21 Metaprogramming library [meta]

21.3 Metaprogramming and type traits [type.traits]

21.3.2 Requirements [meta.rqmts]

1
#
A Cpp17UnaryTypeTrait describes a property of a type.
It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the property being described.
It shall be Cpp17DefaultConstructible, Cpp17CopyConstructible, and publicly and unambiguously derived, directly or indirectly, from its base characteristic, which is a specialization of the template integral_constant ([meta.help]), with the arguments to the template integral_constant determined by the requirements for the particular property being described.
The member names of the base characteristic shall not be hidden and shall be unambiguously available in the Cpp17UnaryTypeTrait.
2
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A Cpp17BinaryTypeTrait describes a relationship between two types.
It shall be a class template that takes two template type arguments and, optionally, additional arguments that help define the relationship being described.
It shall be Cpp17DefaultConstructible, Cpp17CopyConstructible, and publicly and unambiguously derived, directly or indirectly, from its base characteristic, which is a specialization of the template integral_constant ([meta.help]), with the arguments to the template integral_constant determined by the requirements for the particular relationship being described.
The member names of the base characteristic shall not be hidden and shall be unambiguously available in the Cpp17BinaryTypeTrait.
3
#
A Cpp17TransformationTrait modifies a property of a type.
It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the modification.
It shall define a publicly accessible nested type named type, which shall be a synonym for the modified type.
4
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Unless otherwise specified, the behavior of a program that adds specializations for any of the templates specified in [type.traits] is undefined.
5
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Unless otherwise specified, an incomplete type may be used to instantiate a template specified in [type.traits].
The behavior of a program is undefined if:
  • (5.1)
    an instantiation of a template specified in [type.traits] directly or indirectly depends on an incompletely-defined object type T, and
  • (5.2)
    that instantiation could yield a different result were T hypothetically completed.