Tag Dispatch

Tag dispatch is a C++ metaprogramming technique used to select function overloads at compile-time based on type traits or tags. It involves passing lightweight tag objects as function arguments to guide overload resolution, enabling compile-time polymorphism and efficient code generation. This technique is commonly used in template libraries like the C++ Standard Library to implement type-specific behavior without runtime overhead.

Also known as: Tag-based dispatch, Tag overloading, Type tag dispatch, Compile-time dispatch, Tagged dispatch

Why learn Tag Dispatch?

Developers should learn tag dispatch when implementing generic libraries in C++ that require compile-time decisions based on type properties, such as iterator categories or type traits. It is particularly useful for optimizing algorithms (e.g., in the STL) by selecting specialized implementations for different data types, improving performance without sacrificing generality. Use cases include custom iterator adapters, type-safe APIs, and template metaprogramming scenarios where runtime polymorphism is inefficient.

Learning Resources

• docs: C++ Tag Dispatch Explained
• tutorial: Tag Dispatch in C++ - A Tutorial
• book: C++ Templates: The Complete Guide (Book)
• video: Tag Dispatch vs SFINAE - Video Explanation