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Is there a reason for missing transparent (template <class K> at(K&& key);) in std::map?
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They won't use operator==() . The only comparison used to locate objects is the third template argument to std::map<K, V, Compare, Allocator> .
map find() function in C++ STL Return Value: The function returns an iterator or a constant iterator which refers to the position where the key is present in the map. If the key is not present in the map container, it returns an iterator or a constant iterator which refers to map. end().
map::clear() clear() function is used to remove all the elements from the map container and thus leaving it's size 0.
Each element in a map is uniquely identified by its key value. Aliased as member type map::key_type. Type of the mapped value. Each element in a map stores some data as its mapped value.
My guess is that std::map::at() must be a "bounds-checked" version of std::map::operator[](). Providing a transparent version of std::map::operator[]() imposes an additional requirement on std::map::key_type and the query key type K - if the query key is not in the map, it must be inserted (with default constructed value), which means that std::map::key_type must be constructible from the the query key type.
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