การเขียน traits แบบกำหนดเอง

#include <iostream>
#include <type_traits>

template <typename T>
struct is_ptr : std::false_type {};

template <typename T>
struct is_ptr<T*> : std::true_type {};

int main() {
    std::cout << is_ptr<int>::value << "\n";
    std::cout << is_ptr<int*>::value << "\n";
    return 0;
}

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

template <typename T, typename = void>
struct has_value_type : std::false_type {};

template <typename T>
struct has_value_type<T, std::void_t<typename T::value_type>> : std::true_type {};

int main() {
    std::cout << has_value_type<std::vector<int>>::value << "\n";
    std::cout << has_value_type<int>::value << "\n";
    return 0;
}

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

template <typename T, typename = void>
struct has_size : std::false_type {};

template <typename T>
struct has_size<T, std::void_t<decltype(std::declval<T>().size())>> : std::true_type {};

int main() {
    std::cout << has_size<std::string>::value << "\n";
    std::cout << has_size<int>::value << "\n";
    return 0;
}

#include <iostream>
#include <type_traits>

template <typename T>
struct remove_one_pointer { using type = T; };

template <typename T>
struct remove_one_pointer<T*> { using type = T; };

int main() {
    using R = remove_one_pointer<int*>::type;
    std::cout << std::is_same_v<R, int> << "\n";
    return 0;
}

#include <iostream>
#include <type_traits>

template <typename T>
struct is_char : std::false_type {};
template <>
struct is_char<char> : std::true_type {};

template <typename T>
inline constexpr bool is_char_v = is_char<T>::value;

int main() {
    std::cout << is_char_v<char> << "\n";
    std::cout << is_char_v<int> << "\n";
    return 0;
}

#include <type_traits>

template <typename T>
struct is_void_like : std::false_type {};
template <>
struct is_void_like<void> : std::true_type {};

static_assert(is_void_like<void>::value);
static_assert(!is_void_like<int>::value);

int main() { return 0; }