Tag dispatch vs `if constexpr`?

Tag dispatch uses overload sets; `if constexpr` branches inside one function—different ergonomics and compile-time trade-offs.

Iterator category tags drive different algorithm paths (`advance`, `distance`, etc.).

SFINAE removes invalid signatures early; tag dispatch chooses among valid implementations once types are known.

Template bloat and long builds—factor common bases, add concepts in C++20.

C++ Tag Dispatch | Compile-Time Algorithm Selection (Iterators and Traits)

2026년 3월 12일 · 17분 읽기 · 수정 2026년 3월 12일 Advanced Tutorial

이 글의 핵심

Tag dispatch selects the best `*_impl` overload at compile time using iterator categories or custom tag hierarchies.

What is tag dispatch? Why use it?

Problem: iterator-specific optimizations

std::advance should use pointer arithmetic for random-access iterators but step iterators for weaker categories.

Solution: pass a tag object

Use std::iterator_traits<Iter>::iterator_category{} to pick an overload of advance_impl.

flowchart TD
    start["my_advance(it, n)"]
    traits["iterator_traits::iterator_category"]
    tag1["random_access_iterator_tag"]
    tag2["bidirectional_iterator_tag"]
    impl1["advance_impl(..., random_access)\n it += n O(1)"]
    impl2["advance_impl(..., bidirectional)\n step loop O(n)"]
    
    start --> traits
    traits --> tag1
    traits --> tag2
    tag1 --> impl1
    tag2 --> impl2

The article continues with minimal tag dispatch, std::distance-style examples, type-trait tags (signed/unsigned), SFINAE comparison, iterator hierarchy pitfalls, production patterns (multi-level tags, concepts + tags), and a container insert example with reserve for random-access iterator ranges.

Summary

Concept	Meaning
Tag dispatch	Select overloads via empty tag parameters
Purpose	Compile-time algorithm selection
Pros	Often clearer than nested `enable_if`
Cons	Requires tag design; still templates

FAQ

Resources: C++ Templates: The Complete Guide, Effective STL, cppreference iterator tags.

SFINAE
Type traits
CRTP

Keywords

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