TITLE: examples of using static cast

(Newsgroups: comp.lang.c++.moderated, 4 Jun 99)


PHILIP:


> > > Now here's one reason to use 'static_cast<>' to down-cast: Profiling
> > > your app revealed that virtual RTTI was too expensive for a
> > > time-critical section of code.


BECKER: Pete Becker

> > Plus one other condition: you know that the conversion will always be
> > valid. If you don't, then you must have the checking that dynamic_cast
> > does.


PHILIP:

> Well, that's the thing now. One example of "time critical" code is that
> inside an inner loop.
> 
> But if the conversion is "always valid", then it might also be "always
> the same". So move it out of the loop, take the (negligible) performance
> hit, and use 'dynamic_cast<>'.
> 
> Now if it's "always valid", but _not_ "always the same", you can still
> move it out of the loop; just validate all the casts outside the loop
> with 'dynamic_cast<>', and use 'static_cast<>' inside the loop.


BECKER: Pete Becker <petebecker@acm.org>

There's a much simpler case: you've written a container that holds
pointers to a base type, and you have used that container inside a
wrapper that only accepts pointers to objects of a derived type. In that
wrapper, you can access base pointers from the container with assurance
that they will always in fact point to objects of the derived type.

class Base
{
};

class Derived : public Base
{
};

class Wrapper
{
public:
 void insert(Derived *dp) { data[n++] = dp; }
 Derived *get(int i) { return static_cast<Derived*>(data[i]); }
private:
 Base *data[20];
};

The static_cast<Derived*> is always valid, because the only objects that
elements of data can point to are objects of type Derived. Having
virtual functions in Base and Derived doesn't affect this.