dynamic_cast inside a template function problem

[jfradley]

Hello,

Here's my problem. I have a template function where I'd like to set a flag if the passed in data type is derived from a particular base class. The way to typically do this is to use dynamic_cast, unfortunately with PODs and non-polymorphic types coming I can't use the dynamic_cast.

Right now I have a work around that puts the responsibility on the programmer to know whether the input data type is derived from this class.

Any ideas?

class Indexable
{
public:
    virtual ~Indexable();
    virtual void doSomething();

}


template <class T>
void myDesiredFunction(T *ptr)
{
    if(dynamic_cast<Indexable*>(ptr))
    {
         // this is a data type derived from Indexable
    }
    else
    {
         // PODS, non-polymorphic, and classes not derived from  Indexable
     }
}

[Banfa]

Create a specialisation of the template function for that type.

A specialisation is where you write an implementation of a template function for a specific type (or set of types) that is different from the normal template function, something like this

[CODE=cpp]
class Indexable
{
public:
virtual ~Indexable();
virtual void doSomething();

}

template <class T>
void myDesiredFuncti on(T *ptr)
{
// PODS, non-polymorphic, and classes not derived from Indexable
}

template <>
void myDesiredFuncti on(Indexable *ptr)
{
// this is a data type derived from Indexable
}
[/CODE]

[jfradley]

Create a specialisation of the template function for that type.

A specialisation is where you write an implementation of a template function for a specific type (or set of types) that is different from the normal template function, something like this

[CODE=cpp]
class Indexable
{
public:
virtual ~Indexable();
virtual void doSomething();

}

template <class T>
void myDesiredFuncti on(T *ptr)
{
// PODS, non-polymorphic, and classes not derived from Indexable
}

template <>
void myDesiredFuncti on(Indexable *ptr)
{
// this is a data type derived from Indexable
}
[/CODE]

I did try that. But when passing derived data types the non-specialized template function gets called instead of the specialized. Is there a way to force it to check the specialized function first even for these cases?

[Banfa]

static_cast you derived class pointer to a base class pointer when calling the function.

[fual]

I did try that. But when passing derived data types the non-specialized template function gets called instead of the specialized. Is there a way to force it to check the specialized function first even for these cases?

What you need is a little boost magic.

http://cci.lbl.gov/~rwgk/shortcuts/boost_hot/libs/type_traits/doc/html/boost_typetraits/reference/is_base_of.html

http://www.boost.org/libs/mpl/doc/refmanual/if.html

What you can do in this case is get the "if" object to return an empty function object if the input isn't derived from your base, and it will return the function object that does whatever it is you want it to if the input is derived. Since this is all done at compile time it is pretty efficient I reckon.

If you don't have boost installed, then you will probably find it pretty useful generally so it is well worth it.