Using function pointer to change function signatures

[Ryancit2]

Hey guys, i have a pointer-to-function variable which is supposed to store 'any type of function' with 'any amount of parameters'. I think this is possible so here is what i've tried so far;

#include <iostream>
using namespace std;

void Output(int num = 0) {
	cout << num << endl;
}

class A {
private:
	void (*funcptr)(void);
public:
	template<typename ... Args>
	void set(void func(Args... temp), Args... args) {
		funcptr = new ((void) func(args...));
	}
	void invoke() {
		funcptr(5); // should send the 'args...' but thats a question for later
	}
};

void main() {
	A firstfunc;
	firstfunc.set(Output, 4);
	firstfunc.invoke();
	system("pause");
}

The code is obviously not working but it may help you understand what exactly i am going for in this. The question, in simpler words, is How do i store a function with any number of parameters, to a variable, with which i can call that function later?.

[Peter87]

Instead of storing a function pointer you could use std::function to store a lambda that captures the function and all the arguments and calls the function when invoked.

#include <iostream>
#include <functional>

using namespace std;

void Output(int num = 0) {
	cout << num << endl;
}

class A {
private:
	std::function<void()> fun;
public:
	template<typename ... Args>
	void set(void func(Args... temp), Args... args) {
		fun = [=](){ func(args...); };
	}
	void invoke() {
		fun();
	}
};

int main() {
	A firstfunc;
	firstfunc.set(Output, 4);
	firstfunc.invoke();
}

[Ryancit2]

Wow, thanks @Peter87, that worked like a charm. Just a small complication, still, i want to set() member function to take 'any' type of parameters, for some reason, it does not work for char * parameter;

#include <iostream>
#include <functional>
using namespace std;

class A {
private:
	std::function<void()> fun;
public:
	template<typename ... Args>
	void set(void func(Args... temp), Args... args) {
		fun = [=]() { func(args...); };
	}
	void invoke() {
		fun();
	}
	void test()
	{
		cout << "Passed! " << endl;
	}
};

void Output(int num = 0) {
	cout << num << endl;
}

void names(char *name)
{
	cout << name << endl;
}

void objects(A obj)
{
	obj.test();
}

int main() {
	A firstfunc, secfunc, thirdfunc;
	firstfunc.set(Output, 6); // works
	firstfunc.invoke(); // works
	secfunc.set(objects, firstfunc); // works
	secfunc.invoke(); // works
	thirdfunc.set(names, "Ryan"); // does not work
	thirdfunc.invoke(); // does not work
	system("pause");
}

Actually, i checked int, custom class, float and some other parameters, it works in all of them, but not sure why it does not work for char *, and maybe there is some limitation to this. If so, please let me know.

Thanks once again.

[Peter87]

The characters of a string literal are const (i.e. they cannot be modified) so the parameter of your names function should be a pointer to a const char.

void names(const char *name)
{
	cout << name << endl;
}

[Peter87]

Note that the way the set function has been written the parameter types must match the types of arguments exactly. This will prevent you from passing an int to a function that takes a double, or a string literal to function that takes a std::string. To fix this you can add another template argument for the function type so that it becomes independent of the variadic argument list.

	template<typename F, typename ... Args>
	void set(F func, Args... args) {
		fun = [=]() { func(args...); };
	}

[Ryancit2]

Thanks a lot Peter87, that information is pretty darn good. Solved.

[Ryancit2]

Oh wait, how do i provide default values for following parameters?

template<typename ... Args>
	void set(void func(Args... temp), Args... args) {
		fun = [=]() { func(args...); };
	}

And in my actual code, i can not use those 2 parameters, the compilation stops with error;

Error LNK2019 unresolved external symbol "public: __thiscall Rects::Rects<int>(unsigned int,unsigned int,unsigned int,unsigned int,char *,void (__cdecl*)(int),int)" (??$?0H@Rects@@QAE@IIIIPADP6AXH@ZH@Z) referenced in function _main Project11 ...path\Project11\Project11\Source.obj 1

Here is my actual constructor;

public:
	template<typename ... Args>
	Rects(unsigned int xPos, unsigned int yPos, unsigned int width, unsigned int height, char *text, void BoundTo(Args...), Args... args);

// Definition;
template<typename ... Args>
Rects::Rects(unsigned int xPos, unsigned int yPos, unsigned int width, unsigned int height, char *text, void BoundTo(Args...), Args... args)
{
        /* ... */
	func = [=]() { BoundTo(args...); };
        /* ... */
}

[Peter87]

Making the function type a template argument as I showed in my last post should fix this problem as well.

[Ryancit2]

Alright i figured that out. Moreover, i was using typename template in .cpp (function definitions) file which was causing linker error. Fixed that as well.

Thanks for the help once again Peter!