std::flat_map (C++23)

In a nutshell

An ordered associative container that uses a contiguous array instead of a red-black tree—faster lookups (cache-friendly) and more compact memory, but with O(n) insertion/deletion.

Header

#include <flat_map>

Core API Cheat Sheet

Operation	Signature	Description
Access Element	V& operator[](const K& key)	Access by key; inserts default value if not found
Find	iterator find(const K& key)	Returns an iterator to the element
Insert	pair<iterator, bool> insert(const value_type&)	Inserts a key-value pair
Erase	size_t erase(const K& key)	Removes an element by key
Element Count	size_t size() const	Returns the number of elements
Is Empty	bool empty() const	Checks if 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 search for boundaries
Contains	bool contains(const K& key) const	(Since C++20) Checks if 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  (按键序排列)

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

Embedded Applicability: Medium

• Contiguous storage is CPU cache-friendly; lookup performance on small datasets is far superior to std::map
• No node allocator overhead and less memory fragmentation, suitable for embedded environments with limited heap space
• Insertion/deletion is O(n), making it unsuitable for large, frequently modified datasets
• Compiler support is still evolving (GCC 15+, Clang 20+, MSVC 19.51+); evaluate toolchains for production use

Compiler Support

GCC	Clang	MSVC
15	20	19.51

See Also

• cppreference: std::flat_map

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