C++ named requirements: UnaryTypeTrait (since C++11)

A UnaryTypeTrait is a class template that describes a property of its template type parameter with an integral_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 integral_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)
std::tuple_size<std::tuple> (C++11)	obtains the size of tuple at compile time (class template specialization)
std::tuple_size<std::array> (C++11)	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_sizevariant_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)