CODEWITHSUNDEEP
c++templatesfriend
Alex Martin
Alex Martin

Reputation: 33

C++ Template Inner Class Friend Operator Overload

I am trying to implement an overload for operator!= that compares two objects of different types (InnerA and InnerB). Both of the types are defined as nested classes within a template class (Outer). The overload needs to be a friend of both classes as it accesses private fields from each.

template<typename Type> class Outer
{
public:
    class InnerA;
    class InnerB;
};


template<typename Type> bool operator!=(const typename Outer<Type>::InnerA& lhs, const typename Outer<Type>::InnerB& rhs);


template<typename Type> class Outer<Type>::InnerA
{
    const int val = 0;
    friend bool operator!=<>(const InnerA& lhs, const typename Outer<Type>::InnerB& rhs);
};


template<typename Type> class Outer<Type>::InnerB
{
    const int val = 1;
    friend bool operator!=<>(const typename Outer<Type>::InnerA& lhs, const InnerB& rhs);
};


template<typename Type> bool operator!=(const typename Outer<Type>::InnerA& lhs, const typename Outer<Type>::InnerB& rhs)
{
    return lhs.val != rhs.val;
}


int main()
{
    bool b = Outer<int>::InnerA() != Outer<int>::InnerB();
}

The above code fails to compile, emitting:

 In instantiation of 'class Outer<int>::InnerA':
34:33: required from here 
15:17: error: template-id 'operator!=<>' for 'bool operator!=(const Outer<int>::InnerA&, const Outer<int>::InnerB&)' does not match any template declaration 
 In instantiation of 'class Outer<int>::InnerB': 
34:57: required from here 
22:17: error: template-id 'operator!=<>' for 'bool operator!=(const Outer<int>::InnerA&, const Outer<int>::InnerB&)' does not match any template declaration 
 In function 'int main()': 
34:35: error: no match for 'operator!=' (operand types are 'Outer<int>::InnerA' and 'Outer<int>::InnerB') 
34:35: note: candidate is: 
26:30: note: template<class Type> bool operator!=(const typename Outer<Type>::InnerA&, const typename Outer<Type>::InnerB&) 
26:30: note: template argument deduction/substitution failed: 
34:57: note: couldn't deduce template parameter 'Type'

While there might be better ways to achieve a similar result, I'm curious as to what precisely is wrong with my code. Thanks!

Upvotes: 2

Views: 919

Answers (3)

Alex Martin
Alex Martin

Reputation: 33

The issue with the code in the question ended up being that template deduction is not attempted on type names nested inside a dependent type (e.g. Outer<Type>::Inner).

This question is essentially a duplicate of Nested template and parameter deducing. A detailed explanation on why this is a problem can be found here.

Upvotes: 0

Alex Martin
Alex Martin

Reputation: 33

I found the following workaround, refactoring the overload as a method of one of the nested classes:

template<typename Type> class Outer
{
public:
    class InnerA;
    class InnerB;
};


template<typename Type> class Outer<Type>::InnerA
{
    const int val = 0;
public:
    bool operator!=(const typename Outer<Type>::InnerB& other);
};


template<typename Type> class Outer<Type>::InnerB
{
    const int val = 1;
    friend bool Outer<Type>::InnerA::operator!=(const InnerB& other);
};


template<typename Type> bool Outer<Type>::InnerA::operator!=(const typename Outer<Type>::InnerB& other)
{
    return val != other.val;
}


int main()
{
    bool b = Outer<void>::InnerA() != Outer<void>::InnerB();
}

However, I am still curious if the same can be accomplished using a non-member friend function as in the question.

Upvotes: 1

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275190

template<typename Type> class Outer<Type>::InnerA
{
   friend bool operator!=(const InnerA& lhs, const typename Outer<Type>::InnerB& rhs) { return true; }
};
template<typename Type> class Outer<Type>::InnerB
{
  friend bool operator!=(const typename Outer<Type>::InnerA& lhs, const InnerB& rhs) { return true; }
};

these are non-template friends. They also conflict. So implement one of them, omit the other.

They will be found via ADL.

Upvotes: 1

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