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The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T). The value representation of an object is the set of bits that hold the value of type T.

Some types—for example, integral types such as int and wchar_t char—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 access and modify an object soley based on its bit representation, 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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In this noncompliant code example, std::memset() is used to clear the internal state of an object. An implementation may store a vtable within a vtable within the object instance due to the presence of a virtual function, and that vtable is subsequently overwritten by the call to std::memset(), leading to undefined behavior when virtual method dispatch is required.

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