简单Contains()函数

Question

我编写了简单的泛型函数来检查任何STL容器是否有一个元素。好像挺好的。我已经用VC++，GCC和Clang编译器测试了它。它工作得很完美。

我怎样才能进一步改进。

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

// Detect if container has C::find
template <typename C> decltype(std::declval<C>().find(0), std::true_type{}) test(int);
template <typename C> std::false_type test(...);
template <typename C> using has_find = decltype(test<C>(0));

// Based on std::conditional
template <bool B, typename T, typename F>
struct conditional_impl
{
    constexpr static F pick(T&& , F&& f)
    {
        return std::forward<F>(f);
    }
};

template <typename T, typename F>
struct conditional_impl<true, T, F>
{
    constexpr static T pick(T&& t, F&&)
    {
        return std::forward<T>(t);
    }
};

template <bool B, typename T, typename F>
auto conditional(T&& t, F&& f)
{
    return conditional_impl<B,T,F>::pick(std::forward<T>(t), std::forward<F>(f));
}

// one for all and all for one
template <typename C, typename T>
auto contains(const C& container, const T& key)
{
    static auto first = [] (auto&& c, auto&& k)
    {
        return c.end() != c.find(k);
    };

    static auto second = [] (auto&& c, auto&& k)
    {
        return c.end() != std::find(c.begin(), c.end(), k);
    };

    auto op = conditional<has_find<C>::value>(first, second);

    return op(container, 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) << "\n\n";

    std::vector<int> v = { 1, 2, 3 };
    std::cout << contains(v, 0) << "\n";
    std::cout << contains(v, 1) << "\n\n";

    std::set<int> s = { 1, 2, 3 };
    std::cout << contains(s, 0) << "\n";
    std::cout << contains(s, 1) << "\n\n";

    std::map<int, int> m = { { 1, 1}, { 2, 2}, { 3, 3} };
    std::cout << contains(m, 0) << "\n";
    std::cout << contains(m, 1) << "\n";
}

Answer 1

不错的玩具！我花了一段时间才意识到这件事。看上去没问题。我唯一想到的就是为什么不使用std::enable_if，因为它要简单得多。

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

// Detect if container has C::find
template <typename C> decltype(std::declval<C>().find(0), std::true_type{}) test(int);
template <typename C> std::false_type test(...);
template <typename C> using has_find = decltype(test<C>(0));

template <typename C, typename T>
typename std::enable_if<has_find<C>::value, bool>::type contains(const C& container, const T& key) {
    return c.end() != c.find(k);
}

template <typename C, typename T>
typename std::enable_if<!has_find<C>::value, bool>::type contains(const C& container, const T& key) {
    return c.end() != std::find(c.begin(), c.end(), k);
}

int main()
{
    /* */
}