SFINAE to detect operation when it is forbidden to call with temporary

Question

It is possible to use the following pattern to detect whether or not x can be applied to T

template<class T, class Enable = void>
struct CanDoX: std::false_type{};

template<class T>
struct CanDoX<T, std::void_t<decltype(x(std::declval<T>()))>>:std::true_type{};

What if x has been defined as

Foo const& x(Bar const&&) = delete;
Foo const& x(Bar const& val) 
    {return val.get();}

Now CanDoX<Bar>::value is false, because no overload accepts a temporary. Somehow, there has to be a Bar val before the test for x(val).

So, is this code working or not? It's not clear to me exactly what you are asking. — NathanOliver, Aug 28 '19 at 16:51
@NathanOliver Make CanDoX correctly detect the permitted overload. — user877329, Aug 28 '19 at 17:00
_Somehow, there has to be a Bar val before the test for x(val)_ - isn't that what `std::declval()` does? — Maxim Egorushkin, Aug 28 '19 at 17:08

score 0 · Accepted Answer · edited Aug 28 '19 at 17:46

0

std::declval<T>() returns a T&& so the better match is Foo const& x(Bar const&&).

but you can use lvalue reference inside std::declval to get lvalue:

template<class T>
struct CanDoX<T, std::void_t<decltype(x(std::declval<T&>()))>>:std::true_type{};

or

template<class T>
struct CanDoX<T, std::void_t<decltype(x(std::declval<const T&>()))>>:std::true_type{};

Depending of your needs.

edited Aug 28 '19 at 17:46

Brian Bi

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answered Aug 28 '19 at 17:32

Jarod42

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SFINAE to detect operation when it is forbidden to call with temporary

1 Answers1