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std::has_unique_object_representations

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Defined in header <type_traits>
template< class T >
struct has_unique_object_representations;
 (since C++17)

std::has_unique_object_representations is a UnaryTypeTrait.

If T is TriviallyCopyable and if any two objects of type T with the same value have the same object representation, provides the member constant value equal true. For any other type, value is false.

For the purpose of this trait, two arrays have the same value if their elements have the same values, two non-union classes have the same value if their direct subobjects have the same value, and two unions have the same value if they have the same active member and the value of that member is the same.

It is implementation-defined which scalar types satisfy this trait, but unsigned(until C++20) integer types that do not use padding bits are guaranteed to have unique object representations.

The behavior is undefined if T is an incomplete type other than (possibly cv-qualified) void or array of unknown bound.

If the program adds specializations for std::has_unique_object_representations or std::has_unique_object_representations_v, the behavior is undefined.

Contents

[edit] Template parameters

T - a type to check

[edit] Helper variable template

template< class T >

constexpr bool has_unique_object_representations_v =

    has_unique_object_representations<T>::value;
 (since C++17)
[edit]

Inherited from std::integral_constant

Member constants

value
[static]
 true if T has unique object representations, 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>

[edit] Notes

This trait was introduced to make it possible to determine whether a type can be correctly hashed by hashing its object representation as a byte array.

Feature-test macro Value Std Feature
__cpp_lib_has_unique_object_representations 201606L (C++17) std::has_unique_object_representations

[edit] Example

Run this code
#include <cstdint>
#include <type_traits>
 
struct unpadded
{
    std::uint32_t a, b;
};
 
struct likely_padded
{
    std::uint8_t c;
    std::uint16_t st;
    std::uint32_t i;
};
 
int main()
{
    // Every value of a char corresponds to exactly one object representation.
    static_assert(std::has_unique_object_representations_v<char>);
    // For IEC 559 floats, assertion passes because the value NaN has
    // multiple object representations.
    static_assert(!std::has_unique_object_representations_v<float>);
 
    // Should succeed in any sane implementation because unpadded
    // is typically not padded, and std::uint32_t cannot contain padding bits.
    static_assert(std::has_unique_object_representations_v<unpadded>);
    // Fails in most implementations because padding bits are inserted
    // between the data members c and st for the purpose of aligning st to 16 bits.
    static_assert(!std::has_unique_object_representations_v<likely_padded>);
 
    // Notable architectural divergence:
    static_assert(std::has_unique_object_representations_v<bool>);  // x86
 // static_assert(!std::has_unique_object_representations_v<bool>); // ARM
}

[edit] See also

(C++11)
 checks if a type is a standard-layout type
(class template) [edit]
(C++11)
 hash function object
(class template) [edit]
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