Pass variadic std::function

Question

I have a function that accepts a callback. It should work with function ptrs, lambdas (stateless & with state), etc. I could just do the following:

template<typename t_func>
void add_command(const std::string& name, t_func func)

The problem is I need to work with the argument types of func. So I did this:

template<typename... t_args>
void add_command(const std::string& name, const std::function<void(t_args...)>& args)

This creates the following error: no matching function for call to...

note: template argument deduction/substitution failed

Is there any way to pass a generic function type while still having access to it's arguments? I know of std::result_of, is there a similar std::arguments_of?

Answer 1

std::function is a type erasure template. Type deduction is the opposite (almost the inverse) of type erasure.

Type deducing a type erasure template is code smell. And it rarely works.

In c++17 there is a deduction guide, so you can do:

template<typename... t_args>
void add_command(const std::string& name, const std::function<void(t_args...)>& args)        
void add_command(const std::string& name, t_func const& func) {
  std::function f = func;
  add_command(name, f);
}

it is imperfect, but a perfect solution isn't possible.

The deduction guides look like:

template<class R, class... ArgTypes>
function(R(*)(ArgTypes...)) -> function<R(ArgTypes...)>;
template<class F>
function(F) -> function</*see below*/>;

where it extracts the signature of the function via examining &F::operator(). This can fail with overloads or templates. And naturally this doesn't work with function names that are overloaded.

You can replicate this in c++11 with a function traits class:

template<class X>
struct function_traits:function_traits<decltype(&X::operator())> {};
#define MEM_FUN_HELPER2(...) \
  template<class R, class T, class...Args> \
  struct function_traits<R(T::*)(Args...) __VA_ARGS__>:function_traits<R(Args...)>{}; \
  template<class R, class T, class...Args> \
  struct function_traits<R(T::*)(Args..., ...) __VA_ARGS__>:function_traits<R(Args..., ...)>{}; \
  template<class R, class T, class...Args> \
  struct function_traits<R(T::*)(Args...) __VA_ARGS__ noexcept>:function_traits<R(Args...) noexcept>{}; \
  template<class R, class T, class...Args> \
  struct function_traits<R(T::*)(Args..., ...) __VA_ARGS__  noexcept>:function_traits<R(Args..., ...) noexcept>{}
#define MEM_FUN_HELPER1(...) \
  MEM_FUN_HELPER2(__VA_ARGS__); \
  MEM_FUN_HELPER2(__VA_ARGS__ &); \
  MEM_FUN_HELPER2(__VA_ARGS__ &&)
#define MEM_FUN_HELPER0(...) \
  MEM_FUN_HELPER1(__VA_ARGS__); \
  MEM_FUN_HELPER1(const __VA_ARGS__)
#define MEM_FUN_HELPER() \
  MEM_FUN_HELPER0(); \
  MEM_FUN_HELPER0(volatile)

MEM_FUN_HELPER();

template<class R, class...Args>
struct function_traits<R(*)(Args...)>:function_traits<R(Args...)>{};
template<class R, class...Args>
struct function_traits<R(*)(Args..., ...)>:function_traits<R(Args..., ...)>{};
template<class R, class...Args>
struct function_traits<R(*)(Args...) noexcept>:function_traits<R(Args...) noexcept>{};
template<class R, class...Args>
struct function_traits<R(*)(Args..., ...) noexcept>:function_traits<R(Args..., ...) noexcept>{};
template<class R, class...Args>
struct function_traits<R(Args...) noexcept> : function_traits<R(Args...)> {
  enum {is_noexcept=true};
};
template<class R, class...Args>
struct function_traits<R(Args..., ...) noexcept> : function_traits<R(Args..., ...)> {
  enum {is_noexcept=true};
};
template<class R, class...Args>
struct function_traits<R(Args...)> {
  template<template<class...>class Z>
  using transcribe=Z<R(Args...)>;
  using std_function = transcribe<std::function>;
  using result_type = R;
  using arg_tuple = std::tuple<Args...>;
  enum{is_noexcept=false};
};
template<class R, class...Args>
struct function_traits<R(Args..., ...)> {
  template<template<class...>class Z>
  using transcribe=Z<R(Args..., ...)>;
  using std_function = transcribe<std::function>;
  using result_type = R;
  // doesn't really work, but what ya gonna do:
  using arg_tuple = std::tuple<Args...>;
  enum{is_noexcept=false};
};

which is pretty crazy; MEM_FUN_HELPER(); expands into 48 template specializations. 3 ref qualifiers (&, && and nothing), then 4 other things (const, volatile, noexcept and ... C-style varargs) that have to be handled "manually".

In any case, once you have that, you can do:

template<typename... t_args>
void add_command(const std::string& name, const std::function<void(t_args...)>& args)        
template<class t_func>
void add_command(const std::string& name, t_func const& func) {
  typename function_traits<t_func>::std_function f = func;
  add_command(name, f);
}

in c++11. This (roughly and imperfectly) does the same as the c++17 deduction guides.

Live example.