CODEWITHSUNDEEP
c++templatesmetaprogrammingtemplate-meta-programming
arennuit
arennuit

Reputation: 927

Templates inheritance and operators

I have the following code which causes problem when I call operator=() on a Property instance:

// myProperty.h.

template <class T, int typeFamily = TypeFamily<T>::value>
class PropertyImpl : public PropertyBase
{
    // Default template to catch errors.
};

template <class T>
class PropertyImpl<T, 0> : public PropertyBase
{
    // Data is part of the family of base types.
public:
    PropertyImpl(T* dataRef) : PropertyBase(), m_dataRef(dataRef) {}

    void operator=(T const & data) {*m_dataRef = data;}

protected:
    T* m_dataRef;
};

template <class T>
class Property : public PropertyImpl<T> {};

Note that TypeFamily<> is some meta-code computing whether T is a supported base type. TypeFamily<T>::value is 0 if T is a float.

Now I create an aggregated Property

// myNode.h.

class myNode
{
public:
    void setProp(float val) {m_prop = val;}

protected:
    Property<float> m_prop;
}

I was initially thinking that Property<float> deriving from PropertyImpl<float, 0> I would be able to call m_prop = val, as operator=() is defined for PropertyImpl<float, 0>. But my compiler returns the following error:

<myNode_path>(myNode_line) : error C2679: binary '=' : no operator found which takes a right-hand operand of type 'float' (or there is no acceptable conversion)
3>        <myProperty_path>(myProperty_line): could be 'Property<T> &Property<T>::operator =(const MPC::Property<T> &)'
3>        with
3>        [
3>            T=float
3>        ]
3>        while trying to match the argument list '(Property<T>, float)'
3>        with
3>        [
3>            T=float
3>        ]

This is totally unclear to me, and I have the feeling I missed a fundamental behavior of templates. Or it is an easy catch right in front of my eyes...

Anyone understands what is going on?

Thanks!

Upvotes: 2

Views: 117

Answers (1)

Mike Seymour
Mike Seymour

Reputation: 254461

Property contains an implicitly-declared copy-assignment operator, which hides the one in the base class. You'll need a using-declaration to make it accessible:

template <class T>
class Property : public PropertyImpl<T> {
public:
    using PropertyImpl<T>::operator=;
};

Upvotes: 4

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