STL 容器的输出

一个可以输出各种 STL 容器的头文件

// output_container.h
/*
 * Written by Wu Yongwei <wuyongwei AT gmail DOT com>.
 *
 * Using this file requires a C++17-compliant compiler.
 *
 * This is free and unencumbered software released into the public domain.
 *
 * Anyone is free to copy, modify, publish, use, compile, sell, or
 * distribute this software, either in source code form or as a compiled
 * binary, for any purpose, commercial or non-commercial, and by any
 * means.
 *
 * In jurisdictions that recognize copyright laws, the author or authors
 * of this software dedicate any and all copyright interest in the
 * software to the public domain. We make this dedication for the benefit
 * of the public at large and to the detriment of our heirs and
 * successors. We intend this dedication to be an overt act of
 * relinquishment in perpetuity of all present and future rights to this
 * software under copyright law.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 *
 * For more information, please refer to <http://unlicense.org>
 *
 */

#ifndef OUTPUT_CONTAINER_H
#define OUTPUT_CONTAINER_H

#include <ostream>      // std::ostream
#include <type_traits>  // std::false_type/true_type/decay_t/is_same_v
#include <utility>      // std::declval/pair
#include <tuple>

// Type trait to detect std::pair
template <typename T>
struct is_pair : std::false_type {};

template <typename T, typename U>
struct is_pair<std::pair<T, U>> : std::true_type {};

template <typename T>
inline constexpr bool is_pair_v = is_pair<T>::value;

// Type trait to detect whether an output function already exists
template <typename T>
struct has_output_function {
    template <class U>
    static auto output(U* ptr)
        -> decltype(std::declval<std::ostream&>() << *ptr, std::true_type());

    template <class U>
    static std::false_type output(...);

    static constexpr bool value = decltype(output<T>(nullptr))::value;
};
template <typename T>
inline constexpr bool has_output_function_v = has_output_function<T>::value;

/* NB: Visual Studio 2017 (or below) may have problems with
 *     has_output_function_v<T>: you should then use
 *     has_output_function<T>::value instead, or upgrade to
 *     Visual Studio 2019. */

// Output function for std::pair
template <typename T, typename U>
std::ostream& operator<<(std::ostream& os, const std::pair<T, U>& pr);

// Element output function for containers that define a key_type and
// have its value type as std::pair
template <typename T, typename Cont>
auto output_element(std::ostream& os, const T& element,
                    const Cont&, std::true_type)
    -> decltype(std::declval<typename Cont::key_type>(), os);
// Element output function for other containers
template <typename T, typename Cont>
auto output_element(std::ostream& os, const T& element,
                    const Cont&, ...)
    -> decltype(os);

// Main output function, enabled only if no output function already exists
template <typename T, 
          typename = std::enable_if_t<!has_output_function_v<T>>>
auto operator<<(std::ostream& os, const T& container)
    -> decltype(std::begin(container), std::end(container), os)
{
    using std::decay_t;
    using std::is_same_v;

    using element_type = decay_t<decltype(*std::begin(container))>;
    constexpr bool is_char_v = is_same_v<element_type, char>;
    if constexpr (!is_char_v) {
        os << '{';
    }
    auto end = std::end(container);
    bool on_first_element = true;
    for (auto it = std::begin(container); it != end; ++it) {
        if constexpr (is_char_v) {
            if (*it == '\0') {
                break;
            }
        } else {
            if (!on_first_element) {
                os << ", ";
            } else {
                os << ' ';
                on_first_element = false;
            }
        }
        output_element(os, *it, container, is_pair<element_type>());
    }
    if constexpr (!is_char_v) {
        if (!on_first_element) {  // Not empty
            os << ' ';
        }
        os << '}';
    }
    return os;
}

template <typename T, typename Cont>
auto output_element(std::ostream& os, const T& element,
                    const Cont&, std::true_type)
    -> decltype(std::declval<typename Cont::key_type>(), os)
{
    os << element.first << " => " << element.second;
    return os;
}

template <typename T, typename Cont>
auto output_element(std::ostream& os, const T& element,
                    const Cont&, ...)
    -> decltype(os)
{
    os << element;
    return os;
}

template <typename T, typename U>
std::ostream& operator<<(std::ostream& os, const std::pair<T, U>& pr)
{
    os << '(' << pr.first << ", " << pr.second << ')';
    return os;
}

// output tuple
template<typename Ch, typename Tr, typename Tuple, std::size_t... Is>
void print_tuple_impl(std::basic_ostream<Ch, Tr>& os, const Tuple& t,
                      std::index_sequence<Is...>)
{
    ((os << (Is == 0 ? "" : ", ") << std::get<Is>(t)), ...);
}

template<typename Ch, typename Tr, typename... Args>
auto& operator<<(std::basic_ostream<Ch, Tr>& os, const std::tuple<Args...>& t)
{
    os << "(";
    print_tuple_impl(os, t, std::index_sequence_for<Args...>{});
    return os << ")";
}


#endif // OUTPUT_CONTAINER_H

原生数组可以使用 std::span 包装一下后使用

#include "output_container.h"
#include <iostream>
#include <span>

int main()
{
    int arr[] = { 1, 3, 5, 7, 9 };
    std::cout << std::span{arr} << std::endl;
}