Oppiskele Header-Only vs Compiled Libraries | Library Design Fundamentals

When developing C++ libraries, you can choose between two major approaches: header-only libraries and compiled libraries. Each approach has unique advantages, disadvantages, and best-use scenarios. Understanding these differences is essential for designing robust and user-friendly libraries.

Header-only libraries contain all their code in header files (.h or .hpp). You include these headers directly in your projects, and the compiler processes the library code alongside your code. This approach is especially common for libraries that use templates, as template code must be visible to the compiler during compilation.

Compiled libraries, on the other hand, separate interface and implementation. The interface is defined in header files, while the implementation resides in source files (.cpp). The source files are compiled into either static libraries (.a or .lib) or shared libraries (.so or .dll). You link against the compiled binary and include the headers to use the library.

main.cpp

compiled_math.cpp

header_only_math.hpp

compiled_math.hpp


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#include <iostream>
#include "header_only_math.hpp"
#include "compiled_math.hpp"

int main()
{
    int x = 5;
    std::cout << "Square of " << x << " (header-only): " << square(x) << "\n";
    std::cout << "Sum of 3 and 4 (compiled): " << add(3, 4) << "\n";
}

Study More

Header-only libraries:

Eigen (linear algebra);
Catch2 (unit testing);
fmt (formatting);
Range-v3 (ranges).

Compiled libraries:

Boost.Filesystem (part of Boost, but requires compilation);
OpenCV (computer vision);
SQLite (database);
Qt (GUI framework).

These examples show how leading C++ libraries use both approaches, depending on their goals and design requirements.

The choice between header-only and compiled depends on your library's design, intended audience, and use cases. Template-heavy utilities and libraries intended for maximum portability often use the header-only approach. Libraries with complex implementation or a need to hide details typically use the compiled approach.

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