Metaprogramming library (since C++11)

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C++
Metaprogramming library
Type traits
Type categories
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++11)
(C++11)
(C++14)
(C++11)
(C++17)
(C++23)
(C++11) (deprecated in C++26)
(C++11)
(C++11)
(C++11) ( until C++20* )
(C++11) (deprecated in C++20)
(C++11)
(C++11)
(C++20)
(C++20)
(C++23)
(C++17)
Type trait constants
(C++11) (C++17) (C++11) (C++11)
Metafunctions
(C++17)
(C++17)
(C++17)
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11) (deprecated in C++23)
(C++11) (deprecated in C++23)
(C++11)
(C++20)
(C++11)
(C++17)

(C++11)
(C++11)
(C++20)
(C++11)
(C++11) ( until C++20* ) (C++17)
(C++20)
Compile-time rational arithmetic
Compile-time integer sequences
(C++14)

C++ provides metaprogramming facilities, such as type traits, compile-time rational arithmetic, and compile-time integer sequences.

Definitions

The following types are collectively called referenceable types :

For any referenceable type T , a reference to it can be created [1] .

  1. ↑ For reference types, this can be done via reference collapsing .

Type traits

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 listed in this page 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.

Base classes

Most of non-transforming type traits need to be publicly and unambiguously derived from std::integral_constant in order to satisfy the requirements of UnaryTypeTrait or BinaryTypeTrait .

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>
Type Definition
true_type std:: integral_constant < bool , true >
false_type std:: integral_constant < bool , false >

Unary type traits

Unary type traits can be used to query the boolean properties of a type at compile time.

All these type traits satisfy UnaryTypeTrait , the base characteristic of each type trait is either std::true_type or std::false_type , depending on whether the corresponding condition is met.

Primary type categories
Defined in header <type_traits>
(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
Defined in header <type_traits>
(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
Defined in header <type_traits>
(C++11)
checks if a type is const-qualified
(class template)
(C++11)
checks if a type is volatile-qualified
(class template)
(C++11) (deprecated in C++26)
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)
(C++17)
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++23)
checks if a type is an implicit-lifetime 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
Defined in header <type_traits>
(C++11) (C++11) (C++11)
checks if a type has a constructor for specific arguments
(class template)
(C++11) (C++11) (C++11)
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)
(C++23)
checks if a reference is bound to a temporary in direct-initialization
(class template)
(C++23)
checks if a reference is bound to a temporary in copy-initialization
(class template)

Property queries

Property query traits can be used to query the integral properties of a type at compile time.

All these type traits satisfy UnaryTypeTrait , the base characteristic of each type trait is std:: integral_constant < std:: size_t , Value > , where Value is the query result of the corresponding property.

Defined in header <type_traits>
(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

Type relationship traits can be used to query relationships between types at compile time.

All these type traits satisfy BinaryTypeTrait , the base characteristic of each type trait is either std::true_type or std::false_type , depending on whether the corresponding condition is met.

Defined in header <type_traits>
(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++26)
checks if a type is a virtual base of the other type
(class template)
(C++11) (C++20)
checks if a type can be converted to the other type
(class template)
(C++20)
checks if two types are layout-compatible
(class template)
(C++20)
checks if a type is a pointer-interconvertible (initial) base of another type
(class template)
(C++17)
checks if a type can be invoked (as if by std::invoke ) with the given argument types
(class template)

Type transformations

Type transformation traits transform one type to another following some predefined rules.

All these type traits satisfy TransformationTrait .

Const-volatility specifiers
Defined in header <type_traits>
(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
Defined in header <type_traits>
(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)
Sign modifiers
Defined in header <type_traits>
(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
Defined in header <type_traits>
(C++11)
removes one extent from the given array type
(class template)
(C++11)
removes all extents from the given array type
(class template)
Pointers
Defined in header <type_traits>
(C++11)
removes a pointer from the given type
(class template)
(C++11)
adds a pointer to the given type
(class template)
Other 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)
(C++20)
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)

Logical operations

Logical operator traits apply logical operators to other type 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)

Member relationships

Defined in header <type_traits>
(C++20)
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)

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)
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