c++ sfinae在模板元编程中的高级应用案例

1. 判断类型是否有指定成员函数

#include 

template <typename T>
struct has_member_function_foo
{
private:
    template <typename U>
    static auto test(int) -> decltype(std::declval().foo(), std::true_type{});
    
    template <typename>
    static std::false_type test(...);
    
public:
    static constexpr bool value = std::is_same_v<decltype(test(0)), std::true_type>;
};

struct A
{
    void foo() {}
};

struct B
{
    // No foo()
};

int main()
{
    std::cout << has>::value << std class="hljs-comment">// 1
    std::cout << has>::value << std class="hljs-comment">// 0
    return 0;
}

#include 

template <typename T>
struct is_callable
{
private:
    // SFINAE test
    template <typename U>
    static auto test(int) -> decltype(std::declval()(), std::true_type{});

    template <typename>
    static std::false_type test(...);

public:
    static constexpr bool value = std::is_same_v<decltype(test(0)), std::true_type>;
};

struct F
{
    void operator()() {}
};

int main()
{
    std::cout << is>::value << std class="hljs-comment">// 1
    std::cout << is class="hljs-type">int>::value << std class="hljs-comment">// 0
    return 0;
}