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Item 47: Use traits classes for information about types

Concept​

Traits classes make information about types available at compile time, enabling template code to make decisions based on type properties without runtime overhead. The iterator_traits pattern is the canonical example: it provides information like value_type, iterator_category, etc., for any iterator type — including raw pointers — through template specialization. Combined with overload resolution or if constexpr, traits enable writing a single algorithm that adapts optimally to different type categories at compile time.

Code Example​

#include <iterator>
#include <type_traits>

// Using iterator_traits to dispatch based on iterator category
template <typename IterT, typename DistT>
void doAdvance(IterT& iter, DistT d, std::random_access_iterator_tag) {
    iter += d;  // O(1) for random access
}

template <typename IterT, typename DistT>
void doAdvance(IterT& iter, DistT d, std::input_iterator_tag) {
    while (d--) ++iter;  // O(n) for input iterators
}

template <typename IterT, typename DistT>
void advance(IterT& iter, DistT d) {
    // traits class provides compile-time type information
    doAdvance(iter, d,
        typename std::iterator_traits<IterT>::iterator_category());
}

Full source code

Things to Remember​

  • Traits classes make information about types available during compilation. They're implemented using templates and template specializations.
  • In conjunction with overloading, traits classes make it possible to perform compile-time if...else tests on types.
  • Item 42 — typename required for dependent type names (used with traits)
  • Item 48 — Template metaprogramming (traits enable TMP)
  • Item 41 — Compile-time polymorphism (traits are a key mechanism)