include/nlohmann/detail/iterators/iteration_proxy.hpp

#pragma once

#include <cstddef> // size_t
#include <iterator> // input_iterator_tag
#include <string> // string, to_string
#include <tuple> // tuple_size, get, tuple_element

#include <nlohmann/detail/meta/type_traits.hpp>
#include <nlohmann/detail/value_t.hpp>

namespace nlohmann
{
namespace detail
{
template<typename string_type>
void int_to_string( string_type& target, std::size_t value )
{
    // For ADL
    using std::to_string;
    target = to_string(value);
}
template<typename IteratorType> class iteration_proxy_value
{
  public:
    using difference_type = std::ptrdiff_t;
    using value_type = iteration_proxy_value;
    using pointer = value_type * ;
    using reference = value_type & ;
    using iterator_category = std::input_iterator_tag;
    using string_type = typename std::remove_cv< typename std::remove_reference<decltype( std::declval<IteratorType>().key() ) >::type >::type;

  private:
    /// the iterator
    IteratorType anchor;
    /// an index for arrays (used to create key names)
    std::size_t array_index = 0;
    /// last stringified array index
    mutable std::size_t array_index_last = 0;
    /// a string representation of the array index
    mutable string_type array_index_str = "0";
    /// an empty string (to return a reference for primitive values)
    const string_type empty_str = "";

  public:
    explicit iteration_proxy_value(IteratorType it) noexcept : anchor(it) {}

    /// dereference operator (needed for range-based for)
    iteration_proxy_value& operator*()
    {
        return *this;
    }

    /// increment operator (needed for range-based for)
    iteration_proxy_value& operator++()
    {
        ++anchor;
        ++array_index;

        return *this;
    }

    /// equality operator (needed for InputIterator)
    bool operator==(const iteration_proxy_value& o) const
    {
        return anchor == o.anchor;
    }

    /// inequality operator (needed for range-based for)
    bool operator!=(const iteration_proxy_value& o) const
    {
        return anchor != o.anchor;
    }

    /// return key of the iterator
    const string_type& key() const
    {
        JSON_ASSERT(anchor.m_object != nullptr);

        switch (anchor.m_object->type())
        {
            // use integer array index as key
            case value_t::array:
            {
                if (array_index != array_index_last)
                {
                    int_to_string( array_index_str, array_index );
                    array_index_last = array_index;
                }
                return array_index_str;
            }

            // use key from the object
            case value_t::object:
                return anchor.key();

            // use an empty key for all primitive types
            default:
                return empty_str;
        }
    }

    /// return value of the iterator
    typename IteratorType::reference value() const
    {
        return anchor.value();
    }
};

/// proxy class for the items() function
template<typename IteratorType> class iteration_proxy
{
  private:
    /// the container to iterate
    typename IteratorType::reference container;

  public:
    /// construct iteration proxy from a container
    explicit iteration_proxy(typename IteratorType::reference cont) noexcept
        : container(cont) {}

    /// return iterator begin (needed for range-based for)
    iteration_proxy_value<IteratorType> begin() noexcept
    {
        return iteration_proxy_value<IteratorType>(container.begin());
    }

    /// return iterator end (needed for range-based for)
    iteration_proxy_value<IteratorType> end() noexcept
    {
        return iteration_proxy_value<IteratorType>(container.end());
    }
};
// Structured Bindings Support
// For further reference see https://blog.tartanllama.xyz/structured-bindings/
// And see https://github.com/nlohmann/json/pull/1391
template<std::size_t N, typename IteratorType, enable_if_t<N == 0, int> = 0>
auto get(const nlohmann::detail::iteration_proxy_value<IteratorType>& i) -> decltype(i.key())
{
    return i.key();
}
// Structured Bindings Support
// For further reference see https://blog.tartanllama.xyz/structured-bindings/
// And see https://github.com/nlohmann/json/pull/1391
template<std::size_t N, typename IteratorType, enable_if_t<N == 1, int> = 0>
auto get(const nlohmann::detail::iteration_proxy_value<IteratorType>& i) -> decltype(i.value())
{
    return i.value();
}
}  // namespace detail
}  // namespace nlohmann

// The Addition to the STD Namespace is required to add
// Structured Bindings Support to the iteration_proxy_value class
// For further reference see https://blog.tartanllama.xyz/structured-bindings/
// And see https://github.com/nlohmann/json/pull/1391
namespace std
{
#if defined(__clang__)
    // Fix: https://github.com/nlohmann/json/issues/1401
    #pragma clang diagnostic push
    #pragma clang diagnostic ignored "-Wmismatched-tags"
#endif
template<typename IteratorType>
class tuple_size<::nlohmann::detail::iteration_proxy_value<IteratorType>>
            : public std::integral_constant<std::size_t, 2> {};

template<std::size_t N, typename IteratorType>
class tuple_element<N, ::nlohmann::detail::iteration_proxy_value<IteratorType >>
{
  public:
    using type = decltype(
                     get<N>(std::declval <
                            ::nlohmann::detail::iteration_proxy_value<IteratorType >> ()));
};
#if defined(__clang__)
    #pragma clang diagnostic pop
#endif
} // namespace std