# C++ named requirements: UnaryTypeTrait

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< cpp‎ | named req

A UnaryTypeTrait is a class template that describes a property of its template type parameter with a intergral_constant (typically bool_constant) specialization.

### Requirements

- DefaultConstructible and CopyConstructible
- Takes one template type parameter (additional template parameters are optional and allowed)
- Publicly and unambiguously derived from a specialization of intergral_constant, known as its base characteristic
- The member names of the base characteristic are not hidden and are unambiguously available

### Standard library

The following type properties all satisfy UnaryTypeTrait.

- Primary type categories
- Composite type categories
- Type properties
- Supported operations
- Property queries

The following standard library class templates also satisfy UnaryTypeTrait.

integral_constant (C++11) | compile-time constant of specified type with specified value (class template)
negation (C++17) | logical NOT metafunction (class template)
tuple_size | obtains the size of `tuple` at compile time (class template specialization)
std::tuple_size<std::array> | obtains the size of an `array` (class template specialization)
std::tuple_size<std::pair> (C++11) | obtains the size of a `pair` (class template specialization)
variant_size variant_size_v (C++17) | obtains the size of the variant's list of alternatives at compile time (class template) (variable template)
is_bind_expression (C++11) | indicates that an object is `std::bind` expression or can be used as one (class template)
is_placeholder (C++11) | indicates that an object is a standard placeholder or can be used as one (class template)
is_execution_policy (C++17) | test whether a class represents an execution policy (class template)

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