Why should you be careful of the equality operator of the base class in C++
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Reason (reason)
It's really hard to write simple and easy-to-use equality operators for inheritance systems.
Example, bad (negative example) class B {
String name
Int number
Virtual bool operator== (const Ba) const
{
Return name = = a.name & & number = = a.number
}
/ /...
}
The second Operand of the equality comparison operator of B accepts type conversions, but the first one does not.
Class D: B {
Char character
Virtual bool operator== (const Da) const
{
Return name = = a.name & & number = = a.number & & character = = a.character
}
/ /...
}
B b =.
D d =.
B = = d; / / compares name and number, ignores d's character
D = = b; / / error: no = = defined
D d2
D = = D2; / / compares name, number, and character
B & b2 = D2
B2 = = d; / / compares name and number, ignores d2s and d's character
Of course, there is a way to make the equality comparison operator act in the inheritance system, but the simple method is not.
Note (Note)
This rule applies to all common comparison operators:! =, =.
Enforcement (implementation recommendations)
The hint is defined as the equality comparison operator of the virtual function; the same is true of other comparison operators:! =, =.
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