How Can I Preserve Insertion Order in a Map Efficiently?

Preserving Insertion Order in a Map

In the domain of data structures, a map is a container that stores key-value pairs. A common requirement for maps is to maintain the order in which these pairs were inserted, ensuring that when iterating through the map, the elements are accessed in the order they were added. However, the default implementation of a standard map does not guarantee this preservation of insertion order.

To address this need, several alternatives can be considered. One option is to utilize a vector of pairs, which allows for the maintenance of insertion order. However, for scenarios involving a large number of operations, such as iterating over 10,000,000 key-value pairs, a vector might not be the optimal choice due to performance concerns.

Alternatively, for systems with a limited number of key-value pairs, such as the scenario in the question with approximately 50 pairs, converting the map into a vector and employing the standard sorting library (std::sort) with a suitable ordering comparator, such as a functor, can be a viable approach.

Another option for preserving insertion order in maps is to leverage the Boost Multi-Index Library. This library provides various index types that can be combined to create multi-index containers. For instance, in the question's scenario, a multi-index map could be employed with two indices: one for random access (preserving insertion order) and another hashed unique index for efficient string lookups. The following code snippet illustrates how a multi-index map could be implemented for this scenario:

struct value_t {
  string s;
  int i;
};

struct string_tag {};

typedef multi_index_container<
    value_t,
    indexed_by<
        random_access<>, // this index represents insertion order
        hashed_unique<tag<string_tag>, member<value_t, string, &value_t::s>>
    >
> values_t;

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