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Similarly, the std::string::back() and std::string::front() functions are unchecked as they are defined to call through to the appropriate operator[]() overload without without throwing.
Do not pass an out-of-range value as an argument to std::string::opperator[](). Similarly, do not call std::string::back() or std::string::front() on an empty string. This rule is a specific instance of CTR50-CPP. Guarantee that container indices and iterators are within the valid range.
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