تخصص قالب و ارسال برچسب (Template Specialization & Tag Dispatch)

#include <iostream>
#include <cstring>

// Primary template
template <typename T>
class Formatter {
public:
    static std::string format(const T& value) {
        return std::to_string(value);
    }
};

// Full specialization for const char*
template <>
class Formatter<const char*> {
public:
    static std::string format(const char* value) {
        return std::string("\"" ) + value + "\"";
    }
};

// Full specialization for bool
template <>
class Formatter<bool> {
public:
    static std::string format(bool value) {
        return value ? "true" : "false";
    }
};

int main() {
    std::cout << Formatter<int>::format(42)            << '\n'; // 42
    std::cout << Formatter<const char*>::format("hi")  << '\n'; // "hi"
    std::cout << Formatter<bool>::format(true)          << '\n'; // true
}

#include <iostream>
#include <string>
#include <typeinfo>

// Primary template
template <typename T>
struct TypeInfo {
    static std::string name() { return "general type"; }
};

// Partial specialization for all pointer types
template <typename T>
struct TypeInfo<T*> {
    static std::string name() {
        return "pointer to " + TypeInfo<T>::name();
    }
};

// Partial specialization for all references
template <typename T>
struct TypeInfo<T&> {
    static std::string name() {
        return "reference to " + TypeInfo<T>::name();
    }
};

// Partial specialization for arrays
template <typename T, std::size_t N>
struct TypeInfo<T[N]> {
    static std::string name() {
        return "array of " + std::to_string(N) + " " + TypeInfo<T>::name();
    }
};

int main() {
    std::cout << TypeInfo<int>::name()     << '\n'; // general type
    std::cout << TypeInfo<int*>::name()    << '\n'; // pointer to general type
    std::cout << TypeInfo<int**>::name()   << '\n'; // pointer to pointer to general type
    std::cout << TypeInfo<int[5]>::name()  << '\n'; // array of 5 general type
}

#include <iostream>
#include <iterator>
#include <vector>
#include <list>

namespace detail {
    // Tags are empty structs used purely for overload selection
    template <typename Iter>
    void advance_impl(Iter& it, int n, std::random_access_iterator_tag) {
        it += n;  // O(1) — random access
        std::cout << "random access advance\n";
    }

    template <typename Iter>
    void advance_impl(Iter& it, int n, std::bidirectional_iterator_tag) {
        // O(n) — step one at a time, forward or backward
        while (n > 0) { ++it; --n; }
        while (n < 0) { --it; ++n; }
        std::cout << "bidirectional advance\n";
    }

    template <typename Iter>
    void advance_impl(Iter& it, int n, std::input_iterator_tag) {
        while (n > 0) { ++it; --n; }
        std::cout << "input iterator advance\n";
    }
}

template <typename Iter>
void my_advance(Iter& it, int n) {
    // iterator_traits extracts the category tag
    detail::advance_impl(it, n,
        typename std::iterator_traits<Iter>::iterator_category{});
}

int main() {
    std::vector<int> v{1, 2, 3, 4, 5};
    auto vit = v.begin();
    my_advance(vit, 3);  // random access advance

    std::list<int> l{1, 2, 3, 4, 5};
    auto lit = l.begin();
    my_advance(lit, 3);  // bidirectional advance
}

#include <iostream>
#include <type_traits>
#include <string>
#include <vector>

template <typename T>
std::string stringify(const T& value) {
    if constexpr (std::is_arithmetic_v<T>) {
        return std::to_string(value);
    } else if constexpr (std::is_same_v<T, std::string>) {
        return '"' + value + '"';
    } else if constexpr (std::is_same_v<T, const char*>) {
        return std::string("\"" ) + value + "\"";
    } else {
        return "[non-printable]";
    }
}

int main() {
    std::cout << stringify(42)              << '\n'; // 42
    std::cout << stringify(3.14)            << '\n'; // 3.140000
    std::cout << stringify(std::string("hello")) << '\n'; // "hello"
    std::cout << stringify("world")         << '\n'; // "world"
}