C++ function call wrapper with function as template argument

Question

I'm trying to create a generic wrapper function that takes a function as a template argument and takes the same arguments as that function as its arguments. For example:

template <typename F, F func>
/* return type of F */ wrapper(Ts... Args /* not sure how to get Ts*/)
{
    // do stuff
    auto ret = F(std::forward<Ts>(args)...);
    // do some other stuff
    return ret;
}

The solution needs to be castable to a function pointer with the same type as func so that I can pass it to a C api. In other words, the solution needs to be a function and not a function object. Most importantly, I need to be able to do work in the wrapper function.

If the inline comments aren't clear, I'd like to be able to do something like the following:

struct c_api_interface {
    int (*func_a)(int, int);
    int (*func_b)(char, char, char);
};

int foo(int a, int b)
{
    return a + b;
}

int bar(char a, char b, char c)
{
    return a + b * c;
}

c_api_interface my_interface;
my_interface.func_a = wrapper<foo>;
my_interface.func_b = wrapper<bar>;

I looked for related posts and found these, but none of them are quite what I'm trying to do. Most of these posts concern function objects. Is what I'm trying to do even possible?

Function passed as template argument

Function wrapper via (function object) class (variadic) template

How does wrapping a function pointer and function object work in generic code?

How do I get the argument types of a function pointer in a variadic template class?

Generic functor for functions with any argument list

C++ Functors - and their uses

In response to the first 2 responses, I edited the question to make it clear that I need to be able to do work in the wrapper function (i.e. modify some global state before and after the call to the wrapped function)

Answer 1

template<class F, F f> struct wrapper_impl;
template<class R, class... Args, R(*f)(Args...)>
struct wrapper_impl<R(*)(Args...), f> {
    static R wrap(Args... args) {
        // stuff
        return f(args...);
    }
};

template<class F, F f>
constexpr auto wrapper = wrapper_impl<F, f>::wrap;

Use as wrapper<decltype(&foo), foo>.

Answer 2

#include <utility>
#include <iostream>

struct c_api_interface { int (*func_a)(int, int); int (*func_b)(char, char, char); };
int foo(int a, int b) { return a + b; }
int bar(char a, char b, char c) { return a + b * c; }


template<typename Fn, Fn fn, typename... Args>
typename std::result_of<Fn(Args...)>::type
wrapper(Args... args) {
   std::cout << "and ....it's a wrap ";
   return fn(std::forward<Args>(args)...);
}
#define WRAPIT(FUNC) wrapper<decltype(&FUNC), &FUNC>

int main() {
  c_api_interface my_interface;
  my_interface.func_a = WRAPIT(foo);
  my_interface.func_b = WRAPIT(bar);

  std:: cout << my_interface.func_a(1,1) << std::endl;
  std:: cout << my_interface.func_b('a','b', 1) << std::endl;

  return 0;
}

see http://rextester.com/ZZD18334

Answer 3

you may try something like that (Ugly, but works)

#include <iostream>
#include <functional>

struct wrapper_ctx
{
  wrapper_ctx ()
  {
    std::cout << "Before" << std::endl;
  }
  ~wrapper_ctx ()
  {
    std::cout << "after" << std::endl;
  }
};

template <typename F, typename... Args>
auto executor (F&& f, Args&&... args) -> typename std::result_of<F(Args...)>::type
{
  wrapper_ctx ctx;
  return std::forward<F>(f)( std::forward<Args>(args)...);

}

template <typename F>
class wrapper_helper;


template<typename Ret, typename... Args>
class wrapper_helper <std::function<Ret(Args...)>>
{
  std::function<Ret(Args...)> m_f;
public:
  wrapper_helper( std::function<Ret(Args...)> f ) 
      : m_f(f) {}
  Ret operator()(Args... args) const 
  { 
    return executor (m_f, args...); 
  }
};

template <typename T>
wrapper_helper<T> wrapper (T f)
{
  return wrapper_helper <T>(f);
}


int sum(int x, int y)
{
  return x + y;
}


int main (int argc, char* argv [])
{

  std::function<int(int, int)> f = sum;
  auto w = wrapper (f);

  std::cout << "Executing the wrapper" << std::endl;
  int z = w(3, 4);

  std::cout << "z = " << z << std::endl;
}

Answer 4

you probably need something like

template <typename F>
class Wrapper {
public:
    Wrapper(F *func) : function(func) {}
    operator F* () { return function; }
    F *function;
};

Which you can use like void (*funcPtr)(int) = Wrapper<void(int)>(&someFunction);

Answer 5

I think that will be the concise way to do what you want:

template <typename F>
F* wrapper(F* pFunc)
{
    return pFunc;
}

and use it like this:

my_interface.func_a = wrapper(foo);
my_interface.func_a(1, 3);

Answer 6

You may try this

template <class R, class... Args>
struct wrap
{
    using funct_type = R(*)(Args...);
    funct_type func;
    wrap(funct_type f): func(f) {};
    R operator()(Args&&... args)
    {
          //before code block
          std::cout << "before calling\n";
          R ret=func(std::forward<Args>(args)...);
          //after code block
          std::cout << "After calling\n";
    }
};

use like this for example:

int somefunc(double &f, int x);
auto wrapped_somefunc=wrap{somefunc};
double f=1.0;
int x = 2;
auto result=wrapped_somefunc(f,x);

Answer 7

This one is for c++17 and newer uses auto template parameters:

template <auto func, class... Args>
auto wrap_func(Args... args)
{
    std::cout << "before calling wrapped func\n";
    auto ret = func(args...);
    std::cout << "after calling wrapped func\n";
    return ret;
}

use for example:

int some_func(int a, int b);
auto ret = wrap_func<some_func>(2, 3);