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is_[un]signed
unclear for non-arithmetic typesSection: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2012-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The pre-conditions for the trait is_signed
allow for any types as template arguments,
including non-arithmetic ones.
But the current wording in Table 49 defining the predicate condition,
is_arithmetic<T>::value && T(-1) < T(0)
looks like real code and so leaves it open whether such argument types would create a well-formed instantiation of the trait template or not. As written this definition would lead to a hard instantiation error for a non-arithmetic type like e.g.
struct S {};
I would suggest that the wording clarifies that the instantiation would be valid for such types as well, by means of a specification that is not an exact code pattern. This also reflects how existing implementations behave.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change Table 49 as indicated:
Template | Condition | Preconditions |
---|---|---|
template <class T> struct is_signed;
|
If is_arithmetic<T>::value is true, the same result asintegral_constant<bool, T(-1) < T(0)>::value ;otherwise, false. |
|
template <class T> struct is_unsigned;
|
If is_arithmetic<T>::value is true, the same result asintegral_constant<bool, T(0) < T(-1)>::value ;otherwise, false. |