plus side: It is very simple to understand. The downside? It feels a little bit like cheating, because you didn't really process the callback in your C++ code, you just used the results. If you think of the callback as an event and want each event to be properly processed in your member methods this approch won't be enough.

Another simple approach: Use C++ static objects

I guess many of us have already done this. Certainly I have. Thinking: Wait, we have a C++ concept of globals, that is using static. But we can keep the discussion short here: It may be more C++ style than using the C style from previous example, but the problems are the same - we still have globals, that are hard to bring together with non-static, regular member methods. For completeness, it would look like this in our class declaration:

class Presentation
{
public:
	struct KeyInput {
		int pressedKey;
	};
	static KeyInput KeyInputGlobal;

	static void globalKeyHandler(GLFWwindow* window, int key, int scancode, int action, int mods) {
		KeyInputGlobal.pressedKey = key;
	}
	int getCurrentKey()
	{
		return KeyInputGlobal.pressedKey;
	}
...
}

Activating our callback would look the same, but we also have to define the static struct that receives the key pressed in our implementation:

void Presentation::initGLFW()
{
    glfwInit();
    glfwSetKeyCallback(window, globalKeyHandler);
}
//static
Presentation::KeyInput Presentation::KeyInputGlobal;

You might be inclined to just remove the static keyword from our callback method globalKeyHandler: The compiler will immediately tell you that you can no longer pass this to GLFW in glfwSetKeyCallback(). Now, if we only could connect static methods with regular methods somehow...

C++11 Event driven approach with statics and lambdas

The best solution I could find out is the following. It works and is somewhat elegant, but I still do not consider it perfect. Let's look at it and discuss:

void Presentation::initGLFW()
{
    glfwInit();
    static auto callback_static = [this](
           GLFWwindow* window, int key, int scancode, int action, int mods) {
        // because we have a this pointer we are now able to call a non-static member method:
        key_callbackMember(window, key, scancode, action, mods);
    };
    glfwSetKeyCallback(window,
    [](GLFWwindow* window, int key, int scancode, int action, int mods)
      {
        // only static methods can be called here as we cannot change glfw function parameter list to include instance pointer
        callback_static(window, key, scancode, action, mods);
      }
    );
}

void Presentation::key_callbackMember(GLFWwindow* window, int key, int scancode, int action, int mods)
{
    // we can now call anywhere in our code to process the key input:
    app->handleInput(key, action);
}

The definition of callback_static is where we connect a static object with instance data, in this case this is an instance of our Presentation class. You can read the definition as follows: If callback_static is called anytime after this definition, all parameters will be passed to the member method key_callbackMember called at the Presentation instance just used. This definition has nothing to do with the GLFW library yet - it is just the preparation for the next step.

We now use a second lambda to register our callback with the library in glfwSetKeyCallback(). Again, if callback_static would not have been defined as static we could not pass it to GLFW here.

This is what happens at runtime after all the initializations, when GLFW calls our code:

1. GLFW recognizes a key event and calls our static object callback_static
2. callback_static has access to an instance of Presentation class and calls it's instance method key_callbackMember
3. Now that we are in 'object world' we can process the key event somewhere else. In this case we call the method handleInput on some arbitrary object app, that has been setup somewhere else in our code.

The good: We have achieved what we wanted with no need to define global objects outside our initialization method initGLFW. No need for C style globals.

The bad: Don't be fooled just because everything is neatly packed into one method. We still have static objects. And with them all the problems global objects have. E.g. multiple calls to our initialization method (with different instances of Presentation) would probably not have the effect you intended.

Summary

It is possible to connect C style callbacks of existing libraries to instances of classes in your own code. You can try to minimize houeskeeping code by defining the necessary objects in member methods of your code. But you still need one static object for each callback. If you want to connect several instances of your C++ code with a C style callback be prepared to introduce a more complicated management of your static objects than in the example above.

Hope this helps someone. Happy coding.

Looks like std::mem_fn (C++11) does exactly what you need:

> Function template std::mem_fn generates wrapper objects for pointers to members, which can store, copy, and invoke a pointer to member. Both references and pointers (including smart pointers) to an object can be used when invoking a std::mem_fn.