为“make_function"推断 lambda 或任意可调用的调用签名;
在某些情况下,希望能够对可调用对象进行类型擦除(例如,函数、函数指针、带有 operator()
、lambda、mem_fn
的对象实例),例如在 使用带有 C++11 lambdas 的 Boost 适配器 中,其中复制-需要可赋值和默认构造的类型.
In some situations it's desirable to be able to type-erase a callable (e.g. function, function pointer, object instance with operator()
, lambda, mem_fn
), for instance in Using Boost adaptors with C++11 lambdas where a copy-assignable and default-constructible type is required.
std::function
是理想的,但似乎没有办法自动确定实例化类模板的签名std::function
与.有没有一种简单的方法来获取任意可调用的函数签名和/或将其包装在适当的 std::function
实例化实例(即 make_function
函数模板)中?
std::function
would be ideal, but there seems to be no way to automatically determine what signature to instantiate the class template std::function
with. Is there an easy way to get the function signature of an arbitrary callable and/or wrap it in an appropriate std::function
instantiation instance (i.e. a make_function
function template)?
具体来说,我正在寻找其中之一
Specifically, I'm looking for one or other of
template<typename F> using get_signature = ...;
template<typename F> std::function<get_signature<F>> make_function(F &&f) { ... }
这样 make_function([](int i) { return 0; })
返回一个 std::function
.显然,如果一个实例可以用多个签名调用(例如,具有多个、模板或默认参数 operator()
s 的对象),这将不会起作用.
such that make_function([](int i) { return 0; })
returns a std::function<int(int)>
. Obviously this wouldn't be expected to work if an instance is callable with more than one signature (e.g. objects with more than one, template or default-parameter operator()
s).
Boost 很好,虽然不是过于复杂的非 Boost 解决方案是首选.
Boost is fine, although non-Boost solutions that aren't excessively complex are preferred.
回答我自己的问题.
推荐答案
我想出了一个相当讨厌的非库解决方案,利用 lambdas 具有 operator()
的事实:>
I've come up with a fairly nasty non-library solution, using the fact that lambdas have operator()
:
template<typename T> struct remove_class { };
template<typename C, typename R, typename... A>
struct remove_class<R(C::*)(A...)> { using type = R(A...); };
template<typename C, typename R, typename... A>
struct remove_class<R(C::*)(A...) const> { using type = R(A...); };
template<typename C, typename R, typename... A>
struct remove_class<R(C::*)(A...) volatile> { using type = R(A...); };
template<typename C, typename R, typename... A>
struct remove_class<R(C::*)(A...) const volatile> { using type = R(A...); };
template<typename T>
struct get_signature_impl { using type = typename remove_class<
decltype(&std::remove_reference<T>::type::operator())>::type; };
template<typename R, typename... A>
struct get_signature_impl<R(A...)> { using type = R(A...); };
template<typename R, typename... A>
struct get_signature_impl<R(&)(A...)> { using type = R(A...); };
template<typename R, typename... A>
struct get_signature_impl<R(*)(A...)> { using type = R(A...); };
template<typename T> using get_signature = typename get_signature_impl<T>::type;
template<typename F> using make_function_type = std::function<get_signature<F>>;
template<typename F> make_function_type<F> make_function(F &&f) {
return make_function_type<F>(std::forward<F>(f)); }
有什么可以简化或改进的想法吗?有什么明显的错误吗?
Any ideas where this can be simplified or improved? Any obvious bugs?
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