CODEWITHSUNDEEP
c++functionpointersreference
akarshkumar0101
akarshkumar0101

Reputation: 367

C++ passing temporary stack object as reference to function

template<class D>
class Animal{
public:
    D a;
    Animal(){
        a=0;
    }
};
template<class D>
Animal<D>& addAnimal(Animal<D>& ans, const Animal<D>& a, const Animal<D>& b){
    ans.a = a.a + b.a;
    return ans;
}

//an overloaded simpler method for the one above
template<class D>
Animal<D> addAnimal(const Animal<D>& a, const Animal<D>& b){
    return addAnimal( Animal<D>(),a,b);
}
int main() {    
    Animal<int> a;
    Animal<int> b;
    Animal<int> c = addAnimal(a,b);
}

The following doesn't compile. Why not? I know I can fix this by making the ans parameter pass by value OR changing the call to that method by doing:

template<class D>
Animal<D> addAnimal(const Animal<D>& a, const Animal<D>& b){
    Animal<D> ans;
    return addAnimal(ans,a,b);
}

I want a better understanding of pass by reference and how to call the default constructor of a stack object. Thanks!

Upvotes: 0

Views: 44

Answers (1)

R Sahu
R Sahu

Reputation: 206607

The line

return addAnimal( Animal<D>(),a,b);

does not work because Animal<D>() is a temporary object, which cannot be used in a function call that expects a non-const lvalue reference.

More importantly, it is not clear to me why you need the overload

template<class D>
Animal<D>& addAnimal(Animal<D>& ans, const Animal<D>& a, const Animal<D>& b) { ... }

You can implement the simpler, two argument, function as:

template<class D>
Animal<D> addAnimal(const Animal<D>& a, const Animal<D>& b){
   Animal<D> ans;
   ans.a = a.a + b.b;
   return ans;
}

and get rid of the other one altogether.

Upvotes: 2

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