C++ Tag Dispatching | Pattern Guide with Iterator and Serialization Tags

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C++ Tag Dispatching | Pattern Guide with Iterator and Serialization Tags

이 글의 핵심

Tag dispatching dispatches to different `*_impl` functions using tag types—classic STL pattern with iterator categories.

What is tag dispatching?

Use empty tag types and overload resolution to branch at compile time.

The Korean article demonstrates IntTag/FloatTag, iterator advanceImpl with input_iterator_tag vs random_access_iterator_tag, distance computation, serialization tags (PrimitiveTag, ContainerTag, CustomTag), trivial vs non-trivial copy tags, and small-array vs large-array sort selection.

if constexpr vs tag dispatching

  • C++17+: if constexpr can express many branches in one function.
  • C++11/14: tag dispatching is a natural fit.
  • Complex branching: sometimes clearer as multiple *_impl overloads.

Pitfalls

  • Missing iterator tag dispatch → wrong it += n on non-random-access iterators.
  • Wrong conditional_t picking tags—double-check predicates.

FAQ

Performance: Both tag dispatch and if constexpr compile away—no inherent runtime difference.

  • Type traits
  • Tag dispatch
  • Expression templates

Keywords

C++, tag dispatch, metaprogramming, templates, STL.

See also

  • Tag dispatch
  • Expression templates
  • SFINAE & Concepts
  • Type traits
  • algorithm::copy