how to invoke function in c++ from function pointer and arguments

Question

I want to call function by function pointer that have unknown arguments. store input arguments and function and run it later. some thing like call_user_func_array in php

for example:

// example function definition
void foo(int arg1, const char *arg2,const char *arg3){
    //some other code
}
void bar(int arg1, int arg2){
    //some other code
}

// function definition
void store_function_call(int param,void *function,... args){
    // store param , function, and other arguments
    // in php save to global variable for use later
}
void call_later(){
    // execute stored param, function, arguments
    // in PHP use call_user_func_array
}
// use:
int main(){
    store_function_call(10,bar,4,5);
    // store_function_call(5,foo,5,"yyy","sss");

    call_later();
}

You probably want to use Boost::bind.

n. m. could be an AI
– n. m. could be an AI

2013-02-10 09:42:12 +00:00
Commented Feb 10, 2013 at 9:42 — n. m. could be an AI
– n. m. could be an AI, Commented Feb 10, 2013 at 9:42

Community · Accepted Answer · 2017-05-23 12:27:02Z

2

You can do what you're trying to do in C++11 with a little bit of template metaprogramming:

#include <tuple>
#include <iostream>

template<int ...> struct seq {};
template<int N, int ...S> struct gens : gens<N-1, N-1, S...> {};
template<int ...S> struct gens<0, S...>{ typedef seq<S...> type; };

double foo(int x, float y, double z) {
  return x + y + z;
}

template <typename R, typename ...Args>
struct save_it_for_later {
  std::tuple<Args...> params;
  R (*func)(Args...);

  R call_later() {
    return callFunc(typename gens<sizeof...(Args)>::type());
  }

  template<int ...S>
  R callFunc(seq<S...>) {
    return func(std::get<S>(params) ...);
  }
};

template <typename R, typename ...Args>
save_it_for_later<R, Args...> store_function_call(R (*func)(Args...), Args&& ...args) {
  return save_it_for_later<R, Args...>{std::tuple<Args...>(std::forward<Args>(args)...), func};
}

int main() {
  auto saved = store_function_call(foo,1,1.f,1.);
  std::cout << saved.call_later() << "\n";
}

This answer is modified to match your scenario from this answer to a similar question I asked. I've added return type deduction and a helper to deduce the types as your store_function_call.

(I would have used std::forward_as_tuple instead of the clunky tuple constructor+forward construct, but the compiler I tested on doesn't have that)

edited May 23, 2017 at 12:27

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answered Feb 10, 2013 at 9:57

Flexo - Save the data dump♦

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3 Comments

Flexo - Save the data dump Over a year ago

@seyed You will need to enable C++11 on your compiler. For gcc -std=c++0x is sufficient.

seyed Over a year ago

how i can store array (or vector) of return values of store_function_call?

Flexo - Save the data dump Over a year ago

@seyed - you need some type erasure. std::function in C++11 gives you exactly that in a handy form, e.g. std::vector<std::function<void ()>> stored_functions; stored_functions.push_back(store_function_call(foo, 1));

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