Function pointer to member function

Question

I'd like to set up a function pointer as a member of a class that is a pointer to another function in the same class. The reasons why I'm doing this are complicated.

In this example, I would like the output to be "1"

class A {
public:
 int f();
 int (*x)();
}

int A::f() {
 return 1;
}


int main() {
 A a;
 a.x = a.f;
 printf("%d\n",a.x())
}

But this fails at compiling. Why?

Answer 1

The syntax is wrong. A member pointer is a different type category from a ordinary pointer. The member pointer will have to be used together with an object of its class:

class A {
public:
 int f();
 int (A::*x)(); // <- declare by saying what class it is a pointer to
};

int A::f() {
 return 1;
}


int main() {
 A a;
 a.x = &A::f; // use the :: syntax
 printf("%d\n",(a.*(a.x))()); // use together with an object of its class
}

a.x does not yet say on what object the function is to be called on. It just says that you want to use the pointer stored in the object a. Prepending a another time as the left operand to the .* operator will tell the compiler on what object to call the function on.

Answer 2

int (*x)() is not a pointer to member function. A pointer to member function is written like this: int (A::*x)(void) = &A::f;.

Answer 3

Call member function on string command

#include <iostream>
#include <string>


class A 
{
public: 
    void call();
private:
    void printH();
    void command(std::string a, std::string b, void (A::*func)());
};

void A::printH()
{
    std::cout<< "H\n";
}

void A::call()
{
    command("a","a", &A::printH);
}

void A::command(std::string a, std::string b, void (A::*func)())
{
    if(a == b)
    {
        (this->*func)();
    }
}

int main()
{
    A a;
    a.call();
    return 0;
}

Pay attention to (this->*func)(); and the way to declare the function pointer with class name void (A::*func)()

Answer 4

You need to use a pointer to a member function, not just a pointer to a function.

class A { 
    int f() { return 1; }
public:
    int (A::*x)();

    A() : x(&A::f) {}
};

int main() { 
   A a;
   std::cout << (a.*a.x)();
   return 0;
}

Answer 5

While you unfortunately cannot convert an existing member function pointer to a plain function pointer, you can create an adapter function template in a fairly straightforward way that wraps a member function pointer known at compile-time in a normal function like this:

template <class Type>
struct member_function;

template <class Type, class Ret, class... Args>
struct member_function<Ret(Type::*)(Args...)>
{
    template <Ret(Type::*Func)(Args...)>
    static Ret adapter(Type &obj, Args&&... args)
    {
        return (obj.*Func)(std::forward<Args>(args)...);
    }
};

template <class Type, class Ret, class... Args>
struct member_function<Ret(Type::*)(Args...) const>
{
    template <Ret(Type::*Func)(Args...) const>
    static Ret adapter(const Type &obj, Args&&... args)
    {
        return (obj.*Func)(std::forward<Args>(args)...);
    }
};

 

int (*func)(A&) = &member_function<decltype(&A::f)>::adapter<&A::f>;

Note that in order to call the member function, an instance of A must be provided.

Answer 6

While this is based on the sterling answers elsewhere on this page, I had a use case which wasn't completely solved by them; for a vector of pointers to functions do the following:

#include <iostream>
#include <vector>
#include <stdio.h>
#include <stdlib.h>

class A{
public:
  typedef vector<int> (A::*AFunc)(int I1,int I2);
  vector<AFunc> FuncList;
  inline int Subtract(int I1,int I2){return I1-I2;};
  inline int Add(int I1,int I2){return I1+I2;};
  ...
  void Populate();
  void ExecuteAll();
};

void A::Populate(){
    FuncList.push_back(&A::Subtract);
    FuncList.push_back(&A::Add);
    ...
}

void A::ExecuteAll(){
  int In1=1,In2=2,Out=0;
  for(size_t FuncId=0;FuncId<FuncList.size();FuncId++){
    Out=(this->*FuncList[FuncId])(In1,In2);
    printf("Function %ld output %d\n",FuncId,Out);
  }
}

int main(){
  A Demo;
  Demo.Populate();
  Demo.ExecuteAll();
  return 0;
}

Something like this is useful if you are writing a command interpreter with indexed functions that need to be married up with parameter syntax and help tips etc. Possibly also useful in menus.

Answer 7

@Johannes Schaub - litb has the correct solution, but I thought it would be beneficial to post a generic example of using a pointer to a member function as well.

std::string myString{ "Hello World!" };
auto memberFunctionPointer{ &std::string::length };
auto myStringLength{ (myString.*memberFunctionPointer)() };

C++17 has a template function for calling a pointer to a member function, which looks like this.

std::invoke(memberFunctionPointer, myString);

Answer 8

Building on @IllidanS4 's answer, I have created a template class that allows virtually any member function with predefined arguments and class instance to be passed by reference for later calling.

template<class RET, class... RArgs> class Callback_t {
public:
    virtual RET call(RArgs&&... rargs) = 0;
    //virtual RET call() = 0;
};

template<class T, class RET, class... RArgs> class CallbackCalltimeArgs : public Callback_t<RET, RArgs...> {
public:
    T * owner;
    RET(T::*x)(RArgs...);
    RET call(RArgs&&... rargs) {
        return (*owner.*(x))(std::forward<RArgs>(rargs)...);
    };
    CallbackCalltimeArgs(T* t, RET(T::*x)(RArgs...)) : owner(t), x(x) {}
};

template<class T, class RET, class... Args> class CallbackCreattimeArgs : public Callback_t<RET> {
public:
    T* owner;
    RET(T::*x)(Args...);
    RET call() {
        return (*owner.*(x))(std::get<Args&&>(args)...);
    };
    std::tuple<Args&&...> args;
    CallbackCreattimeArgs(T* t, RET(T::*x)(Args...), Args&&... args) : owner(t), x(x),
        args(std::tuple<Args&&...>(std::forward<Args>(args)...)) {}
};

Test / example:

class container {
public:
    static void printFrom(container* c) { c->print(); };
    container(int data) : data(data) {};
    ~container() {};
    void print() { printf("%d\n", data); };
    void printTo(FILE* f) { fprintf(f, "%d\n", data); };
    void printWith(int arg) { printf("%d:%d\n", data, arg); };
private:
    int data;
};

int main() {
    container c1(1), c2(20);
    CallbackCreattimeArgs<container, void> f1(&c1, &container::print);
    Callback_t<void>* fp1 = &f1;
    fp1->call();//1
    CallbackCreattimeArgs<container, void, FILE*> f2(&c2, &container::printTo, stdout);
    Callback_t<void>* fp2 = &f2;
    fp2->call();//20
    CallbackCalltimeArgs<container, void, int> f3(&c2, &container::printWith);
    Callback_t<void, int>* fp3 = &f3;
    fp3->call(15);//20:15
}

Obviously, this will only work if the given arguments and owner class are still valid. As far as readability... please forgive me.

Edit: removed unnecessary malloc by making the tuple normal storage. Added inherited type for the reference. Added option to provide all arguments at calltime instead. Now working on having both....

Edit 2: As promised, both. Only restriction (that I see) is that the predefined arguments must come before the runtime supplied arguments in the callback function. Thanks to @Chipster for some help with gcc compliance. This works on gcc on ubuntu and visual studio on windows.

#ifdef _WIN32
#define wintypename typename
#else
#define wintypename
#endif

template<class RET, class... RArgs> class Callback_t {
public:
    virtual RET call(RArgs... rargs) = 0;
    virtual ~Callback_t() = default;
};

template<class RET, class... RArgs> class CallbackFactory {
private:
    template<class T, class... CArgs> class Callback : public Callback_t<RET, RArgs...> {
    private:
        T * owner;
        RET(T::*x)(CArgs..., RArgs...);
        std::tuple<CArgs...> cargs;
        RET call(RArgs... rargs) {
            return (*owner.*(x))(std::get<CArgs>(cargs)..., rargs...);
        };
    public:
        Callback(T* t, RET(T::*x)(CArgs..., RArgs...), CArgs... pda);
        ~Callback() {};
    };
public:
    template<class U, class... CArgs> static Callback_t<RET, RArgs...>* make(U* owner, CArgs... cargs, RET(U::*func)(CArgs..., RArgs...));
};
template<class RET2, class... RArgs2> template<class T2, class... CArgs2> CallbackFactory<RET2, RArgs2...>::Callback<T2, CArgs2...>::Callback(T2* t, RET2(T2::*x)(CArgs2..., RArgs2...), CArgs2... pda) : x(x), owner(t), cargs(std::forward<CArgs2>(pda)...) {}
template<class RET, class... RArgs> template<class U, class... CArgs> Callback_t<RET, RArgs...>* CallbackFactory<RET, RArgs...>::make(U* owner, CArgs... cargs, RET(U::*func)(CArgs..., RArgs...)) {
    return new wintypename CallbackFactory<RET, RArgs...>::Callback<U, CArgs...>(owner, func, std::forward<CArgs>(cargs)...);
}

Edit 3: clang compliance, greater flexibility and examples. (Ripped from my active hobby project, which I plan to open source... eventually.)

//CallbackFactory.h
#pragma once

#ifdef _WIN32
#define wintypename typename
#else
#define wintypename
#endif

namespace WITE {

  template<class RET, class... RArgs> class Callback_t {
  public:
    virtual RET call(RArgs... rargs) const = 0;
    virtual ~Callback_t() = default;
  };

  template<class RET, class... RArgs> class CallbackFactory {
  private:
    template<class T, class... CArgs> class Callback : public Callback_t<RET, RArgs...> {
    private:
      RET(T::*x)(CArgs..., RArgs...);
      T * owner;
      std::tuple<CArgs...> cargs;
    public:
      Callback(T* t, RET(T::*x)(CArgs..., RArgs...), CArgs... pda);
      ~Callback() {};
      RET call(RArgs... rargs) const override {
        return (*owner.*(x))(std::get<CArgs>(cargs)..., rargs...);
      };
    };
    template<class... CArgs> class StaticCallback : public Callback_t<RET, RArgs...> {
    private:
      RET(*x)(CArgs..., RArgs...);
      std::tuple<CArgs...> cargs;
    public:
      StaticCallback(RET(*x)(CArgs..., RArgs...), CArgs... pda);
      ~StaticCallback() {};
      RET call(RArgs... rargs) const override {
        return (*x)(std::get<CArgs>(cargs)..., rargs...);
      };
    };
  public:
    typedef Callback_t<RET, RArgs...>* callback_t;
    template<class U, class... CArgs> static callback_t make(U* owner, CArgs... cargs, RET(U::*func)(CArgs..., RArgs...));
    template<class... CArgs> static callback_t make(CArgs... cargs, RET(*func)(CArgs..., RArgs...));//for non-members or static members
  };
  template<class RET2, class... RArgs2> template<class T2, class... CArgs2>
  CallbackFactory<RET2, RArgs2...>::Callback<T2, CArgs2...>::Callback(T2* t, RET2(T2::*x)(CArgs2..., RArgs2...), CArgs2... pda) :
    x(x), owner(t), cargs(std::forward<CArgs2>(pda)...) {}

  template<class RET2, class... RArgs2> template<class... CArgs2>
  CallbackFactory<RET2, RArgs2...>::StaticCallback<CArgs2...>::StaticCallback(RET2(*x)(CArgs2..., RArgs2...), CArgs2... pda) :
    x(x), cargs(std::forward<CArgs2>(pda)...) {}

  template<class RET, class... RArgs> template<class U, class... CArgs> Callback_t<RET, RArgs...>*
  CallbackFactory<RET, RArgs...>::make(U* owner, CArgs... cargs, RET(U::*func)(CArgs..., RArgs...)) {
    return new wintypename CallbackFactory<RET, RArgs...>::Callback<U, CArgs...>(owner, func, std::forward<CArgs>(cargs)...);
  };

  template<class RET, class... RArgs> template<class... CArgs> Callback_t<RET, RArgs...>*
  CallbackFactory<RET, RArgs...>::make(CArgs... cargs, RET(*func)(CArgs..., RArgs...)) {
    return new wintypename CallbackFactory<RET, RArgs...>::StaticCallback<CArgs...>(func, std::forward<CArgs>(cargs)...);
  };

  #define typedefCB(name, ...) typedef WITE::CallbackFactory<__VA_ARGS__> name## _F; typedef typename name## _F::callback_t name ;

  typedefCB(rawDataSource, int, void*, size_t)

};

//example:
class Integer {
public:
  typedefCB(oneInOneOut, int, int);
  typedefCB(twoInOneOut, int, int, int);
  int value;
  Integer(int v) : value(v) {};
  int plus(int o) {
    return value + o;
  };
  int plus(int a, int b, int c) {
    return value + a + b + c;
  };
  static int simpleSum(int a, int b) {
    return a + b;
  };
};

int main(int argc, char** argv) {
  Integer::twoInOneOut sumOfTwo = Integer::twoInOneOut_F::make(&Integer::simpleSum);
  std::cout << sumOfTwo->call(5, 6) << std::endl;//11
  //
  Integer seven(7);
  Integer::oneInOneOut sevenPlus = Integer::oneInOneOut_F::make<Integer>(&seven, &Integer::plus);
  std::cout << sevenPlus->call(12) << std::endl;//19
  //
  Integer::twoInOneOut seventeenPlus = Integer::twoInOneOut_F::make<Integer, int>(&seven, 10, &Integer::plus);//provide the 1st arg here, and the other 2 when called
  std::cout << seventeenPlus->call(52, 48) << std::endl;//117
}

While writing this up, I ran into libstdc++ known bug #71096 which breaks std::get when >1 argument is given at callback construction time. This bug has been marked fix in gcc 11, which has unfortunately not made it into the ubuntu repo at this time (apt says I'm up to date with 9.3.0)