Metaprogramming library

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Metaprogramming library

Type traits
Type categories is_void (C++11) is_null_pointer (C++14) is_array (C++11) is_pointer (C++11) is_enum (C++11) is_union (C++11) is_class (C++11) is_function (C++11) is_object (C++11) is_scalar (C++11) is_compound (C++11) is_integral (C++11) is_floating_point (C++11) is_fundamental (C++11) is_arithmetic (C++11) is_reference (C++11) is_lvalue_reference (C++11) is_rvalue_reference (C++11) is_member_pointer (C++11) is_member_object_pointer (C++11) is_member_function_pointer (C++11) Type properties is_const (C++11) is_volatile (C++11) is_empty (C++11) is_polymorphic (C++11) is_final (C++14) is_abstract (C++11) is_aggregate (C++17) is_trivial (C++11) is_trivially_copyable (C++11) is_standard_layout (C++11) is_literal_type (C++11)(until C++20) is_pod (C++11)(deprecated in C++20) is_signed (C++11) is_unsigned (C++11) is_bounded_array (C++20) is_unbounded_array (C++20) is_scoped_enum (C++23) has_unique_object_representations (C++17) Type trait constants integral_constantbool_constanttrue_typefalse_type (C++11)(C++17)(C++11)(C++11) Metafunctions conjunction (C++17) disjunction (C++17) negation (C++17)	Supported operations is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11) is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11) is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11) is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11) is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11) is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11) is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11) is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11) has_virtual_destructor (C++11) is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17) Relationships and property queries is_same (C++11) is_base_of (C++11) is_convertibleis_nothrow_convertible (C++11)(C++20) is_layout_compatible (C++20) is_pointer_interconvertible_base_of (C++20) is_pointer_interconvertible_with_class (C++20) is_corresponding_member (C++20) reference_constructs_from_temporary (C++23) alignment_of (C++11) rank (C++11) extent (C++11) is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17) reference_converts_from_temporary (C++23) Type modifications remove_cvremove_constremove_volatile (C++11)(C++11)(C++11) add_cvadd_constadd_volatile (C++11)(C++11)(C++11) make_signed (C++11) make_unsigned (C++11) remove_reference (C++11) add_lvalue_referenceadd_rvalue_reference (C++11)(C++11) remove_pointer (C++11) add_pointer (C++11) remove_extent (C++11) remove_all_extents (C++11) Type transformations aligned_storage (C++11)(deprecated in C++23) aligned_union (C++11)(deprecated in C++23) decay (C++11) remove_cvref (C++20) enable_if (C++11) void_t (C++17) conditional (C++11) common_type (C++11) common_reference (C++20) underlying_type (C++11) result_ofinvoke_result (C++11)(until C++20)(C++17) type_identity (C++20)
Compile-time rational arithmetic
Compile-time integer sequences
integer_sequence (C++14)

 

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>
Primary type categories
is_void (C++11)	checks if a type is void (class template)
is_null_pointer (C++14)	checks if a type is std::nullptr_t (class template)
is_integral (C++11)	checks if a type is an integral type (class template)
is_floating_point (C++11)	checks if a type is a floating-point type (class template)
is_array (C++11)	checks if a type is an array type (class template)
is_enum (C++11)	checks if a type is an enumeration type (class template)
is_union (C++11)	checks if a type is an union type (class template)
is_class (C++11)	checks if a type is a non-union class type (class template)
is_function (C++11)	checks if a type is a function type (class template)
is_pointer (C++11)	checks if a type is a pointer type (class template)
is_lvalue_reference (C++11)	checks if a type is a lvalue reference (class template)
is_rvalue_reference (C++11)	checks if a type is a rvalue reference (class template)
is_member_object_pointer (C++11)	checks if a type is a pointer to a non-static member object (class template)
is_member_function_pointer (C++11)	checks if a type is a pointer to a non-static member function (class template)
Composite type categories
is_fundamental (C++11)	checks if a type is a fundamental type (class template)
is_arithmetic (C++11)	checks if a type is an arithmetic type (class template)
is_scalar (C++11)	checks if a type is a scalar type (class template)
is_object (C++11)	checks if a type is an object type (class template)
is_compound (C++11)	checks if a type is a compound type (class template)
is_reference (C++11)	checks if a type is either a lvalue reference or rvalue reference (class template)
is_member_pointer (C++11)	checks if a type is a pointer to an non-static member function or object (class template)
Type properties
is_const (C++11)	checks if a type is const-qualified (class template)
is_volatile (C++11)	checks if a type is volatile-qualified (class template)
is_trivial (C++11)	checks if a type is trivial (class template)
is_trivially_copyable (C++11)	checks if a type is trivially copyable (class template)
is_standard_layout (C++11)	checks if a type is a standard-layout type (class template)
is_pod (C++11)(deprecated in C++20)	checks if a type is a plain-old data (POD) type (class template)
is_literal_type (C++11)(deprecated in C++17)(removed in C++20)	checks if a type is a literal type (class template)
has_unique_object_representations (C++17)	checks if every bit in the type's object representation contributes to its value (class template)
is_empty (C++11)	checks if a type is a class (but not union) type and has no non-static data members (class template)
is_polymorphic (C++11)	checks if a type is a polymorphic class type (class template)
is_abstract (C++11)	checks if a type is an abstract class type (class template)
is_final (C++14)	checks if a type is a final class type (class template)
is_aggregate (C++17)	checks if a type is an aggregate type (class template)
is_signed (C++11)	checks if a type is a signed arithmetic type (class template)
is_unsigned (C++11)	checks if a type is an unsigned arithmetic type (class template)
is_bounded_array (C++20)	checks if a type is an array type of known bound (class template)
is_unbounded_array (C++20)	checks if a type is an array type of unknown bound (class template)
is_scoped_enum (C++23)	checks if a type is a scoped enumeration type (class template)

Supported operations
is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)	checks if a type has a constructor for specific arguments (class template)
is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11)	checks if a type has a default constructor (class template)
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)	checks if a type has a copy constructor (class template)
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11)	checks if a type can be constructed from an rvalue reference (class template)
is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)	checks if a type has a assignment operator for a specific argument (class template)
is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11)	checks if a type has a copy assignment operator (class template)
is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11)	checks if a type has a move assignment operator (class template)
is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11)	checks if a type has a non-deleted destructor (class template)
has_virtual_destructor (C++11)	checks if a type has a virtual destructor (class template)
is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (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
alignment_of (C++11)	obtains the type's alignment requirements (class template)
rank (C++11)	obtains the number of dimensions of an array type (class template)
extent (C++11)	obtains the size of an array type along a specified dimension (class template)

Type relationships
is_same (C++11)	checks if two types are the same (class template)
is_base_of (C++11)	checks if a type is derived from the other type (class template)
is_convertibleis_nothrow_convertible (C++11)(C++20)	checks if a type can be converted to the other type (class template)
is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)	checks if a type can be invoked (as if by std::invoke) with the given argument types (class template)
reference_constructs_from_temporary (C++23)	checks if a reference is bound to a temporary in direct-initialization (class template)
reference_converts_from_temporary (C++23)	checks if a reference is bound to a temporary in copy-initialization (class template)
is_layout_compatible (C++20)	checks if two types are layout-compatible (class template)
is_pointer_interconvertible_base_of (C++20)	checks if a type is a pointer-interconvertible (initial) base of another type (class template)
is_pointer_interconvertible_with_class (C++20)	checks if objects of a type are pointer-interconvertible with the specified subobject of that type (function template)
is_corresponding_member (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>
conjunction (C++17)	variadic logical AND metafunction (class template)
disjunction (C++17)	variadic logical OR metafunction (class template)
negation (C++17)	logical NOT metafunction (class template)

Base classes

Defined in header <type_traits>
integral_constantbool_constant (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
remove_cvremove_constremove_volatile (C++11)(C++11)(C++11)	removes const or/and volatile specifiers from the given type (class template)
add_cvadd_constadd_volatile (C++11)(C++11)(C++11)	adds const or/and volatile specifiers to the given type (class template)
References
remove_reference (C++11)	removes a reference from the given type (class template)
add_lvalue_referenceadd_rvalue_reference (C++11)(C++11)	adds a lvalue or rvalue reference to the given type (class template)
Pointers
remove_pointer (C++11)	removes a pointer from the given type (class template)
add_pointer (C++11)	adds a pointer to the given type (class template)
Sign modifiers
make_signed (C++11)	makes the given integral type signed (class template)
make_unsigned (C++11)	makes the given integral type unsigned (class template)
Arrays
remove_extent (C++11)	removes one extent from the given array type (class template)
remove_all_extents (C++11)	removes all extents from the given array type (class template)

Miscellaneous transformations

Defined in header <type_traits>
aligned_storage (C++11)(deprecated in C++23)	defines the type suitable for use as uninitialized storage for types of given size (class template)
aligned_union (C++11)(deprecated in C++23)	defines the type suitable for use as uninitialized storage for all given types (class template)
decay (C++11)	applies type transformations as when passing a function argument by value (class template)
remove_cvref (C++20)	combines std::remove_cv and std::remove_reference (class template)
enable_if (C++11)	conditionally removes a function overload or template specialization from overload resolution (class template)
conditional (C++11)	chooses one type or another based on compile-time boolean (class template)
common_type (C++11)	determines the common type of a group of types (class template)
common_referencebasic_common_reference (C++20)	determines the common reference type of a group of types (class template)
underlying_type (C++11)	obtains the underlying integer type for a given enumeration type (class template)
result_ofinvoke_result (C++11)(removed in C++20)(C++17)	deduces the result type of invoking a callable object with a set of arguments (class template)
void_t (C++17)	void variadic alias template (alias template)
type_identity (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>
integer_sequence (C++14)	implements compile-time sequence of integers (class template)