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nlohmann::ordered_map

template<class Key, class T, class IgnoredLess = std::less<Key>,
         class Allocator = std::allocator<std::pair<const Key, T>>>
struct ordered_map : std::vector<std::pair<const Key, T>, Allocator>;

A minimal map-like container that preserves insertion order for use within nlohmann::ordered_json (nlohmann::basic_json<ordered_map>).

Template parameters

Key
key type
T
mapped type
IgnoredLess
comparison function (ignored and only added to ensure compatibility with std::map)
Allocator
allocator type

Iterator invalidation

The type uses a std::vector to store object elements. Therefore, adding elements can yield a reallocation in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

Member types

  • key_type - key type (Key)
  • mapped_type - mapped type (T)
  • Container - base container type (std::vector<std::pair<const Key, T>, Allocator>)
  • iterator
  • const_iterator
  • size_type
  • value_type
  • key_compare - key comparison function 
    std::equal_to<Key>  // until C++14

std::equal_to<>     // since C++14

Member functions

  • (constructor)
  • (destructor)
  • emplace
  • operator[]
  • at
  • erase
  • count
  • find
  • insert

Examples

Example

The example shows the different behavior of std::map and nlohmann::ordered_map.

#include <iostream>
#include <nlohmann/json.hpp>

// simple output function
template<typename Map>
void output(const char* prefix, const Map& m)
{
    std::cout << prefix << " = { ";
    for (auto& element : m)
    {
        std::cout << element.first << ":" << element.second << ' ';
    }
    std::cout << "}" << std::endl;
}

int main()
{
    // create and fill two maps
    nlohmann::ordered_map<std::string, std::string> m_ordered;
    m_ordered["one"] = "eins";
    m_ordered["two"] = "zwei";
    m_ordered["three"] = "drei";

    std::map<std::string, std::string> m_std;
    m_std["one"] = "eins";
    m_std["two"] = "zwei";
    m_std["three"] = "drei";

    // output: m_ordered is ordered by insertion order, m_std is ordered by key
    output("m_ordered", m_ordered);
    output("m_std", m_std);

    // erase and re-add "one" key
    m_ordered.erase("one");
    m_ordered["one"] = "eins";

    m_std.erase("one");
    m_std["one"] = "eins";

    // output: m_ordered shows newly added key at the end; m_std is again ordered by key
    output("m_ordered", m_ordered);
    output("m_std", m_std);
}

Output:

m_ordered = { one:eins two:zwei three:drei }
m_std = { one:eins three:drei two:zwei }
m_ordered = { two:zwei three:drei one:eins }
m_std = { one:eins three:drei two:zwei }

See also

Version history