SFINAE، ویژگی‌های نوع و enable_if

#include <iostream>
#include <type_traits>

// Only enabled for integral types
template <typename T>
std::enable_if_t<std::is_integral_v<T>, T>
safe_divide(T a, T b) {
    if (b == 0) return 0;  // safe fallback for integers
    return a / b;
}

// Only enabled for floating-point types
template <typename T>
std::enable_if_t<std::is_floating_point_v<T>, T>
safe_divide(T a, T b) {
    if (b == 0.0) return std::numeric_limits<T>::quiet_NaN();
    return a / b;
}

// enable_if in template parameter (alternative placement)
template <typename T,
         std::enable_if_t<std::is_arithmetic_v<T>, int> = 0>
void process(T value) {
    std::cout << "Processing arithmetic value: " << value << '\n';
}

int main() {
    std::cout << safe_divide(10, 3)    << '\n'; // 3 (integer)
    std::cout << safe_divide(10.0, 3.0) << '\n'; // 3.33333 (double)
    std::cout << safe_divide(10, 0)    << '\n'; // 0 (safe fallback)
    std::cout << safe_divide(10.0, 0.0) << '\n'; // nan

    process(42);
    // process(std::string("hello")); // ERROR: no matching function
}

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

// Primary template: fallback, T does NOT have .size()
template <typename T, typename = void>
struct has_size : std::false_type {};

// Specialization: matches only if T has a .size() method
template <typename T>
struct has_size<T, std::void_t<decltype(std::declval<T>().size())>>
    : std::true_type {};

template <typename T>
constexpr bool has_size_v = has_size<T>::value;

// Use the trait to branch behavior
template <typename T>
void describe(const T& value) {
    if constexpr (has_size_v<T>) {
        std::cout << "Container with " << value.size() << " elements\n";
    } else {
        std::cout << "Not a container: " << value << '\n';
    }
}

int main() {
    static_assert(has_size_v<std::string>);     // true
    static_assert(has_size_v<std::vector<int>>); // true
    static_assert(!has_size_v<int>);             // false

    describe(std::vector<int>{1, 2, 3}); // Container with 3 elements
    describe(42);                        // Not a container: 42
}

#include <iostream>
#include <type_traits>

// Only callable if a + b is valid
template <typename T, typename U>
auto add(T a, U b) -> decltype(a + b) {
    return a + b;
}

// Only callable if t.begin() and t.end() are valid
template <typename Container>
auto element_count(const Container& c)
    -> decltype(c.begin(), c.end(), std::size_t{}) {
    std::size_t count = 0;
    for (auto it = c.begin(); it != c.end(); ++it) ++count;
    return count;
}

int main() {
    std::cout << add(1, 2.5)   << '\n'; // 3.5
    std::cout << add(std::string("a"), std::string("b")) << '\n'; // ab

    std::vector<int> v{1, 2, 3, 4};
    std::cout << element_count(v) << '\n'; // 4
    // add(std::string("a"), 5); // SFINAE: no match, a+b invalid
}