问题描述
鉴于以下模板化函数,如何更改它以利用可变参数模板?也就是说,要用可变参数代替std :: bind占位符而不是P1和P2?目前,我对每个Arity拥有这些功能之一,其中Arity零没有P参数,直到Arity 9具有P1到P9参数。我希望尽可能将其折叠为一个函数。
Given the following templated function, how can I change it to take advantage of variadic templates? That is to say, to replace std::bind placeholders with a variadic parameter instead of P1 and P2? At the moment I have one of these functions per arity, with arity zero having no P parameter, up to arity 9 having P1 to P9 parameters. I was hoping to collapse this into a single function if possible.
template<typename R, typename T, typename U, typename P1, typename P2>
void Attach(R (T::*f)(P1, P2), U p)
{
AttachInternal(p, std::bind(f,
p.get(),
std::placeholders::_1,
std::placeholders::_2));
}
推荐答案
您可以(部分)专业化 std :: is_placeholder
用于自定义模板的特殊化。这样,您可以通过常用的 int_sequence
技术引入占位符生成器。
You can (partially) specialize std::is_placeholder
for specializations of a custom template. This way, you can introduce a placeholder generator via the usual int_sequence
technique.
来自[func.bind.isplace ] / 2
From [func.bind.isplace]/2
通常的 int_sequence
:
#include <cstddef>
template<int...> struct int_sequence {};
template<int N, int... Is> struct make_int_sequence
: make_int_sequence<N-1, N-1, Is...> {};
template<int... Is> struct make_int_sequence<0, Is...>
: int_sequence<Is...> {};
自定义占位符模板和 is_placeholder
的特化:
The custom placeholder template and specialization of is_placeholder
:
template<int> // begin with 0 here!
struct placeholder_template
{};
#include <functional>
#include <type_traits>
namespace std
{
template<int N>
struct is_placeholder< placeholder_template<N> >
: integral_constant<int, N+1> // the one is important
{};
}
我不确定在哪里引入 1
;
I'm not sure where to introduce the 1
; the places I considered are all not optimal.
使用它编写一些活页夹:
Using it to write some binder:
template<class Ret, class... Args, int... Is>
void my_bind(Ret (*p)(Args...), int_sequence<Is...>)
{
auto x = std::bind(p, placeholder_template<Is>{}...);
x( Args(42)... );
}
template<class Ret, class... Args>
void my_bind(Ret (*p)(Args...))
{
my_bind(p, make_int_sequence< sizeof...(Args) >{});
}
活页夹的用法示例:
#include <iostream>
void foo(double, char, int) { std::cout << __PRETTY_FUNCTION__ << "\n"; }
void bar(bool, short) { std::cout << __PRETTY_FUNCTION__ << "\n"; }
int main()
{
my_bind(foo);
my_bind(bar);
}
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