可變參數函數包裝器，用於任何返回類型

Question

我正在嘗試為要傳遞給它的任何函數編寫可變參數模板包裝，並且（可能）要返回其返回值。

現在，我想出了下面的代碼，但我真的很討厭我需要6種不同的函數重載，其中3種用於void返回類型，而3種用於非void返回類型。

// Void return type
// Template for non-member functions
template <typename Callable, typename... Args>
std::enable_if_t<std::is_void<std::result_of_t<Callable(Args...)> >::value
    && std::is_member_function_pointer<Callable>::value == false>
call(Callable worker, Args... params)
{
    worker(std::forward<Args>(params)...);
}

// Template for member functions (general)
template <typename Callable, typename... Args>
std::enable_if_t<std::is_void<std::result_of_t<Callable(Args...)> >::value
    && std::is_member_function_pointer<Callable>::value>
call(Callable worker, Args... params)
{
    call(std::forward<Callable>(worker), std::forward<Args>(params)...);
}

// Template for member functions (object instance extractor)
template <typename Callable, typename Object, typename... Args>
std::enable_if_t<std::is_void<std::result_of_t<Callable(Args...)> >::value
    && std::is_member_function_pointer<Callable>::value>
call(Callable worker, Object object, Args... params)
{
    auto fn = std::mem_fn(worker);
    fn(std::forward<Object>(object), std::forward<Args>(params)...);
}

// Non-void return types
// Template for non-member functions
template <typename Callable, typename... Args>
std::enable_if_t<std::is_void<std::result_of_t<Callable(Args...)> >::value == false
    && std::is_member_function_pointer<Callable>::value == false
    , std::result_of_t<Callable(Args...)> >
call(Callable worker, Args... params)
{
    return worker(std::forward<Args>(params)...);
}

// Template for member functions (general)
template <typename Callable, typename... Args>
std::enable_if_t<std::is_void<std::result_of_t<Callable(Args...)> >::value == false
    && std::is_member_function_pointer<Callable>::value
    , std::result_of_t<Callable(Args...)> >
call(Callable worker, Args... params)
{
    return call(std::forward<Callable>(worker), std::forward<Args>(params)...);
}

// Template for member functions (object instance extractor)
template <typename Callable, typename Object, typename... Args>
std::enable_if_t<std::is_void<std::result_of_t<Callable(Args...)> >::value == false
    && std::is_member_function_pointer<Callable>::value
    , std::result_of_t<Callable(Args...)> >
call(Callable worker, Object object, Args... params)
{
    auto fn = std::mem_fn(worker);
    return fn(std::forward<Object>(object), std::forward<Args>(params)...);
}

有什么辦法可以消除其中的一些重載？ 事實證明，這種方法可以很好地工作，但是從代碼方面來說，我真的希望更短一些。

Answer 1

這是SFINAE的較短版本（如果不需要，可以用decltype(auto)代替）。

template <typename R, typename T, typename... Args, typename U,
          typename... Params>
auto call(R (T::*arg)(Args...), U &&first, Params &&... params)
    -> decltype((std::forward<T>(first).*
                 arg)(std::forward<Params>(params)...)) {
  return (std::forward<T>(first).*arg)(std::forward<Params>(params)...);
}

template <typename F, typename... Ts>
auto call(F &&f, Ts &&... args)
    -> decltype(std::forward<F>(f)(std::forward<Ts>(args)...)) {
  return std::forward<F>(f)(std::forward<Ts>(args)...);
}