选择正确功能的更好方法

对于记录，您可以写：

#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包含：

#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...>;

演示

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，告诉你是否可以应用它

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{};

我们现在在myfind:上标记调度

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);
}

另外，上面的版本可以使用原始的C风格数组。

我要做的下一个增强将是自动ADLstd::begin，使begin扩展在非dot_find的情况下工作。

我的个人等价物返回适当类型的std::optional<iterator>。这既提供了一个快速的"是否存在"，也提供了对迭代器的轻松访问(如果不存在的话)。

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

或

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

但这既不在这里也不在那里。

因此，如果可能，您希望调用c.find，否则调用std::find。但也要注意std::set中的类型歧义。

以下是解决这个问题的代码(为了可读性，去掉了冗长和微观优化)：

#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
#include <type_traits>
#include <set>
#include <cstdarg>
using namespace std;
template <typename T, typename Ret>
struct dummy {
typedef Ret type;
};
template <class Container>
auto contains(const Container &c, typename Container::key_type const &key) ->
typename dummy<decltype(c.find(key)), bool>::type {
cout << "c.find" << endl;
return c.end() != c.find(key);
}
template <class Container, typename ...T>
typename std::enable_if<sizeof...(T)==1, bool>::type contains(const Container &c, const T&... args) {
typename Container::value_type const &val = std::get<0>(std::tuple<const T&...>(args...));
cout << "std::find" << endl;
return c.cend() != find(c.cbegin(), c.cend(), val);
}
int main() {
vector<int> v = {1,2,3};
cout << contains(v,4) << contains(v,2) << endl;
map<int, int> m;
m[1] = 1;
m[2] = 2;
m[3] = 3;
cout << contains(m,4) << contains(m,2) << endl;
set<int> s;
cout << contains(s,4) << contains(s,2) << endl;
return 0;
}

我做了什么：

• 我使依赖于c.find()的第一个contains函数是可调用的。如果没有，编译器就看不到函数，也不会出现任何问题
• 当key_type和value_type相同时，我通过引入使用std::find及其第二个强制参数作为变元模板的函数来解决歧义。我还强制使用了大小为1的可变模板

如果您只是假设key_type存在意味着container.find与OP中一样存在，那么您可以简化代码并删除dummy结构：

template <class Container>
bool contains(const Container &c, typename Container::key_type const &key)
{
return c.end() != c.find(key);
}
template <class Container, typename ...T>
typename std::enable_if<sizeof...(T)==1, bool>::type contains(const Container &c, const T&... args) {
typename Container::value_type const &val = std::get<0>(std::tuple<const T&...>(args...));
return c.cend() != find(c.cbegin(), c.cend(), val);
}

如果Container::find确实存在，则可以完全禁用第二个函数，而不必以这种方式解决歧义。这个和那个答案都显示了不同的方法。然后使用std::enable_if<! (Does Container have the find method?) , bool>::type作为第二个函数的返回类型就可以了。

由于find存在于关联容器中，因此可以显式地将它们设为true类型。

元函数：

template <class> struct has_find_impl:std::false_type{};
template <class T, class... Args> struct has_find_impl<std::set<T, Args...>>:std::true_type{};
template <class T, class... Args> struct has_find_impl<std::map<T, Args...>>:std::true_type{};
template <class T, class... Args> struct has_find_impl<std::multiset<T, Args...>>:std::true_type{};
template <class T, class... Args> struct has_find_impl<std::multimap<T, Args...>>:std::true_type{};
template <class T> using has_find = has_find_impl<typename std::decay<T>::type>;

并像这样使用：

template <class Container>
bool contains_impl(const Container& c, const typename Container::key_type& key, std::true_type)
{
return c.find(key) != c.cend();
}
template <class Container>
bool contains_impl(const Container& c, typename Container::const_reference key, std::false_type)
{
return std::find(c.cbegin(), c.cend(), key) != c.cend();
}
template <class Container, class T>
bool contains(const Container& c, const T& key)
{
return contains_impl(c, key, has_find<Container>{});
}

或与SFINAE一起使用。这里有一个完整的例子：

#include <iostream>
#include <algorithm>
#include <utility>
#include <map>
#include <set>
#include <vector>
#include <array>
#include <type_traits>

template <class> struct has_find_impl:std::false_type{};
template <class T, class... Args> struct has_find_impl<std::set<T, Args...>>:std::true_type{};
template <class T, class... Args> struct has_find_impl<std::map<T, Args...>>:std::true_type{};
template <class T, class... Args> struct has_find_impl<std::multiset<T, Args...>>:std::true_type{};
template <class T, class... Args> struct has_find_impl<std::multimap<T, Args...>>:std::true_type{};
template <class T> using has_find = has_find_impl<typename std::decay<T>::type>;

template <class Container>
typename std::enable_if<has_find<Container>::value, bool>::type
contains_impl(const Container& c, const typename Container::key_type& key)
{
return c.find(key) != c.cend();
}
template <class Container>
typename std::enable_if<!has_find<Container>::value, bool>::type
contains_impl(const Container& c, typename Container::const_reference key)
{
return std::find(c.cbegin(), c.cend(), key) != c.cend();
}
template <class Container, class T>
bool contains(const Container& c, const T& key)
{
return contains_impl(c, key);
}
int main()
{
std::cout << std::boolalpha;
std::array<int, 3> a = {{ 1, 2, 3 }};
std::cout << contains(a, 0) << "n";
std::cout << contains(a, 1) << "nn";
std::vector<int> v = { 1, 2, 3 };
std::cout << contains(v, 0) << "n";
std::cout << contains(v, 1) << "nn";
std::set<int> s = { 1, 2, 3 };
std::cout << contains(s, 0) << "n";
std::cout << contains(s, 1) << "nn";
std::map<int, int> m = { { 1, 1}, { 2, 2}, { 3, 3} };
std::cout << contains(m, 0) << "n";
std::cout << contains(m, 1) << "nn";
}