Component bdlb_transparenthash [Package bdlb]

Provide a transparent hash functor. More...

Namespaces
namespace	bdlb

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Detailed Description

Outline

• Purpose
• Classes
• Description
  • Usage
    • Example 1: Basic Use of bdlb::TransparentHash

Purpose:

Provide a transparent hash functor.

Classes:

bdlb::TransparentHash

a transparent hash functor

See also:

bsl_map, bsl_set

Description:

This component provides a struct, bdlb::TransparentHash, that defines a functor to generate a hash code for different types and can be used as transparent hash functor for heterogeneous lookup.

Usage:

This section illustrates intended use of this component.

Example 1: Basic Use of bdlb::TransparentHash:

Suppose we need a container to store set of bsl::string unique objects. bsl::unordered_set is designed exactly for this purpose. But imagine that we want to use bsl::string_view objects for search operations within our container. bsl::unordered_set uses bsl::hash as default hash functor. The problem is that even though the hash function for bsl::string_view exists, compiler tries to convert bsl::string_view objects to the bsl::string since bsl::hash is parameterized by bsl::string. And compilation fails, because there is no such implicit conversion. In addition, implicit conversions where they are available, may lead to additional memory allocation for temporary objects. The following code illustrates how to use bdlb::TransparentHash as a hash functor for the standard container unordered_set, in this case to allow a bsl::unordered_set<bsl::string> to be searched with a bsl::string_view.

First, we create a container that usesn'bdlbTransparentHash'. Note that to avoid implicit conversions we also have to use a transparent comparator:

  typedef bsl::unordered_set<bsl::string,
                             bdlb::TransparentHash,
                             bdlb::TransparentEqualTo> TransparentHashSet;

  TransparentHashSet transparentSet;

Now, we fill the container with the strings:

  transparentSet.insert("NY");
  transparentSet.insert("LA");

Finally, we observe that the container allows to use bsl::string_view objects as a key and does not make any implicit conversions:

  bsl::string_view newYork     ("NY");
  bsl::string_view losAngeles  ("LA");
  bsl::string_view sanFrancisco("SF");

  assert(transparentSet.end() != transparentSet.find(newYork     ));
  assert(transparentSet.end() != transparentSet.find(losAngeles  ));
  assert(transparentSet.end() == transparentSet.find(sanFrancisco));