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Revision as of 23:39, 26 August 2022
C++ provides metaprogramming facilities, such as type traits, compile-time rational arithmetic, and compile-time integer sequences.
Contents |
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> | |
Primary type categories | |
| (C++11) |
checks if a type is void (class template) |
| (C++14) |
checks if a type is std::nullptr_t (class template) |
| (C++11) |
checks if a type is an integral type (class template) |
| (C++11) |
checks if a type is a floating-point type (class template) |
| (C++11) |
checks if a type is an array type (class template) |
| (C++11) |
checks if a type is an enumeration type (class template) |
| (C++11) |
checks if a type is a union type (class template) |
| (C++11) |
checks if a type is a non-union class type (class template) |
| (C++11) |
checks if a type is a function type (class template) |
| (C++11) |
checks if a type is a pointer type (class template) |
| (C++11) |
checks if a type is an lvalue reference (class template) |
| (C++11) |
checks if a type is an rvalue reference (class template) |
| (C++11) |
checks if a type is a non-static member object pointer (class template) |
| (C++11) |
checks if a type is a non-static member function pointer (class template) |
Composite type categories | |
| (C++11) |
checks if a type is a fundamental type (class template) |
| (C++11) |
checks if a type is an arithmetic type (class template) |
| (C++11) |
checks if a type is a scalar type (class template) |
| (C++11) |
checks if a type is an object type (class template) |
| (C++11) |
checks if a type is a compound type (class template) |
| (C++11) |
checks if a type is either an lvalue reference or rvalue reference (class template) |
| (C++11) |
checks if a type is a pointer to a non-static member function or object (class template) |
Type properties | |
| (C++11) |
checks if a type is const-qualified (class template) |
| (C++11) |
checks if a type is volatile-qualified (class template) |
| (C++11) |
checks if a type is trivial (class template) |
| (C++11) |
checks if a type is trivially copyable (class template) |
| (C++11) |
checks if a type is a standard-layout type (class template) |
| (C++11)(deprecated in C++20) |
checks if a type is a plain-old data (POD) type (class template) |
| (C++11)(deprecated in C++17)(removed in C++20) |
checks if a type is a literal type (class template) |
| checks if every bit in the type's object representation contributes to its value (class template) | |
| (C++11) |
checks if a type is a class (but not union) type and has no non-static data members (class template) |
| (C++11) |
checks if a type is a polymorphic class type (class template) |
| (C++11) |
checks if a type is an abstract class type (class template) |
| (C++14) |
checks if a type is a final class type (class template) |
| (C++17) |
checks if a type is an aggregate type (class template) |
| (C++11) |
checks if a type is a signed arithmetic type (class template) |
| (C++11) |
checks if a type is an unsigned arithmetic type (class template) |
| (C++20) |
checks if a type is an array type of known bound (class template) |
| (C++20) |
checks if a type is an array type of unknown bound (class template) |
| (C++23) |
checks if a type is a scoped enumeration type (class template) |
Supported operations | |
| (C++11)(C++11)(C++11) |
checks if a type has a constructor for specific arguments (class template) |
| checks if a type has a default constructor (class template) | |
| (C++11)(C++11)(C++11) |
checks if a type has a copy constructor (class template) |
| (C++11)(C++11)(C++11) |
checks if a type can be constructed from an rvalue reference (class template) |
| (C++11)(C++11)(C++11) |
checks if a type has an assignment operator for a specific argument (class template) |
| (C++11)(C++11)(C++11) |
checks if a type has a copy assignment operator (class template) |
| (C++11)(C++11)(C++11) |
checks if a type has a move assignment operator (class template) |
| (C++11)(C++11)(C++11) |
checks if a type has a non-deleted destructor (class template) |
| (C++11) |
checks if a type has a virtual destructor (class template) |
| (C++17)(C++17)(C++17)(C++17) |
checks if objects of a type can be swapped with objects of same or different type (class template) |
Property queries | |
| (C++11) |
obtains the type's alignment requirements (class template) |
| (C++11) |
obtains the number of dimensions of an array type (class template) |
| (C++11) |
obtains the size of an array type along a specified dimension (class template) |
Type relationships | |
| (C++11) |
checks if two types are the same (class template) |
| (C++11) |
checks if a type is a base of the other type (class template) |
| (C++11)(C++20) |
checks if a type can be converted to the other type (class template) |
| checks if a type can be invoked (as if by std::invoke) with the given argument types (class template) | |
| checks if a reference is bound to a temporary in direct-initialization (class template) | |
| checks if a reference is bound to a temporary in copy-initialization (class template) | |
| (C++20) |
checks if two types are layout-compatible (class template) |
| checks if a type is a pointer-interconvertible (initial) base of another type (class template) | |
| checks if objects of a type are pointer-interconvertible with the specified subobject of that type (function template) | |
| (C++20) |
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> | |
| (C++17) |
variadic logical AND metafunction (class template) |
| (C++17) |
variadic logical OR metafunction (class template) |
| (C++17) |
logical NOT metafunction (class template) |
Base classes
| Defined in header
<type_traits> | |
| (C++11)(C++17) |
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> | |
Specializations | |
| 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> | |
Const-volatility specifiers | |
| (C++11)(C++11)(C++11) |
removes const and/or volatile specifiers from the given type (class template) |
| (C++11)(C++11)(C++11) |
adds const and/or volatile specifiers to the given type (class template) |
References | |
| (C++11) |
removes a reference from the given type (class template) |
| (C++11)(C++11) |
adds an lvalue or rvalue reference to the given type (class template) |
Pointers | |
| (C++11) |
removes a pointer from the given type (class template) |
| (C++11) |
adds a pointer to the given type (class template) |
Sign modifiers | |
| (C++11) |
obtains the corresponding signed type for the given integral type (class template) |
| (C++11) |
obtains the corresponding signed type for the given integral type (class template) |
Arrays | |
| (C++11) |
removes one extent from the given array type (class template) |
| (C++11) |
removes all extents from the given array type (class template) |
Miscellaneous transformations
| Defined in header
<type_traits> | |
| (C++11)(deprecated in C++23) |
defines the type suitable for use as uninitialized storage for types of given size (class template) |
| (C++11)(deprecated in C++23) |
defines the type suitable for use as uninitialized storage for all given types (class template) |
| (C++11) |
applies type transformations as when passing a function argument by value (class template) |
| (C++20) |
combines std::remove_cv and std::remove_reference (class template) |
| (C++11) |
conditionally removes a function overload or template specialization from overload resolution (class template) |
| (C++11) |
chooses one type or another based on compile-time boolean (class template) |
| (C++11) |
determines the common type of a group of types (class template) |
| determines the common reference type of a group of types (class template) | |
| (C++11) |
obtains the underlying integer type for a given enumeration type (class template) |
| (C++11)(removed in C++20)(C++17) |
deduces the result type of invoking a callable object with a set of arguments (class template) |
| (C++17) |
void variadic alias template (alias template) |
| (C++20) |
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> | |
| (C++14) |
implements compile-time sequence of integers (class template) |
