Fix circular dependency in arithmetic class

Question

I have a set of classes implementing the curiously recurring template pattern. However, the trick is that the base class needs to return instances of the subclasses. Here's an example:

template <typename SubType>
class ArithmeticBase
{
public:
    template <typename OtherType>
    const Addition operator+(const OtherType &other)
        {return Addition(get_subclass(), other);}

    // ...
    // Operators for subtraction, multiplication, division, ...

private:
    const SubType &get_subclass() const
        {return *static_cast<const SubType*>(this);}
};

template <typename OperatorType1, typename OperatorType2>
class Addition : ArithmeticBase<Addition<OperatorType1, OperatorType2>>
{
public:
    Addition(const OperatorType1 &op1, const OperatorType2 &op2)
        : op1(op1)
        , op2(op2)
    {}

private:
    const OperatorType1 &op1;
    const OperatorType2 &op2;
};

// ...
// Additional classes for subtraction, multiplication, division, ...

Compiling this fails because the Addition class is not defined before it's used in the ArithmeticBase class:

arithmetic.cpp:6:8: error: unknown type name 'Addition'
        const Addition operator+(const OtherType &other)
              ^

How can I resolve this?

make it a non-member friend operator? – Alexander Balabin Jul 24 '15 at 15:27 — Alexander Balabin, Jul 24 '15 at 15:27

bhzag · Accepted Answer · 2015-07-24T15:37:36.353

4

You could forward declare Addition before the base class.

template <typename OperatorType1, typename OperatorType2>
class Addition;
template <typename SubType>
class ArithmeticBase
{
...
};

This allows the compiler to know there is a type Addition that exists before it is defined.

edited Jul 24 '15 at 15:37

answered Jul 24 '15 at 15:29

bhzag

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That's not a correct forward declaration of a class template. – Ben Voigt Jul 24 '15 at 15:33
Thanks, I didn't realize that a return type could be an incomplete type. – Joel Jul 24 '15 at 15:40
@Joel: It can be in a declaration, but not in a definition. See JKor's answer. – Ben Voigt Jul 24 '15 at 15:48
@BenVoigt Hmm, then why does this compile fine? https://gist.github.com/anonymous/039e630dd66bc7d0eb43 – Joel Jul 24 '15 at 16:02
@Joel: Oh right, it's because the instantiation of the template function occurs much later than the function template definition. – Ben Voigt Jul 24 '15 at 16:05
@BenVoigt Ah, I see now – Joel Jul 24 '15 at 16:07

Alexander Balabin · Answer 2 · 2015-07-24T16:11:10.623

Or use non-member form declared after Addition:

template <typename OperatorType1, typename OperatorType2>
class Addition;

template <typename SubType>
class ArithmeticBase
{
public:

     template <typename OneType, typename OtherType>
     friend const Addition<OneType, OtherType> operator+(const ArithmeticBase<OneType>& one, const OtherType &other);

private:
    const SubType &get_subclass() const
    {
        return *static_cast<const SubType*>(this);
    }
};

class ArithmeticType : public ArithmeticBase < ArithmeticType > {};

template <typename OperatorType1, typename OperatorType2>
class Addition : ArithmeticBase<Addition<OperatorType1, OperatorType2>>
{
public:
    Addition(const OperatorType1 &op1, const OperatorType2 &op2)
        : op1(op1)
        , op2(op2)
    {}

private:
    const OperatorType1 &op1;
    const OperatorType2 &op2;
};


template <typename OneType, typename OtherType>
const Addition<OneType, OtherType> operator+(const ArithmeticBase<OneType>& one, const OtherType &other)
{
    return Addition<OneType, OtherType>(one.get_subclass(), other);
}

int main()
{
    ArithmeticType a, b;
    a + b;
}

The original restricts the left-hand argument to instantiations of `ArithmeticBase`, yours does not. — Ben Voigt, Jul 24 '15 at 15:49

score 1 · Answer 3 · answered Jul 24 '15 at 15:32

In addition to forward declaring the Addition class (as bhzag's answer shows) you'll need to move the definition of operator+ to after the definition the Addition class. Otherwise you'll get an error on the next line.

Make sure the definition is in the header file. If it isn't, you'll get linker errors.

Fix circular dependency in arithmetic class

3 Answers3

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