Why does an overloaded assignment operator not get inherited?

Question

Why does this code:

class X {
public:
    X& operator=(int p) {
        return *this;
    }
    X& operator+(int p) {
        return *this;
    }
};

class Y : public X { };

int main() {
    X x;
    Y y;
    x + 2;
    y + 3;
    x = 2;
    y = 3;
}

Give the error:

prog.cpp: In function ‘int main()’:
prog.cpp:14:9: error: no match for ‘operator=’ in ‘y = 3’
prog.cpp:14:9: note: candidates are:
prog.cpp:8:7: note: Y& Y::operator=(const Y&)
prog.cpp:8:7: note:   no known conversion for argument 1 from ‘int’ to ‘const Y&’
prog.cpp:8:7: note: Y& Y::operator=(Y&&)
prog.cpp:8:7: note:   no known conversion for argument 1 from ‘int’ to ‘Y&&’

Why is the + operator inherited, but the = operator not?

The answer to your question is [here](http://stackoverflow.com/questions/12009865/operator-and-functions-that-are-not-inherited-in-c) — Andy Prowl, Mar 01 '13 at 13:36

score 9 · Accepted Answer · answered Mar 01 '13 at 13:38

9

Class Y contains implicitly-declared assignment operators, which hide the operator declared in the base class. In general, declaring a function in a derived class hides any function with the same name declared in a base class.

If you want to make both available in Y, use a using-declaration:

class Y : public X {
public:
    using X::operator=;
};

answered Mar 01 '13 at 13:38

Mike Seymour

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What if class `Y` needs to "extend" the operator `=`, i.e. do more things than `return *this` in parent class? – Ignorant Dec 07 '15 at 17:35
Then you override it yourself as you did initially and don't use inheritance. I have personally found that using virtual operator overloads are not always the best thing – Curious Jan 01 '16 at 00:00

score 0 · Answer 2 · answered Mar 01 '13 at 13:37

0

You are forgetting the methods that the compiler automatically generates if they are not declared. Assignment is amongst them.

answered Mar 01 '13 at 13:37

Ed Heal

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Why does automatic generation delete my definition, when they have different signatures? – Eric Mar 01 '13 at 13:38
@Eric: It doesn't delete it, it hides it. – Andy Prowl Mar 01 '13 at 13:38
@AndyProwl: Same question then: why does it hide it, when the signatures don't conflict? – Eric Mar 01 '13 at 13:39
2

@Eric: Because any function declared in a derived class hides any function with the same *name* (not signature) declared in a base class. – Mike Seymour Mar 01 '13 at 13:40
@Eric hiding does not require signatures to match. It works at the level of *names*. – juanchopanza Mar 01 '13 at 13:41

Why does an overloaded assignment operator not get inherited?

2 Answers2

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