Metaprogramming library
C++ provides metaprogramming facilities, such as type traits, compile-time rational arithmetic, and compile-time integer sequences.
Type property
These type traits define compile-time template-based interfaces to query the properties of types.
Attempting to specialize a template defined in the <type_traits> header and described in this section results in undefined behavior.
A template defined in the <type_traits> header may be instantiated with an incomplete type unless otherwise specified, notwithstanding the general prohibition against instantiating standard library templates with incomplete types.
Defined in header <type_traits> |
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Primary type categories |
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(C++11)
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checks if a type is void (class template) |
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(C++14)
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checks if a type is std::nullptr_t (class template) |
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(C++11)
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checks if a type is an integral type (class template) |
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(C++11)
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checks if a type is a floating-point type (class template) |
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(C++11)
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checks if a type is an array type (class template) |
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(C++11)
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checks if a type is an enumeration type (class template) |
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(C++11)
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checks if a type is an union type (class template) |
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(C++11)
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checks if a type is a non-union class type (class template) |
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(C++11)
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checks if a type is a function type (class template) |
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(C++11)
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checks if a type is a pointer type (class template) |
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(C++11)
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checks if a type is a lvalue reference (class template) |
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(C++11)
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checks if a type is a rvalue reference (class template) |
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(C++11)
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checks if a type is a pointer to a non-static member object (class template) |
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(C++11)
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checks if a type is a pointer to a non-static member function (class template) |
Composite type categories |
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(C++11)
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checks if a type is a fundamental type (class template) |
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(C++11)
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checks if a type is an arithmetic type (class template) |
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(C++11)
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checks if a type is a scalar type (class template) |
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(C++11)
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checks if a type is an object type (class template) |
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(C++11)
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checks if a type is a compound type (class template) |
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(C++11)
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checks if a type is either a lvalue reference or rvalue reference (class template) |
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(C++11)
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checks if a type is a pointer to an non-static member function or object (class template) |
Type properties |
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(C++11)
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checks if a type is const-qualified (class template) |
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(C++11)
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checks if a type is volatile-qualified (class template) |
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(C++11)
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checks if a type is trivial (class template) |
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(C++11)
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checks if a type is trivially copyable (class template) |
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(C++11)
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checks if a type is a standard-layout type (class template) |
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(C++11)(deprecated in C++20)
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checks if a type is a plain-old data (POD) type (class template) |
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(C++11)(deprecated in C++17)(removed in C++20)
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checks if a type is a literal type (class template) |
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(C++17)
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checks if every bit in the type's object representation contributes to its value (class template) |
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(C++11)
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checks if a type is a class (but not union) type and has no non-static data members (class template) |
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(C++11)
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checks if a type is a polymorphic class type (class template) |
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(C++11)
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checks if a type is an abstract class type (class template) |
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(C++14)
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checks if a type is a final class type (class template) |
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(C++17)
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checks if a type is an aggregate type (class template) |
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(C++11)
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checks if a type is a signed arithmetic type (class template) |
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(C++11)
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checks if a type is an unsigned arithmetic type (class template) |
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(C++20)
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checks if a type is an array type of known bound (class template) |
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(C++20)
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checks if a type is an array type of unknown bound (class template) |
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(C++23)
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checks if a type is a scoped enumeration type (class template) |
Supported operations |
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(C++11)(C++11)(C++11)
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checks if a type has a constructor for specific arguments (class template) |
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(C++11)(C++11)(C++11)
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checks if a type has a default constructor (class template) |
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(C++11)(C++11)(C++11)
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checks if a type has a copy constructor (class template) |
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(C++11)(C++11)(C++11)
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checks if a type can be constructed from an rvalue reference (class template) |
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(C++11)(C++11)(C++11)
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checks if a type has a assignment operator for a specific argument (class template) |
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(C++11)(C++11)(C++11)
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checks if a type has a copy assignment operator (class template) |
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(C++11)(C++11)(C++11)
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checks if a type has a move assignment operator (class template) |
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(C++11)(C++11)(C++11)
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checks if a type has a non-deleted destructor (class template) |
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(C++11)
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checks if a type has a virtual destructor (class template) |
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(C++17)(C++17)(C++17)(C++17)
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checks if objects of a type can be swapped with objects of same or different type (class template) |
Property queries |
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(C++11)
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obtains the type's alignment requirements (class template) |
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(C++11)
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obtains the number of dimensions of an array type (class template) |
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(C++11)
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obtains the size of an array type along a specified dimension (class template) |
Type relationships |
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(C++11)
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checks if two types are the same (class template) |
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(C++11)
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checks if a type is derived from the other type (class template) |
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(C++11)(C++20)
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checks if a type can be converted to the other type (class template) |
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(C++17)
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checks if a type can be invoked (as if by std::invoke) with the given argument types (class template) |
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(C++23)
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checks if a reference is bound to a temporary in direct-initialization (class template) |
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(C++23)
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checks if a reference is bound to a temporary in copy-initialization (class template) |
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(C++20)
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checks if two types are layout-compatible (class template) |
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(C++20)
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checks if a type is a pointer-interconvertible (initial) base of another type (class template) |
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(C++20)
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checks if objects of a type are pointer-interconvertible with the specified subobject of that type (function template) |
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(C++20)
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checks if two specified members correspond to each other in the common initial subsequence of two specified types (function template) |
Operations on traits
Defined in header <type_traits> |
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(C++17)
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variadic logical AND metafunction (class template) |
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(C++17)
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variadic logical OR metafunction (class template) |
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(C++17)
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logical NOT metafunction (class template) |
Base classes
Defined in header <type_traits> |
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(C++11)(C++17)
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compile-time constant of specified type with specified value (class template) |
Two specializations of std::integral_constant for the type bool are provided:
Defined in header <type_traits> |
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Specializations |
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| Type | Definition |
true_type |
std::integral_constant<bool, true> |
false_type |
std::integral_constant<bool, false> |
Type modifications
These type traits apply modifications on a template parameter, and declare (sometimes conditionally) the type member typedef as the resulting type.
Attempting to specialize a template defined in the <type_traits> header and described in this section results in undefined behavior, except that std::common_type and std::basic_common_reference (since C++20) may be specialized as required in description.
A template defined in the <type_traits> header may be instantiated with an incomplete type unless otherwise specified, notwithstanding the general prohibition against instantiating standard library templates with incomplete types.
Defined in header <type_traits> |
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Const-volatility specifiers |
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(C++11)(C++11)(C++11)
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removes const or/and volatile specifiers from the given type (class template) |
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(C++11)(C++11)(C++11)
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adds const or/and volatile specifiers to the given type (class template) |
References |
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(C++11)
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removes a reference from the given type (class template) |
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(C++11)(C++11)
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adds a lvalue or rvalue reference to the given type (class template) |
Pointers |
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(C++11)
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removes a pointer from the given type (class template) |
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(C++11)
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adds a pointer to the given type (class template) |
Sign modifiers |
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(C++11)
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makes the given integral type signed (class template) |
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(C++11)
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makes the given integral type unsigned (class template) |
Arrays |
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(C++11)
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removes one extent from the given array type (class template) |
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(C++11)
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removes all extents from the given array type (class template) |
Miscellaneous transformations
Defined in header <type_traits> |
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(C++11)(deprecated in C++23)
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defines the type suitable for use as uninitialized storage for types of given size (class template) |
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(C++11)(deprecated in C++23)
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defines the type suitable for use as uninitialized storage for all given types (class template) |
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(C++11)
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applies type transformations as when passing a function argument by value (class template) |
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(C++20)
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combines std::remove_cv and std::remove_reference (class template) |
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(C++11)
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conditionally removes a function overload or template specialization from overload resolution (class template) |
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(C++11)
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chooses one type or another based on compile-time boolean (class template) |
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(C++11)
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determines the common type of a group of types (class template) |
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(C++20)
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determines the common reference type of a group of types (class template) |
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(C++11)
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obtains the underlying integer type for a given enumeration type (class template) |
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(C++11)(removed in C++20)(C++17)
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deduces the result type of invoking a callable object with a set of arguments (class template) |
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(C++17)
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void variadic alias template (alias template) |
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(C++20)
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returns the type argument unchanged (class template) |
Compile-time rational arithmetic
The header <ratio> provides types and functions for manipulating and storing compile-time ratios.
Compile-time integer sequences
Defined in header <utility> |
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(C++14)
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implements compile-time sequence of integers (class template) |
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