C++ Tag Dispatching Complete Guide

What is Tag Dispatching?

Tag dispatching uses empty tag types to drive function overload resolution at compile time, enabling type-specific optimizations without runtime branching.

// Tag types
struct IntTag {};
struct FloatTag {};
// Tag dispatch implementations
void processImpl(int value, IntTag) {
    cout << "Integer: " << value << endl;
}
void processImpl(double value, FloatTag) {
    cout << "Float: " << value << endl;
}
// Unified interface
template<typename T>
void process(T value) {
    if constexpr (is_integral_v<T>) {
        processImpl(value, IntTag{});
    } else {
        processImpl(value, FloatTag{});
    }
}
int main() {
    process(10);    // Integer
    process(3.14);  // Float
}

---

Iterator Tag Dispatching

STL uses iterator category tags to select optimal algorithms. Random-access iterators can jump directly (it += n), while input iterators must increment one-by-one.

#include <iterator>
// Implementation functions
template<typename Iter>
void advanceImpl(Iter& it, int n, input_iterator_tag) {
    // Input iterator: one step at a time
    while (n--) ++it;
}
template<typename Iter>
void advanceImpl(Iter& it, int n, random_access_iterator_tag) {
    // Random access iterator: jump directly
    it += n;
}
// Unified interface
template<typename Iter>
void advance(Iter& it, int n) {
    advanceImpl(it, n, typename iterator_traits<Iter>::iterator_category{});
}
int main() {
    vector<int> v = {1, 2, 3, 4, 5};
    auto it = v.begin();
    advance(it, 3);  // Random access (fast)
    
    list<int> l = {1, 2, 3, 4, 5};
    auto it2 = l.begin();
    advance(it2, 3);  // Input iterator (slow)
}

---

Practical Examples

Example 1: Distance Calculation

// Input iterator
template<typename Iter>
typename iterator_traits<Iter>::difference_type
distanceImpl(Iter first, Iter last, input_iterator_tag) {
    typename iterator_traits<Iter>::difference_type n = 0;
    while (first != last) {
        ++first;
        ++n;
    }
    return n;
}
// Random access iterator
template<typename Iter>
typename iterator_traits<Iter>::difference_type
distanceImpl(Iter first, Iter last, random_access_iterator_tag) {
    return last - first;  // O(1)
}
template<typename Iter>
auto distance(Iter first, Iter last) {
    return distanceImpl(first, last, 
        typename iterator_traits<Iter>::iterator_category{});
}
int main() {
    vector<int> v = {1, 2, 3, 4, 5};
    cout << distance(v.begin(), v.end()) << endl;  // 5 (fast)
    
    list<int> l = {1, 2, 3, 4, 5};
    cout << distance(l.begin(), l.end()) << endl;  // 5 (slow)
}

Example 2: Type-Based Serialization

struct PrimitiveTag {};
struct ContainerTag {};
struct CustomTag {};
// Type traits
template<typename T>
struct SerializeTraits {
    using tag = CustomTag;
};
template<> struct SerializeTraits<int> { using tag = PrimitiveTag; };
template<> struct SerializeTraits<double> { using tag = PrimitiveTag; };
template<typename T> struct SerializeTraits<vector<T>> { using tag = ContainerTag; };
// Implementations
template<typename T>
string serializeImpl(const T& value, PrimitiveTag) {
    return to_string(value);
}
template<typename T>
string serializeImpl(const T& container, ContainerTag) {
    string result = "[";
    for (const auto& item : container) {
        result += serialize(item) + ",";
    }
    result += "]";
    return result;
}
template<typename T>
string serializeImpl(const T& value, CustomTag) {
    return value.toString();  // Call custom method
}
// Unified interface
template<typename T>
string serialize(const T& value) {
    return serializeImpl(value, typename SerializeTraits<T>::tag{});
}
int main() {
    cout << serialize(42) << endl;
    cout << serialize(vector<int>{1, 2, 3}) << endl;
}

Example 3: Copy Optimization

struct TrivialTag {};
struct NonTrivialTag {};
template<typename T>
using CopyTag = conditional_t<
    is_trivially_copyable_v<T>,
    TrivialTag,
    NonTrivialTag
>;
// Trivial types (memcpy)
template<typename T>
void copyImpl(T* dest, const T* src, size_t n, TrivialTag) {
    memcpy(dest, src, n * sizeof(T));
}
// Non-trivial types (constructor)
template<typename T>
void copyImpl(T* dest, const T* src, size_t n, NonTrivialTag) {
    for (size_t i = 0; i < n; i++) {
        new (&dest[i]) T(src[i]);
    }
}
template<typename T>
void copy(T* dest, const T* src, size_t n) {
    copyImpl(dest, src, n, CopyTag<T>{});
}
int main() {
    int arr1[5] = {1, 2, 3, 4, 5};
    int arr2[5];
    copy(arr1, arr2, 5);  // memcpy (fast)
    
    string str1[3] = {"a", "b", "c"};
    string str2[3];
    copy(str1, str2, 3);  // Constructor (safe)
}

Example 4: Algorithm Optimization

struct SmallSizeTag {};
struct LargeSizeTag {};
template<size_t N>
using SizeTag = conditional_t<(N < 10), SmallSizeTag, LargeSizeTag>;
// Small arrays: bubble sort
template<typename T, size_t N>
void sortImpl(T (&arr)[N], SmallSizeTag) {
    for (size_t i = 0; i < N; i++) {
        for (size_t j = i + 1; j < N; j++) {
            if (arr[j] < arr[i]) {
                swap(arr[i], arr[j]);
            }
        }
    }
}
// Large arrays: quick sort
template<typename T, size_t N>
void sortImpl(T (&arr)[N], LargeSizeTag) {
    sort(begin(arr), end(arr));
}
template<typename T, size_t N>
void sort(T (&arr)[N]) {
    sortImpl(arr, SizeTag<N>{});
}
int main() {
    int small[5] = {5, 2, 8, 1, 9};
    sort(small);  // Bubble sort
    
    int large[100];
    sort(large);  // Quick sort
}

---

if constexpr vs Tag Dispatching

// if constexpr (C++17)
template<typename T>
void process(T value) {
    if constexpr (is_integral_v<T>) {
        // Integer processing
    } else {
        // Float processing
    }
}
// Tag Dispatching (C++11)
template<typename T>
void process(T value) {
    processImpl(value, TypeTag<T>{});
}

Selection criteria:

• C++17+: if constexpr (more concise)
• C++11/14: Tag dispatching
• Complex branching: Tag dispatching (better readability)

---

Common Issues

Issue 1: Missing tag type

// ❌ No tags
template<typename Iter>
void advance(Iter& it, int n) {
    it += n;  // Doesn't work for all iterators
}
// ✅ Tag dispatch
template<typename Iter>
void advance(Iter& it, int n) {
    advanceImpl(it, n, typename iterator_traits<Iter>::iterator_category{});
}

Issue 2: Wrong tag selection

// ❌ Wrong condition
template<typename T>
using Tag = conditional_t<sizeof(T) == 4, SmallTag, LargeTag>;
// ✅ Meaningful condition
template<typename T>
using Tag = conditional_t<is_trivially_copyable_v<T>, TrivialTag, NonTrivialTag>;

---

FAQ

Q1: When should I use tag dispatching?

A:

• Type-based optimization
• STL algorithm implementation
• Compile-time branching

Q2: if constexpr vs tag dispatching?

A: In C++17+, if constexpr is more concise. However, for complex cases, tag dispatching can be clearer.

Q3: Performance difference?

A: Both are resolved at compile time, so there’s no runtime difference.

Q4: How do I define tag types?

A: Define as empty structs. No data members needed.

Q5: Does STL use this?

A: Yes, iterator_category is a prime example.

Q6: Learning resources for tag dispatching?

A:

• “Effective STL” (Scott Meyers)
• cppreference.com
• STL source code

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Related Articles

• C++ Type Traits Complete Guide
• C++ Tag Dispatch Complete Guide | Compile-Time Function Selection & STL Optimization
• C++ Expression Templates | Advanced Lazy Evaluation Techniques

Related Posts

• C++ Tag Dispatch Complete Guide
• C++ Expression Templates
• C++ SFINAE & Concepts
• C++ Type Traits
• C++ Algorithm Copy

Keywords

C++, tag dispatch, metaprogramming, templates, STL, compile-time optimization, iterator categories, type traits

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