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C++ Academy · Lesson

Writing a Modern Header-Only Utility Library

Walk through publishing a small modern C++ header-only utility library.

Writing a Modern Header-Only Utility Library is a free C++ Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

A Realistic Goal

Build a small header-only library that demonstrates modern C++ practices: clear API, namespaces, no global state, concepts where helpful.

Project Layout

A typical layout:

  • include/mylib/util.hpp — public headers
  • include/mylib/detail/impl.hpp — internals
  • tests/ — unit tests
  • examples/ — sample programs
  • CMakeLists.txt

Include Guards or Pragma Once

Use #pragma once in modern code — supported by all major compilers and avoids include guard boilerplate.

#pragma once

namespace mylib {
    // ...
}

Namespacing

Wrap everything in a project namespace. Use a nested detail namespace for internals you do not want consumers to use.

namespace mylib {
    namespace detail {
        // private helpers
    }

    // public API
    int public_func();
}

Inline Variables (C++17)

Header-only libraries often need constants. inline variables (C++17) avoid linker errors when a header is included in multiple TUs.

namespace mylib {
    inline constexpr int VERSION = 100;
    inline const std::string LIBRARY_NAME = "mylib";
}

Concepts for Better Errors

Use C++20 concepts to constrain templates — produces helpful error messages.

template <std::integral T>
T clamp_int(T value, T lo, T hi) {
    return std::max(lo, std::min(value, hi));
}

Noexcept Where Possible

Mark non-throwing functions noexcept. Improves optimization and signals intent.

Const Correctness

Mark every read-only method and reference parameter as const. Make immutability the default.

Avoid Macros in Headers

Macros leak across includes. Prefer constexpr functions, templates, and inline constants.

Documentation

Use Doxygen-style comments. Brief description, parameters, return value, examples.

/// Clamp an integer to a range.
/// @param value The value to clamp
/// @param lo Minimum allowed
/// @param hi Maximum allowed
/// @return value clamped to [lo, hi]
template <std::integral T>
T clamp_int(T value, T lo, T hi);

Unit Tests

Test with Catch2, doctest, or GoogleTest. Place tests in a separate directory and integrate with CMake s ctest.

CMake Integration

Provide a CMake config file so consumers can find_package(mylib). Export an INTERFACE target with include directories.

add_library(mylib INTERFACE)
target_include_directories(mylib INTERFACE include)
target_compile_features(mylib INTERFACE cxx_std_20)

Publishing

Publish on GitHub, register with vcpkg or Conan, and tag releases with semantic versions. Provide release notes summarizing changes per version.

Quick Check

Why prefer #pragma once over traditional include guards in a modern header-only library?

Recap

A modern header-only library uses #pragma once, namespaces with a detail subspace, inline constants, concepts for constraints, and noexcept/const correctness. Document with Doxygen, test with a modern framework, expose CMake targets, and publish via vcpkg or Conan.

Frequently asked questions

Is the “Writing a Modern Header-Only Utility Library” lesson free?

Yes — the full text of “Writing a Modern Header-Only Utility Library” is free to read here on the web, and the C++ Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C++ Academy course, upgrade to CoddyKit PRO.

What will I learn in “Writing a Modern Header-Only Utility Library”?

Walk through publishing a small modern C++ header-only utility library. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C++ Academy?

No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Writing a Modern Header-Only Utility Library” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C++ Academy lesson?

Yes. Every C++ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Header-Only vs Compiled Libraries
  2. API Versioning and ABI Stability
  3. Hiding Implementation with PIMPL
  4. Writing a Modern Header-Only Utility Library
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