更好的方法来选择合适的功能？

Question

我正在写一个contains()实用功能,并提出了这个问题.我的问题是:有没有更好的方法来选择正确的函数来处理呼叫？

template <class Container>
inline auto contains(Container const& c,
  typename Container::key_type const& key, int) noexcept(
    noexcept(c.end(), c.find(key))) ->
  decltype(c.find(key), true)
{
  return c.end() != c.find(key);
}

template <class Container>
inline auto contains(Container const& c,
  typename Container::value_type const& key, long) noexcept(
    noexcept(c.end(), ::std::find(c.begin(), c.end(), key))
)
{
  auto const cend(c.cend());

  return cend != ::std::find(c.cbegin(), cend, key);
}

template <class Container, typename T>
inline auto contains(Container const& c, T const& key) noexcept(
  noexcept(contains(c, key, 0))
)
{
  return contains(c, key, 0);
}

Answer 1

namespace details {
  template<template<class...>class Z, class, class...Ts>
  struct can_apply:std::false_type{};
  template<template<class...>class Z, class...Ts>
  struct can_apply<Z,std::void_t<Z<Ts...>>,Ts...>:std::true_type{};
};
template<template<class...>class Z, class...Ts>
using can_apply=typename details::can_apply<Z,void,Ts...>::type;

这需要一个模板和参数,并告诉您是否可以应用它.

template<class T, class...Args>
using dot_find_r = decltype(std::declval<T>().find(std::declval<Args>()...));

template<class T, class...Args>
constexpr can_apply<dot_find_r, T, Args...> can_dot_find{};

我们现在标记调度myfind:

template<class C>
using iterator = decltype( ::std::begin(std::declval<C>()) );

namespace details {
  template<class Container, class Key>
  iterator<Container const&> myfind(
    std::false_type can_dot_find,
    Container const& c,
    Key const& key
  )
  noexcept(
    noexcept( ::std::find(::std::begin(c), ::std::end(c), key) )
  )
  {
    return ::std::find( ::std::begin(c), ::std::end(c), key );
  }

  template <class Container, class Key>
  iterator<Container const&> myfind(
    std::true_type can_dot_find,
    Container const& c,
    Key const& key
  ) noexcept(
    noexcept( c.find(key) )
  )
  {
    return c.find(key);
  }
}
template<class Container, class Key>
iterator<Container const&> myfind(
  Container const& c,
  Key const& k
) noexcept (
  details::myfind( can_dot_find<Container const&, Key const&>, c, k )
)
{
  return details::myfind( can_dot_find<Container const&, Key const&>, c, k );
}
template<class Container, class Key>
bool contains(
  Container const& c,
  Key const& k
) noexcept (
  noexcept( ::std::end(c), myfind( c, k ) )
)
{
  return myfind(c, k) != ::std::end(c);
}

作为奖励,上述版本适用于原始C样式数组.

我要做的下一个改进是自动ADL,std::begin以使begin扩展在非dot_find大小写中起作用.

我个人的等价物返回std::optional<iterator>适当的类型.这两者都提供了一个快速的"就在那里",并且可以轻松访问迭代器(如果不是那样).

if (auto oit = search_for( container, key )) {
  // use *oit here as the iterator to the element, guaranteed not to be `end`
}

要么

if (search_for( container, key )) {
  // key was there
}

但那不是在这里,也不在那里.

Answer 2

为了记录,你可以写:

#include "magic.h"

template <typename T, typename... Us>
using has_find = decltype(std::declval<T>().find(std::declval<Us>()...));

template <class Container, typename T>
auto contains(const Container& c, const T& key)
{
    return static_if<detect<has_find, decltype(c), decltype(key)>{}>
    (
        [&] (auto& cont) { return cont.end() != cont.find(key); },
        [&] (auto& cont) { return cont.end() != std::find(cont.begin(), cont.end(), key); }
    )(c);
}

其中magic.h包括:

#include <type_traits>

template <bool> struct tag {};

template <typename T, typename F>
auto static_if(tag<true>, T t, F f) { return t; }

template <typename T, typename F>
auto static_if(tag<false>, T t, F f) { return f; }

template <bool B, typename T, typename F>
auto static_if(T t, F f) { return static_if(tag<B>{}, t, f); }

template <bool B, typename T>
auto static_if(T t) { return static_if(tag<B>{}, t, [](auto&&...){}); }

template <typename...>
using void_t = void;

template <typename AlwaysVoid, template <typename...> class Operation, typename... Args>
struct detect_impl : std::false_type {};

template <template <typename...> class Operation, typename... Args>
struct detect_impl<void_t<Operation<Args...>>, Operation, Args...> : std::true_type {};

template <template <typename...> class Operation, typename... Args>
using detect = detect_impl<void, Operation, Args...>;

DEMO