Eggs.FlatMap

Introduction

Eggs.FlatMap is a C++23 header-only implementation of a variadic flat_map — a sorted associative container with a Structure of Arrays (SoA) layout.

It extends the std::flat_map design from C++23 to support multiple value types per key, each stored in its own contiguous array for cache-friendly column-wise access.

#include <eggs/flat_map.hpp>
using namespace eggs::flat_map;

// entity database: ID → (position, health, name)
flat_map<uint32_t(Vec3, float, std::string)> entities;

entities.insert({1, Vec3{0, 0, 0}, 100.f, "Goblin"});

// iterate all rows — flat structured bindings
for (auto [id, pos, hp, name] : entities) { ... }

// access a specific value column by index
entities.find(1)->value<1>() -= 25.f;

// iterate entire health column — cache-friendly, no indirection
for (float& hp : entities.values<1>())
    hp = std::min(hp + 5.f, 100.f);

Design

The container is parameterized via a function-type signature:

flat_map<Key(V0, V1, ..., Vn), Compare>

This mirrors the familiar std::function<R(Args...)> syntax. Compare defaults to std::less<Key>. Internally, keys and each value type occupy separate std::vectors kept in parallel sorted order.

Operation	Complexity
Lookup	O(log n)
Insert	O(n)
Iteration	O(n), cache-friendly per-column

The interface follows std::flat_map as closely as possible. Notable extensions:

• it->value<I>() — access the I-th value of an iterator element
• m.values<I>() — direct access to the I-th value column (std::vector)
• auto [k, v0, v1, ...] = *it — flat structured bindings across all columns
• containers struct supports flat structured bindings for extract()/replace()

Requirements

• C++23 compiler (GCC 14+, Clang 17+, MSVC 19.38+)
• CMake 3.23 or later

Usage

Single header

Copy include/eggs/flat_map.hpp into your project.

CMake (FetchContent)

include(FetchContent)
FetchContent_Declare(eggs.flat_map
    GIT_REPOSITORY https://github.com/eggs-cpp/flat_map
    GIT_TAG        main)
FetchContent_MakeAvailable(eggs.flat_map)

target_link_libraries(my_target PRIVATE Eggs::FlatMap)

CMake (installed)

find_package(eggs.flat_map REQUIRED)
target_link_libraries(my_target PRIVATE Eggs::FlatMap)

License

Copyright Agustín Bergé, Fusion Fenix 2026

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE.txt or copy at http://www.boost.org/LICENSE_1_0.txt)