Eggs.KeyedSet

Eggs.KeyedSet is a C++23/26 single-header ordered associative container that stores unique Value objects indexed by a designated member field, satisfying all requirements of the C++26 AssociativeContainer named requirement ([associative.reqmts]).

See the library header at include/eggs/keyed_set.hpp.

---

Synopsis

namespace eggs
{
    template <typename Value, auto Value::*Key>
    class keyed_set;
}

keyed_set<Value, Key> behaves like a std::set<Value> but exposes key_type = decltype(v.*Key) and orders elements by that member. All heterogeneous lookup operations (find, count, contains, lower_bound, upper_bound, equal_range, extract, erase) accept key_type directly, with no Value object construction required.

Example

#include <eggs/keyed_set.hpp>

struct Employee { int id; std::string name; double salary; };

eggs::keyed_set<Employee, &Employee::id> roster;

roster.insert({101, "Alice",  95'000.0});
roster.insert({102, "Bob",    80'000.0});

// Heterogeneous lookup by int — no Employee constructed
if (auto it = roster.find(101); it != roster.end())
    std::cout << it->name << '\n';  // Alice

// Iteration is in ascending key order
for (auto const& e : roster)
    std::cout << e.id << ' ' << e.name << '\n';

// from_range constructor
std::vector<Employee> vec{ {1, "Carol", 70'000.0}, {2, "Dave", 80'000.0} };
eggs::keyed_set<Employee, &Employee::id> from_vec(std::from_range, vec);

Requirements

The library requires a C++23-conformant compiler and standard library. It has been tested with:

• GCC 14+
• Clang 17+ (with libc++ or libstdc++)
• MSVC 19.38+ (VS 2022 17.8+)

There are no external dependencies.

Building

The library is header-only. Copy include/eggs/keyed_set.hpp into your project, or use CMake:

# As a subdirectory
add_subdirectory(eggs-keyed_set)
target_link_libraries(my_target PRIVATE Eggs::KeyedSet)

# Or via find_package after installation
find_package(Eggs.KeyedSet REQUIRED)
target_link_libraries(my_target PRIVATE Eggs::KeyedSet)

CMake options

Option (top-level)	Option (subdirectory)	Default	Description
BUILD_TESTING	EGGS_KEYED_SET_BUILD_TESTING	ON / OFF	Build the test suite
BUILD_EXAMPLE	EGGS_KEYED_SET_BUILD_EXAMPLE	ON / OFF	Build the example
ENABLE_INSTALL	EGGS_KEYED_SET_ENABLE_INSTALL	ON / OFF	Install the library

Configuring and building with presets

The project ships a CMakePresets.json with named configurations for the most common workflows. CMake 3.23 or later is required to use presets.

# List all available presets
cmake --list-presets

# Configure + build + test in one go (system compiler, Debug)
cmake --preset dev
cmake --build --preset dev
ctest --preset dev

# Explicit toolchain variants
cmake --preset dev-gcc    && cmake --build --preset dev-gcc    && ctest --preset dev-gcc
cmake --preset dev-clang  && cmake --build --preset dev-clang  && ctest --preset dev-clang

# Release build
cmake --preset release-gcc && cmake --build --preset release-gcc

# Address + UB sanitizers
cmake --preset asan && cmake --build --preset asan && ctest --preset asan

# Thread sanitizer
cmake --preset tsan && cmake --build --preset tsan && ctest --preset tsan

# Visual Studio 2022 (multi-config)
cmake --preset vs2022
cmake --build --preset vs2022-debug
ctest --preset vs2022

Build trees land in build/<preset-name>/ so all presets coexist without interfering.

Local overrides

Copy the provided CMakeUserPresets.json template (it is .gitignored) to point at a specific compiler installation, set a custom install prefix, or wire in a vcpkg / Conan toolchain:

# Example: use a locally built GCC 15
cmake --preset dev-gcc15   # defined only in your CMakeUserPresets.json

C++26 AssociativeContainer conformance

keyed_set satisfies every expression listed in [associative.reqmts.general], including:

• All required nested types (key_type, value_type, key_compare, value_compare, allocator_type, node_type, insert_return_type, …)
• Constant bidirectional iterators (iterator == const_iterator)
• All constructor forms: default, comparator, iterator-range, from_range, initializer-list, copy, move, allocator-extended variants
• operator=(initializer_list)
• emplace, emplace_hint
• insert (lvalue, rvalue, hint, iterator-range, insert_range, initializer-list, node-handle, hint + node-handle)
• extract (by iterator, by key, transparent)
• erase (by key, transparent, by iterator, by range)
• clear, merge
• key_comp, value_comp
• find, count, contains, lower_bound, upper_bound, equal_range — all with both exact and transparent (K const&) overloads
• operator== and operator<=> (synthesises all six relational operators)
• Member and non-member swap

License

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)

Copyright Agustin K-ballo Berge, Fusion Fenix 2026