How Do Transparent Comparators Enhance Lookup in C 14 Associative Containers?

Transparent Comparators: Enhanced Lookup Capabilities in Associative Containers

In C 14, a significant change has been introduced to associative containers. As stated in [associative.reqmts]/13, the member function templates find, count, lower_bound, upper_bound, and equal_range only participate in overload resolution if the type Compare::is_transparent exists.

Purpose of Transparent Comparators

The purpose behind making a comparator "transparent" is to expand the lookup functionality of associative containers. By allowing comparators to be transparent, developers can utilize types that are comparable with the container's key rather than being constrained to using the key type itself. This enables more flexible and efficient lookup scenarios.

Implementation and Usage

C 14 introduces library templates such as std::less that facilitate the creation of transparent comparators:

template <class T = void> struct less {
    // ...
    typedef *unspecified* is_transparent;
};

template <> struct less<void> {
    // ...
    typedef *unspecified* is_transparent;
};

In this example, std::set> would not have a transparent comparator, while std::set> would have one.

Impact on Associative Containers

Notably, this change does not impact the default operation of standard containers out of the box. They will continue to behave as they always have, using their key type as the primary comparison criteria.

However, by utilizing the std::less<> transparent comparator or other compatible alternatives, developers can unlock the enhanced functionality of heterogeneous lookup in associative containers. This allows them to perform comparisons using types that may differ from the container's key type, increasing flexibility and versatility.

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