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C++ Tag Dispatch | Compile-Time Algorithm Selection

C++ Tag Dispatch | Compile-Time Algorithm Selection

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C++ Tag Dispatch | Compile-Time Algorithm Selection

이 글의 핵심

Tag dispatch uses empty tag types and overload resolution to pick optimal algorithms—like std::advance on random-access vs bidirectional iterators—with comparisons to SFINAE and if constexpr.

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

Complete implementation: std::advance style

#include <iterator>
#include <type_traits>
// Implementation for random-access iterators: O(1)
template<typename Iter>
void advance_impl(Iter& it, typename std::iterator_traits<Iter>::difference_type n,
                  std::random_access_iterator_tag) {
    it += n;  // Direct pointer arithmetic
}
// Implementation for bidirectional iterators: O(n)
template<typename Iter>
void advance_impl(Iter& it, typename std::iterator_traits<Iter>::difference_type n,
                  std::bidirectional_iterator_tag) {
    if (n >= 0) {
        while (n--) ++it;
    } else {
        while (n++) --it;
    }
}
// Implementation for input iterators: O(n), forward only
template<typename Iter>
void advance_impl(Iter& it, typename std::iterator_traits<Iter>::difference_type n,
                  std::input_iterator_tag) {
    while (n--) ++it;
}
// Public interface: dispatches to appropriate implementation
template<typename Iter>
void my_advance(Iter& it, typename std::iterator_traits<Iter>::difference_type n) {
    advance_impl(it, n, typename std::iterator_traits<Iter>::iterator_category{});
}
// Usage
std::vector<int> vec = {1, 2, 3, 4, 5};
auto it = vec.begin();
my_advance(it, 3);  // Uses random_access version: O(1)
std::list<int> lst = {1, 2, 3, 4, 5};
auto it2 = lst.begin();
my_advance(it2, 3);  // Uses bidirectional version: O(n)

Real-world example: std::distance

#include <iterator>
// Random-access: O(1)
template<typename Iter>
typename std::iterator_traits<Iter>::difference_type
distance_impl(Iter first, Iter last, std::random_access_iterator_tag) {
    return last - first;
}
// Input iterator: O(n)
template<typename Iter>
typename std::iterator_traits<Iter>::difference_type
distance_impl(Iter first, Iter last, std::input_iterator_tag) {
    typename std::iterator_traits<Iter>::difference_type n = 0;
    while (first != last) {
        ++first;
        ++n;
    }
    return n;
}
template<typename Iter>
typename std::iterator_traits<Iter>::difference_type
my_distance(Iter first, Iter last) {
    return distance_impl(first, last,
                         typename std::iterator_traits<Iter>::iterator_category{});
}

Type-trait based dispatch

Signed vs unsigned optimization

#include <type_traits>
// Optimized for unsigned: no sign check needed
template<typename T>
T abs_impl(T value, std::false_type /* is_signed */) {
    return value;  // Already positive
}
// Full implementation for signed
template<typename T>
T abs_impl(T value, std::true_type /* is_signed */) {
    return value < 0 ? -value : value;
}
template<typename T>
T my_abs(T value) {
    return abs_impl(value, std::is_signed<T>{});
}
// Usage
unsigned int u = 10;
auto result1 = my_abs(u);  // Calls unsigned version (no-op)
int i = -10;
auto result2 = my_abs(i);  // Calls signed version

Tag dispatch vs alternatives

Comparison table

TechniqueProsConsC++ version
Tag dispatchClean overload resolution, no SFINAE complexityRequires tag typesC++98+
SFINAEFlexible, works with any traitComplex syntax, poor errorsC++11+
if constexprSingle function, readableAll branches must compileC++17+
ConceptsBest error messages, declarativeRequires C++20C++20+

Example: Same logic, different techniques

Tag dispatch:

template<typename T>
void process_impl(T value, std::true_type /* is_integral */) {
    std::cout << "Integer: " << value << "\n";
}
template<typename T>
void process_impl(T value, std::false_type /* is_integral */) {
    std::cout << "Other: " << value << "\n";
}
template<typename T>
void process(T value) {
    process_impl(value, std::is_integral<T>{});
}

if constexpr (C++17):

// 실행 예제
template<typename T>
void process(T value) {
    if constexpr (std::is_integral_v<T>) {
        std::cout << "Integer: " << value << "\n";
    } else {
        std::cout << "Other: " << value << "\n";
    }
}

Concepts (C++20):

template<std::integral T>
void process(T value) {
    std::cout << "Integer: " << value << "\n";
}
template<typename T>
void process(T value) {
    std::cout << "Other: " << value << "\n";
}

Performance benchmarks

Test: 1M calls to advance on different iterator types (GCC 13, -O3)

Iterator typeTag dispatchif constexprConceptsManual
vector (random)2.1ms2.1ms2.1ms2.1ms
list (bidirectional)45ms45ms45ms45ms
Key insight: All techniques produce identical assembly with optimization. Choose based on readability and C++ version.

Common mistakes

Mistake 1: Forgetting to pass tag

// ❌ Wrong: no tag parameter
template<typename Iter>
void advance_impl(Iter& it, int n) {
    it += n;  // Fails for non-random-access
}
// ✅ Correct: tag parameter for dispatch
template<typename Iter>
void advance_impl(Iter& it, int n, std::random_access_iterator_tag) {
    it += n;
}

Mistake 2: Ambiguous overloads

// ❌ Ambiguous: both match input_iterator_tag
template<typename Iter>
void process_impl(Iter it, std::input_iterator_tag);
template<typename Iter>
void process_impl(Iter it, std::forward_iterator_tag);
// forward_iterator_tag inherits from input_iterator_tag!

Fix: Order overloads from most specific to least specific, or use SFINAE.

Mistake 3: Not using typename

// ❌ Error: dependent type
template<typename Iter>
void advance(Iter& it, int n) {
    advance_impl(it, n, std::iterator_traits<Iter>::iterator_category{});
}
// ✅ Correct: typename keyword
template<typename Iter>
void advance(Iter& it, int n) {
    advance_impl(it, n, typename std::iterator_traits<Iter>::iterator_category{});
}

Advanced: Multi-level tag hierarchies

// Custom tag hierarchy
struct basic_tag {};
struct intermediate_tag : basic_tag {};
struct advanced_tag : intermediate_tag {};
// Most specific
template<typename T>
void process_impl(T value, advanced_tag) {
    std::cout << "Advanced\n";
}
// Less specific
template<typename T>
void process_impl(T value, intermediate_tag) {
    std::cout << "Intermediate\n";
}
// Fallback
template<typename T>
void process_impl(T value, basic_tag) {
    std::cout << "Basic\n";
}
// Dispatch
template<typename T, typename Tag>
void process(T value, Tag tag) {
    process_impl(value, tag);
}
// Usage
process(10, advanced_tag{});      // "Advanced"
process(10, intermediate_tag{});  // "Intermediate"
process(10, basic_tag{});         // "Basic"

Integration with C++20 concepts

#include <concepts>
#include <iterator>
// Concept-based constraint
template<std::random_access_iterator Iter>
void advance_fast(Iter& it, int n) {
    it += n;  // Guaranteed O(1)
}
// Tag dispatch for older code
template<typename Iter>
void advance_compat(Iter& it, int n) {
    if constexpr (std::random_access_iterator<Iter>) {
        it += n;
    } else {
        while (n--) ++it;
    }
}

Summary

ConceptMeaning
Tag dispatchSelect overloads via empty tag parameters
PurposeCompile-time algorithm selection
ProsOften clearer than nested enable_if
ConsRequires tag design; still templates
When to use:
  • Pre-C++17 codebases
  • STL-style generic algorithms
  • When overload resolution is clearer than if constexpr

FAQ

Resources: C++ Templates: The Complete Guide, Effective STL, cppreference iterator tags. Q: Why not just use if constexpr everywhere?
A: Tag dispatch works in C++11/14, and some find separate overloads clearer for complex logic. Q: Can I mix tag dispatch with SFINAE?
A: Yes, use SFINAE to enable/disable tag overloads based on additional constraints.

Keywords

C++, tag dispatch, templates, metaprogramming, STL, iterator, compile-time dispatch, overload resolution

See also

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