std::flat_map (C++23)

In a Nutshell

An ordered map backed by a contiguous array instead of a red-black tree -- faster lookups (cache-friendly), more compact memory, but O(n) insertion and deletion.

Header

#include <flat_map>

Core API Quick Reference

Operation	Signature	Description
Access element	V& operator[](const K& key)	Access by key; inserts a default value if not present
Find	iterator find(const K& key)	Returns an iterator to the element
Insert	pair <iterator, bool> insert(const value_type&)	Insert a key-value pair
Erase	size_t erase(const K& key)	Remove element by key
Element count	size_t size() const	Returns the number of elements
Check empty	bool empty() const	Checks whether the container is empty
Clear	void clear()	Removes all elements
Iterate	iterator begin() / end()	Traverse in key order
Lower/upper bound	iterator lower_bound(const K&)	Ordered boundary lookup
Contains	bool contains(const K& key) const	(Since C++20) Check whether a key exists

Minimal Example

// Standard: C++23
#include <flat_map>
#include <iostream>

int main() {
    std::flat_map<int, const char*> m;
    m[1] = "one";
    m[3] = "three";
    m[2] = "two";

    for (const auto& [k, v] : m) {
        std::cout << k << ": " << v << "\n";
    }
    // 1: one  2: two  3: three  (sorted by key)

    std::cout << std::boolalpha << m.contains(2) << "\n"; // true
}

Embedded Applicability: Medium

• Contiguous storage is CPU cache-friendly; lookup performance for small datasets far exceeds std::map
• No per-node allocator overhead, less memory fragmentation -- suitable for embedded environments with limited heap space
• Insertion/deletion is O(n); not suitable for large datasets with frequent modifications
• Compiler support is still evolving (GCC 15+, Clang 20+, MSVC 19.51+); evaluate your toolchain before production use

Compiler Support

GCC	Clang	MSVC
15	20	19.51

See Also

• cppreference: std::flat_map

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