我如何处理 TMP 以避免打印任何内容

Question

I'm falling into a problem, so I want to avoid this problem which is: if I provide a class template, I have to ensure that when I enter string it returns false, it worked for primitive data type but for this class template no.

template<class Y>
class Point
{
    Y x, y;
public:
    Point(){}
    Point(Y _x, Y _y):x(_x),y(_y){}
    ~Point(){}

    Point<Y>& operator++()
    {
        ++x;
        ++y;
        return *this;
    }
};
Point<int>Pi;

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

template< typename T >
struct is_incrementable<T,std::void_t<decltype(++std::declval<T&>())>> : std::true_type { };

int main()
{
    cout << boolalpha;
    cout << is_incrementable<Point<int>>();
    return 0;
}

how can I resolve that?

Answer 1

You need to apply your is_incrementable to operator++ to make sure you do not define that function when Y is not incrementable. You can do that with SFINAE like

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

template< typename T >
struct is_incrementable<T,std::void_t<decltype(++std::declval<T&>())>> : std::true_type { };

template<class Y>
class Point
{
    Y x, y;
public:
    Point(){}
    Point(Y _x, Y _y):x(_x),y(_y){}
    ~Point(){}

    template <typename T = Y, std::enable_if_t<is_incrementable<T>::value, bool> = true>
    //        ^            ^ this is needed as SFINAE only works with the current template parameters
    Point<T>& operator++()
    {
        ++x;
        ++y;
        return *this;
    }
};

and using

int main()
{
    using namespace std;
    cout << boolalpha;
    cout << is_incrementable<Point<int>>() << "\n";
    cout << is_incrementable<Point<string>>();
    return 0;
}

will print

true
false

which you can see in this live example