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 std::hash operator that:, std::hash.

The hash class is default constructible, which means that one can construct this object without any arguments or initialization values. It is used to get the hash value of the argument that is being passed to it. Member functions: This Hash class only has one member function:. Skip to content. Change Language. Open In App.

Std::hash

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. HashMap and HashSet both rely on this behavior. Violating this property is a logic error. The behavior resulting from a logic error is not specified, but users of the trait must ensure that such logic errors do not result in undefined behavior. This means that unsafe code must not rely on the correctness of these methods. Implementations of hash should ensure that the data they pass to the Hasher are prefix-free. That is, values which are not equal should cause two different sequences of values to be written, and neither of the two sequences should be a prefix of the other.

This means that unsafe code must not rely on the correctness of these methods, std::hash. From cppreference.

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:. On top of that, some headers also provide other enabled std::hash specializations for library types see below. For all std::hash specializations provided by the standard library except the following, all their member functions are noexcept :. The actual hash functions are implementation-dependent and are not required to fulfill any other quality criteria except those specified above.

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. The actual hash functions are implementation-dependent and are not required to fulfill any other quality criteria except those specified above.

Std::hash

When we use any associative container we unknowingly use an hash function. The basis of mapping comes from the hashcode generation and the hash function. It is such a class that can be constructed in a more dafault way which in others words means that any user who intends to use the hash class can constuct the objects without any given initial values and arguments. So by default a hash class is a template class. Now lets directly switch over to different objects that can be used in the hash function to get their corresponding hash values. Now for an integer the hash function returns the same value as the number that is given as input. The hash function returns an integer, and the input is an integer, so just returning the input value results in the most unique hash possible for the hash type. Apart from these standard data types, we can also use hash function for many other data types:. So these were some types of data that can be hashed using the std::hash class.

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This article is being improved by another user right now. Trait std :: hash :: Hash 1. General utilities. Improved By :. Enhance the article with your expertise. In particular, they define an operator that:. From cppreference. Due to differences in endianness and type sizes, data fed by Hash to a Hasher should not be considered portable across platforms. Thank you for your valuable feedback! From cppreference. The hash template defines a function object that implements a hash function. Types implementing Hash are able to be hash ed with an instance of Hasher.

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.

When implementing both Hash and Eq , it is important that the following property holds:. Improved By :. There is no specialization for C strings. Disabled specializations do not satisfy Hash , do not satisfy FunctionObject , and following values are all false :. In other words, if two keys are equal, their hashes must also be equal. Notably, some implementations use trivial identity hash functions which map an integer to itself. Types implementing Hash are able to be hash ed with an instance of Hasher. Instances of this function object satisfy Hash. Note: additional specializations for std::pair and the standard container types, as well as utility functions to compose hashes are available in boost. Log in. Suggest Changes. Like Article. The hash class is default constructible, which means that one can construct this object without any arguments or initialization values.