The std::basic_string type uses the traits design pattern to handle implementation details of the various string types, resulting in a series of string-like classes with a common, underlying implementation. Specifically, the std::basic_string class is paired with std::char_traits to create the std::string, std::wstring, std::u16string, and std::u32string classes. The std::char_traits class is explicitly specialized to provide policy-based implementation details to the std::basic_string type. One such implementation detail is the std::char_traits::length() function, which is frequently used when dealing with null-terminated string data such as const char * or const wchar_t * values. According to the C++ Standard, [char.traits.require], Table 62 [ISO/IEC 14882-2014], passing a null pointer to this function leads these functions lead to undefined behavior because it would result in dereferencing a null pointer.
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