Return a reference providing modifiable access to the mapped-value associated with the specified key in this unordered map; if this unordered map does not already contain a value_type object with key, first insert a new value_type object having key and a default-constructed VALUE object. Note that this method requires that the (template parameter) VALUE is "default-constructible" (see {Requirements on value_type}). Note that key may be modified; it is guaranteed to be left in a valid state.
Declared in <bslstl_unorderedmap.h>
add_lvalue_reference<Json>::type
operator[](BloombergLP::bslmf::MovableRef<key_type> key);