Convert C++ function pointer to c function pointer - c++

Question

I am developing a C++ application using a C library. I have to send a pointer to function to the C library.
This is my class:
class MainWindow : public QMainWindow {
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
private:
Ui::MainWindow *ui;
void f(int*);
private slots:
void on_btn_clicked();
};
This is my on_btn_clicked function:
void MainWindow::on_btn_clicked()
{
void (MainWindow::* ptfptr) (int*) = &MainWindow::f;
c_library_function(static_cast<void()(int*)>(ptfptr), NULL);
}
The C function should get a pointer to a such function : void f(int*).
But the code above doesn't work, I cannot succeed to convert my f member function to the desired pointer.
Can anybody please help?

Answer 1

You can't pass a non-static member function pointer as an ordinary function pointer. They're not the same thing, and probably not even the same size.
You can however (usually) pass a pointer to a static member function through C. Usually when registering a callback in a C API, you also get to pass a "user data" pointer which gets passed back to your registered function. So you can do something like:
class MyClass
{
void non_static_func(/* args */);
public:
static void static_func(MyClass *ptr, /* other args */) {
ptr->non_static_func(/* other args */);
}
};
Then register your callback as
c_library_function(MyClass::static_func, this);
i.e. pass the instance pointer to the static method, and use that as a forwarding function.
Strictly speaking for total portability you need to use a free function declared extern "C" rather than a static member to do your forwarding (declared as a friend if necessary), but practically speaking I've never had any problems using this method to interface C++ code with GObject code, which is C callback-heavy.

Answer 2

You can't pass a function pointer to a non-static member function. What you can do is to create a static or global function that makes the call with an instance parameter.
Here's an example I find useful which uses a helper class with two members: a function wrapper and a callback function that calls the wrapper.
template <typename T>
struct Callback;
template <typename Ret, typename... Params>
struct Callback<Ret(Params...)> {
template <typename... Args>
static Ret callback(Args... args) { return func(args...); }
static std::function<Ret(Params...)> func;
};
// Initialize the static member.
template <typename Ret, typename... Params>
std::function<Ret(Params...)> Callback<Ret(Params...)>::func;
Using this you can store any callable, even non-static member functions (using std::bind) and convert to a c-pointer using the Callback::callback function. E.g:
struct Foo {
void print(int* x) { // Some member function.
std::cout << *x << std::endl;
}
};
int main() {
Foo foo; // Create instance of Foo.
// Store member function and the instance using std::bind.
Callback<void(int*)>::func = std::bind(&Foo::print, foo, std::placeholders::_1);
// Convert callback-function to c-pointer.
void (*c_func)(int*) = static_cast<decltype(c_func)>(Callback<void(int*)>::callback);
// Use in any way you wish.
std::unique_ptr<int> iptr{new int(5)};
c_func(iptr.get());
}

Answer 3

If I recall it correctly, Only static methods of a class can be accessed via "normal" C pointer to function syntax. So try to make it static. The pointer to a method of a class needs extra information, such as the "object" (this) which has no meaning for a pure C method.
The FAQ shown here has good explanation and a possible (ugly) solution for your problem.

Answer 4

#Snps answer is great. I extended it with a maker function that creates a callback, as I always use void callbacks without parameters:
typedef void (*voidCCallback)();
template<typename T>
voidCCallback makeCCallback(void (T::*method)(),T* r){
Callback<void()>::func = std::bind(method, r);
void (*c_function_pointer)() = static_cast<decltype(c_function_pointer)>(Callback<void()>::callback);
return c_function_pointer;
}
From then on, you can create your plain C callback from within the class or anywhere else and have the member called:
voidCCallback callback = makeCCallback(&Foo::print, this);
plainOldCFunction(callback);

Answer 5

#Snps answer is perfect! But as #DXM mentioned it can hold only one callback. I've improved it a little, now it can keep many callbacks of the same type. It's a little bit strange, but works perfect:
#include <type_traits>
template<typename T>
struct ActualType {
typedef T type;
};
template<typename T>
struct ActualType<T*> {
typedef typename ActualType<T>::type type;
};
template<typename T, unsigned int n,typename CallerType>
struct Callback;
template<typename Ret, typename ... Params, unsigned int n,typename CallerType>
struct Callback<Ret(Params...), n,CallerType> {
typedef Ret (*ret_cb)(Params...);
template<typename ... Args>
static Ret callback(Args ... args) {
func(args...);
}
static ret_cb getCallback(std::function<Ret(Params...)> fn) {
func = fn;
return static_cast<ret_cb>(Callback<Ret(Params...), n,CallerType>::callback);
}
static std::function<Ret(Params...)> func;
};
template<typename Ret, typename ... Params, unsigned int n,typename CallerType>
std::function<Ret(Params...)> Callback<Ret(Params...), n,CallerType>::func;
#define GETCB(ptrtype,callertype) Callback<ActualType<ptrtype>::type,__COUNTER__,callertype>::getCallback
Now you can just do something like this:
typedef void (cb_type)(uint8_t, uint8_t);
class testfunc {
public:
void test(int x) {
std::cout << "in testfunc.test " <<x<< std::endl;
}
void test1(int x) {
std::cout << "in testfunc.test1 " <<x<< std::endl;
}
};
cb_type* f = GETCB(cb_type, testfunc)(std::bind(&testfunc::test, tf, std::placeholders::_2));
cb_type* f1 = GETCB(cb_type, testfunc)(
std::bind(&testfunc::test1, tf, std::placeholders::_2));
f(5, 4);
f1(5, 7);

Answer 6

The short answer is: you can convert a member function pointer to an ordinary C function pointer using std::mem_fn.
That is the answer to the question as given, but this question seems to have a confused premise, as the asker expects C code to be able to call an instance method of MainWindow without having a MainWindow*, which is simply impossible.
If you use mem_fn to cast MainWindow::on_btn_clicked to a C function pointer, then you still a function that takes a MainWindow* as its first argument.
void (*window_callback)(MainWindow*,int*) = std::mem_fn(&MainWindow::on_btn_clicked);
That is the answer to the question as given, but it doesn't match the interface. You would have to write a C function to wrap the call to a specific instance (after all, your C API code knows nothing about MainWindow or any specific instance of it):
void window_button_click_wrapper(int* arg)
{
MainWindow::inst()->on_btn_clicked(arg);
}
This is considered an OO anti-pattern, but since the C API knows nothing about your object, it's the only way.

Answer 7

I've got an idea (not entirely standard-compliant, as extern "C" is missing):
class MainWindow;
static MainWindow* instance;
class MainWindow
{
public:
MainWindow()
{
instance = this;
registerCallback([](int* arg){instance->...});
}
};
You will have problems if multiple instances of MainWindow are instantiated.