Equality

What does it mean for two objects to have the same state?

A naive answer could be that those objects have the same state if they have the same bit pattern in memory. While it's true that having the same bit pattern means objects have the same state, the converse isn't necessarily true. Think about two std::vector<int>s. They compare equal if they have the same contents, but those contents would exist in two different locations in memory, and so be referenced by different pointers inside each std::vector.

Another answer could be that the objects compare equal with ==. Again, that's true, but not every class defines an operator==, nor does it always make sense to provide one. What would it mean to compare two stream objects, for example?

A better way to think about state equality is in terms of substitutability. Wherever an object is constructed in a program, if it can be substituted with another object without changing the behaviour of the program then those objects have the same state.