CODEWITHSUNDEEP
c++functionc++11return-valueauto
Alex
Alex

Reputation: 402

auto vs auto&& for a function return value

Could you tell me please whether I am right that use of auto&& for a function return value is always a better choice than use of auto. For instance, in

auto val = someObj.getVal();

the val will be a copy if getVal() returns a reference. However, use of the universal reference auto&& does not have such a disadvantage? The only information I need to know is whether getVal() is const or not.

Upvotes: 6

Views: 1765

Answers (1)

Shmuel H.
Shmuel H.

Reputation: 2546

However, use of the universal reference auto&& does not have such a disadvantage?

No, it would not copy the return value.

The problem

However, sometimes copy is necessary, and using auto&& for both prvalue and references can create dangling references. Consider this case: 

struct A {
    // A: Another programmer changed it from:
    // int getVal() { return mInteger; }

    // B
    int &getVal() { return mInteger; }

private:
    int mInteger;
};

int main() {
    A *a = new A;

    auto   integerCopy = a->getVal();

    // int& on case B, int&& on case A.
    auto&& integerRvalueRef  = a->getVal();

    delete a;

    // Ok, no matter what getVal returns.
    std::cout << integerCopy;
    // Dangling Reference dereference if getVal returns a reference. 
    std::cout << integerRvalueRef;
}

As you can see, with auto, there was no problem with changing of this return value. However, with auto&&, it has created a dangling reference.

Conclusion

Use auto&& like a regular reference: treat it with caution. Using it for both prvalue and reference value return values may cause bad surprises.

Upvotes: 6

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