Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Access the specified element, with bounds checking.

value&
at(
    string_view key);
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value&&
at(
    string_view key);
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value const&
at(
    string_view key) const;
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