Std::hash

Disabled specializations do not satisfy Hashdo not satisfy FunctionObjectand following values are all false :. Each header that declares the template std::hash also provides enabled specializations of std::hash for the std::hash types:. On top of that, std::hash, some headers also provide other enabled std::hash specializations for library types see below, std::hash.

The hash template defines a function object that implements a hash function. Instances of this function object define an operator that:. The hash template is both CopyConstructible and Destructible. The actual hash functions are implementation-dependent and are not required to fulfill any other quality criteria except those specified above. Notably, some implementations use trivial identity hash functions which map an integer to itself.

Std::hash

The hash template defines a function object that implements a hash function. Instances of this function object satisfy Hash. In particular, they define an operator that:. The hash template is both CopyConstructible and Destructible. The actual hash functions are implementation-dependent and are not required to fulfill any other quality criteria except those specified above. Notably, some implementations use trivial identity hash functions which map an integer to itself. In other words, these hash functions are designed to work with unordered associative containers, but not as cryptographic hashes, for example. There is no specialization for C strings. Note: additional specializations for std::pair and the standard container types, as well as utility functions to compose hashes are available in boost. Demonstrates the computation of a hash for std::string , a type that already has a hash specialization.

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Types implementing Hash are able to be hash ed with an instance of Hasher. You can derive Hash with [derive Hash ] if all fields implement Hash. The resulting hash will be the combination of the values from calling hash on each field. If you need more control over how a value is hashed, you can of course implement the Hash trait yourself:. When implementing both Hash and Eq , it is important that the following property holds:. In other words, if two keys are equal, their hashes must also be equal.

Each specialization of this template is either enabled "untainted" or disabled "poisoned". In other words, they exist, but cannot be used. Instances of this function object satisfy Hash. In particular, they define an operator const that:. All explicit and partial specializations of hash provided by the standard library are DefaultConstructible , CopyAssignable , Swappable and Destructible. User-provided specializations of hash also must meet those requirements.

Std::hash

Each specialization of this template is either enabled "untainted" or disabled "poisoned". In other words, they exist, but cannot be used. Instances of this function object satisfy Hash.

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Hire With Us. From cppreference. Demonstrates how to specialize std::hash for a user defined type. Note: additional specializations for std::pair and the standard container types, as well as utility functions to compose hashes are available in boost. Save Article. Feeds this value into the given Hasher. Work Experiences. Please go through our recently updated Improvement Guidelines before submitting any improvements. Demonstrates the computation of a hash for std:: string , a type that already has a hash specialization. Demonstrates creation of a hash function for a user defined type. Submit your entries in Dev Scripter today. Vote for difficulty :. For all std::hash specializations provided by the standard library except the following, all their member functions are noexcept :.

Disabled specializations do not satisfy Hash , do not satisfy FunctionObject , and following values are all false :. Each header that declares the template std::hash also provides enabled specializations of std::hash for the following types:.

Thank you for your valuable feedback! The actual hash functions are implementation-dependent and are not required to fulfill any other quality criteria except those specified above. Improve Improve. Demonstrates the computation of a hash for std:: string , a type that already has a hash specialization. Add Other Experiences. Last Updated : 24 Oct, Each header that declares the template std::hash also provides enabled specializations of std::hash for the following types:. Language support library. In other words, these hash functions are designed to work with unordered associative containers, but not as cryptographic hashes, for example. Types implementing Hash are able to be hash ed with an instance of Hasher. Instances of this function object satisfy Hash.