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

// 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&, const 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(container.begin(), container.end(), os)
{
    using std::decay_t;
    using std::is_same_v;

    using element_type = decay_t<decltype(*container.begin())>;
    constexpr bool is_char_v = is_same_v<element_type, char>;
    if constexpr (!is_char_v) {
        os << "{ ";
    }
    if (!container.empty()) {
        auto end = container.end();
        bool on_first_element = true;
        for (auto it = container.begin(); it != end; ++it) {
            if constexpr (is_char_v) {
                if (*it == '\0') {
                    break;
                }
            }
            if constexpr (!is_char_v) {
                if (!on_first_element) {
                    os << ", ";
                } else {
                    on_first_element = false;
                }
            }
            output_element(os, *it, container, is_pair<element_type>());
        }
    }
    if constexpr (!is_char_v) {
        os << " }";
    }
    return os;
}

template <typename T, typename Cont>
auto output_element(std::ostream& os, const T& element,
                    const Cont&, const 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;
}

#endif // OUTPUT_CONTAINER_H