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Some types—for example, integral types such as int and wchar_t—have an object representation composed solely of the bits from the object's value representation. For such types, accessing any of the bits of the value representation is well-defined behavior. This form of object representation allows a programmer to use byte-wise bytewise access of the object, such as by calling std::memcmp() on its object representation. Other types, such as classes, may not have an object representation composed solely of the bits from the object's value representation. For instance, classes may have bit-field data members, padding inserted between data members, a vtable to support virtual method dispatch, or data members declared with different access privileges. For such types, accessing bits of the object representation that are not part of the object's value representation may result in undefined behavior depending on how those bits are accessed.
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The effects of accessing bits of an object representation that are not part of the object's value representation can range from implementation-defined behavior (such as assuming the layout of fields with differing access controls) to code execution vulnerabilities (such as overwriting the vtable pointer).
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