bdlb_transparentequalto

Detailed Description

Outline

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

Purpose

Provide a transparent equality predicate.

Classes

• bdlb::TransparentEqualTo: a transparent equality predicate.

See also

bdlb_transparentless, bdlb_transparenthash

Description

This component provides a struct, bdlb::TransparentEqualTo, that defines a functor that can be used as transparent equality comparator for heterogeneous lookup.

Usage

This section illustrates intended use of this component.

Example 1: Basic Use of bdlb::TransparentEqualTo

Suppose we need a container to store set of unique bsl::string 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::equal_to as default comparator. The problem is that even though the equality operator between bsl::string and bsl::string_view exists, the compiler will try to convert the bsl::string_view objects to bsl::string since bsl::equal_to is parameterized by bsl::string. The compilation will fail because there is no such implicit conversion. Even if the compilation were to succeed (because an implicit conversion were available), such an operation can lead to additional memory allocation for temporary objects. The following code illustrates how to use bdlb::TransparentEqualTo as a comparator 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 uses bdlb::TransparentEqualTo as a comparator. Note that to avoid implicit conversions we also have to use transparent hash functor:

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