SFINAE 特定のメンバ変数を持っているかどうかの trait

SFINAE-has-value.cpp

#include <iostream>
#include <type_traits>

struct has_value_impl {
    template <class T, class U = decltype(T::value)>
    constexpr static std::is_same<U, int> test_variable(int);
    
    template <typename...>
    constexpr static std::false_type test_variable(...);
    
    template <class T, class U = decltype(std::declval<T&>().value())>
    constexpr static std::is_same<U, int> test_func(int);
    
    template <typename...>
    constexpr static std::false_type test_func(...);
};

template <class T>
constexpr bool has_variable_value = decltype(has_value_impl::test_variable<T>(0))::value;

template <class T>
constexpr bool has_func_value = decltype(has_value_impl::test_func<T>(0))::value;

template <class T>
constexpr bool has_value = has_variable_value<T> || has_func_value<T>;

struct value_trait {
    template <typename T, std::enable_if_t<has_variable_value<T>, std::nullptr_t> = nullptr>
    static int get(const T &t) { return t.value; }
    
    template <typename T, std::enable_if_t<has_func_value<T>, std::nullptr_t> = nullptr>
    static int get(const T &t) { return t.value(); }
};

struct foo {
    int value;
};

struct bar {
    int value() const { return 456; }
};

struct baz {
    float value;
};

template <typename T, std::enable_if_t<has_value<std::decay_t<T>>, std::nullptr_t> = nullptr>
void print_value(const T &t) {
    std::cout
        << " has value: "
        << value_trait::get(t)
        << std::endl;
}

template <typename T, std::enable_if_t<!has_value<std::decay_t<T>>, std::nullptr_t> = nullptr>
void print_value(const T &t) {
    (void)t;
    std::cout
        << " doesn't have value"
        << std::endl;
}

int main()
{
    foo a{123};
    bar b;
    baz c{789.0f};
    int d = 1000;
    
    std::cout << "a";
    print_value(a);
    
    std::cout << "b";
    print_value(b);
    
    std::cout << "c";
    print_value(c);
    
    std::cout << "d";
    print_value(d);
    
    return 0;
}