What is the meaning of the error message "std::unordered_map' with Key class" and why is it indicating that a function has been deleted?

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The error message "std::unorderedmap' with Key class" indicates that there is an issue with using an object of a particular class as a key in a std::unorderedmap. The error message is usually accompanied by a note that the compiler has deleted a function. This is because the use of the class as a key requires the class to have a valid hash function and a valid equality operator. If the class does not define these functions, then the compiler will automatically delete the default functions. The error message is a way of telling the programmer that they need to define these functions in order to use the class as a key in the unordered_map.