std::is_const

 

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Type traits
Type categories is_void (C++11) is_null_pointer (C++11)(DR*) 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_reference (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_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_implicit_lifetime (C++23) is_trivial (C++11)(deprecated in C++26) 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_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) reference_converts_from_temporary (C++23) is_base_of (C++11) is_virtual_base_of (C++26) 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) 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) result_ofinvoke_result (C++11)(until C++20*)(C++17) conditional (C++11) common_type (C++11) common_reference (C++20) underlying_type (C++11) type_identity (C++20) enable_if (C++11) void_t (C++17)
Compile-time rational arithmetic
Compile-time integer sequences
integer_sequence (C++14)

 

Defined in header <type_traits>
template<class T > struct is_const;		(since C++11)

std::is_const is a UnaryTypeTrait.

If T is a const-qualified type (that is, const, or constvolatile), provides the member constant value equal to true. For any other type, value is false.

If the program adds specializations for std::is_const or std::is_const_v, the behavior is undefined.

Template parameters

T

-

a type to check

Helper variable template

template<class T > constexprbool is_const_v = is_const<T>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

true if T is a const-qualified type, false otherwise (public static member constant)

Member functions

operator bool	converts the object to bool, returns value (public member function)
operator() (C++14)	returns value (public member function)

Member types

Type	Definition
value_type	bool
type	std::integral_constant<bool, value>

Notes

If T is a reference type then is_const<T>::value is always false. The proper way to check a potentially-reference type for constness is to remove the reference: is_const<typename remove_reference<T>::type>.

Possible implementation

template<class T>struct is_const :std::false_type{};template<class T>struct is_const<const T>:std::true_type{};

Example

Run this code

#include <type_traits>   static_assert(std::is_same_v<constint*, intconst*>, "Remember, constness binds tightly inside pointers."); static_assert(!std::is_const_v<int>); static_assert(std::is_const_v<constint>); static_assert(!std::is_const_v<int*>); static_assert(std::is_const_v<int*const>, "Because the pointer itself can't be changed but the int pointed at can."); static_assert(!std::is_const_v<constint*>, "Because the pointer itself can be changed but not the int pointed at."); static_assert(!std::is_const_v<constint&>); static_assert(std::is_const_v<std::remove_reference_t<constint&>>);   struct S {void foo()const{}void bar()const{}};   int main(){// A const member function is const in a different way:   static_assert(!std::is_const_v<decltype(&S::foo)>, "Because &S::foo is a pointer.");   using S_mem_fun_ptr =void(S::*)()const;   S_mem_fun_ptr sfp =&S::foo; sfp =&S::bar;// OK, can be re-pointed static_assert(!std::is_const_v<decltype(sfp)>, "Because sfp is the same pointer type and thus can be re-pointed.");   const S_mem_fun_ptr csfp =&S::foo;// csfp = &S::bar; // Error static_assert(std::is_const_v<decltype(csfp)>, "Because csfp cannot be re-pointed.");}

See also

is_volatile (C++11)	checks if a type is volatile-qualified (class template)
as_const (C++17)	obtains a reference to const to its argument (function template)