How Can I Maintain Insertion Order in a std::map, Especially with a Limited Number of Elements?

std::Map with Insertion Order

Keeping track of insertion order in a std::map is crucial when the order of elements matters. While std::map primarily sorts elements based on key values, there are two potential solutions to address your requirement:

Solution 1: Using std::Vector

Although you mentioned concerns about performance when using std::vector due to frequent lookups and increments, a std::vector coupled with a custom comparator can maintain the insertion order. However, it is important to note that this approach may indeed introduce performance drawbacks if the number of elements increases significantly.

Solution 2: Boost::Multi-Index

For your specific scenario with a limited number of (50) elements, the Boost multi-index library provides a robust solution. It allows the creation of a container with multiple indexes. In your case, the following code snippet demonstrates how to define a multi-index container that tracks both insertion order and unique string identifiers:

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;

This multi-index container enables efficient lookups based on the string identifier while preserving the insertion order through the "random_access" index.

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