C++ STL打印

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output.hpp

#ifndef OUTPUT_RANGE_H
#define OUTPUT_RANGE_H

#include <cstddef>    // std::byte
#include <iterator>   // std::begin/end
#include <ostream>    // std::ostream
#include <tuple>      // std::tuple/tuple_size
#include <type_traits>// std::false_type/true_type/decay_t/is_same_v/remove_...
#include <utility>    // std::declval/pair

#ifndef OUTPUT_RANGE_NO_STRING_QUOTE
#include <string>     // std::string
#include <string_view>// std::string_view
#endif

namespace output_range {

    using std::begin;
    using std::end;

    template<class Rng>
    auto adl_begin(Rng&& rng) -> decltype(begin(rng))
    {
        // Intentionally not using std::forward, as std::begin does not
        // accept an rvalue reference.
        return begin(rng);
    }

    template<class Rng>
    auto adl_end(Rng&& rng) -> decltype(end(rng))
    {
        // Intentionally not using std::forward, as std::end does not
        // accept an rvalue reference.
        return end(rng);
    }

    // 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 for tuple-like objects
    template<typename T, typename = void>
    struct is_tuple_like : std::false_type {};
    template<typename T>
    struct is_tuple_like<T, std::void_t<decltype(std::tuple_size<T>::value)>> : std::true_type {};
    template<typename T>
    inline constexpr bool is_tuple_like_v = is_tuple_like<T>::value;

    // Type traits for ranges
    template<typename T, typename = void>
    struct is_range : std::false_type {};
    template<typename T>
    struct is_range<T, std::void_t<decltype(adl_begin(std::declval<T>()), adl_end(std::declval<T>()))>> : std::true_type {};
    template<typename T>
    inline constexpr bool is_range_v = is_range<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;
    };
#ifndef OUTPUT_RANGE_NO_ARRAY_OUTPUT
    template<typename T, std::size_t N>
    struct has_output_function<T[N]> : std::false_type {};
    template<std::size_t N>
    struct has_output_function<char[N]> : std::true_type {};
    template<std::size_t N>
    struct has_output_function<signed char[N]> : std::true_type {};
    template<std::size_t N>
    struct has_output_function<unsigned char[N]> : std::true_type {};
    template<std::size_t N>
    struct has_output_function<const char[N]> : std::true_type {};
    template<std::size_t N>
    struct has_output_function<const signed char[N]> : std::true_type {};
    template<std::size_t N>
    struct has_output_function<const unsigned char[N]> : std::true_type {};
#endif
    template<typename T>
    inline constexpr bool has_output_function_v = has_output_function<T>::value;

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

    // Member output function for tuple-like objects
    template<typename Tup, std::size_t... Is>
    void output_tuple_members(std::ostream& os, const Tup& tup, std::index_sequence<Is...>);

}// namespace output_range

// Output function for tuple-like objects
template<typename T, std::enable_if_t<(output_range::is_tuple_like_v<T> && !output_range::is_range_v<T>), bool> = true>
std::ostream& operator<<(std::ostream& os, const T& tup);

// Main output function, enabled only if no output function already exists
template<typename Rng,
    std::enable_if_t<(output_range::is_range_v<Rng> &&
        !output_range::has_output_function_v<std::remove_cv_t<std::remove_reference_t<Rng>>>),
    bool> = true>
std::ostream& operator<<(std::ostream& os, Rng&& rng)
{
    using std::decay_t;
    using std::is_same_v;

    using element_type = decay_t<decltype(*output_range::adl_begin(rng))>;
    os << '{';
    auto end = output_range::adl_end(rng);
    bool on_first_element = true;
    for (auto it = output_range::adl_begin(rng); it != end; ++it) {
        if (!on_first_element) {
            os << ", ";
        }
        else {
            os << ' ';
            on_first_element = false;
        }
        output_range::output_element(os, *it, rng, output_range::is_pair<element_type>{});
    }
    if (!on_first_element) {// Not empty
        os << ' ';
    }
    os << '}';
    return os;
}

template<typename T, std::enable_if_t<(output_range::is_tuple_like_v<T> && !output_range::is_range_v<T>), bool>>
std::ostream& operator<<(std::ostream& os, const T& tup)
{
    os << '(';
    output_range::output_tuple_members(os, tup, std::make_index_sequence<std::tuple_size_v<T>>{});
    os << ')';
    return os;
}

namespace output_range {

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

    template<typename T, typename Rng>
    auto output_element(std::ostream& os, const T& element, const Rng&, ...) -> decltype(os)
    {
        if constexpr (std::is_same_v<T, char> || std::is_same_v<T, signed char>) {
            os << '\'' << element << '\'';
        }
        else if constexpr (std::is_same_v<T, unsigned char> || std::is_same_v<T, std::byte>) {
            os << static_cast<unsigned>(element);
        }
        else
#ifndef OUTPUT_RANGE_NO_STRING_QUOTE
        {
            using DT = std::decay_t<T>;
            using PT = std::remove_cv_t<std::remove_pointer_t<DT>>;
            if constexpr (std::is_same_v<T, std::string> || std::is_same_v<T, std::string_view> ||
                (std::is_pointer_v<DT> && (std::is_same_v<PT, char> || std::is_same_v<PT, signed char> ||
                    std::is_same_v<PT, unsigned char>))) {
                os << '"' << element << '"';
            }
            else {
                os << element;
            }
        }
#else
        {
            os << element;
        }
#endif
        return os;
    }

    template<typename Tup, std::size_t... Is>
    void output_tuple_members(std::ostream& os, const Tup& tup, std::index_sequence<Is...>)
    {
        using std::get;
        ((os << (Is != 0 ? ", " : ""), output_element(os, get<Is>(tup), tup)), ...);
    }

}// namespace output_range

#endif// OUTPUT_RANGE_H

测试

#include "output.hpp"
#include <iostream>
#include <map>
int main(int argc, char* argv[]) {
    std::vector<int> v{ 1,2,3,4 };
    std::tuple<char, int, float> t{ 'a', 1, 2.0f };
    std::map<int, std::string> m{ {1,"one"},{2,"two"} };

    std::cout << v << std::endl;
    std::cout << m << std::endl;
    std::cout << t << std::endl;
	return 0;
}

输出

{ 1, 2, 3, 4 }

{ 1 => “one”, 2 => “two” }

(‘a’, 1, 2)

参考

https://github.com/adah1972/output_range

---

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