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- [#predicate]
- [section Predicate]
- A ['predicate] (or unary predicate) is a
- [link metafunction_class template metafunction class] taking one argument and
- returning a [link boxed_value boxed value] of type `bool`.
- For example the following predicate checks if its argument is the boxed `char`
- value `x`:
- struct is_x
- {
- template <class C>
- struct apply
- {
- static constexpr bool value = (C::value == 'x');
- using type = apply;
- };
-
- using type = is_x;
- };
- [endsect]
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