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The C++ Standard, [basic.types], paragraph 9 [ISO/IEC 14882-2014], states:

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.

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In this noncompliant code example, the complete object representation is accessed when comparing two objects of type S. Per the C++ Standard, [class], paragraph 13 [ISO/IEC 14882-2014], classes may be padded with data to ensure that they are properly aligned in memory. The contents of the padding and the amount of padding added is implementation-defined. This can lead to incorrect results when comparing the object representation of classes instead of the value representation, as the padding may assume different unspecified values for each object instance.

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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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Bibliography

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