Checking runtime object type

Steven Schveighoffer schveiguy at yahoo.com
Wed Feb 8 12:44:08 PST 2012


On Wed, 08 Feb 2012 14:41:51 -0500, Jonathan M Davis <jmdavisProg at gmx.com>  
wrote:

> On Wednesday, February 08, 2012 20:21:45 Johannes Pfau wrote:
>> Am Wed, 8 Feb 2012 11:20:39 -0800
>>
>> schrieb "H. S. Teoh" <hsteoh at quickfur.ath.cx>:
>> > What's the correct syntax for checking the runtime type of a derived
>> >
>> > object given its base class pointer? I tried:
>> > Base f() { return new Derived(); }
>> > Base b = f();
>> > assert(is(typeof(b)==Derived));
>> >
>> > but it throws an error. Apparently typeof(b)==Base; so typeof returns
>> > only compile-time information? How do I get at the runtime type?
>> >
>> >
>> > T
>>
>> I think using casts is the only way:
>>
>> Base f() { return new Derived(); }
>> Base b = f();
>> auto c = cast(Derived)b;
>> assert(c !is null);
>
> Casting is definitely the way that you're supposed to do it. If the cast
> results in null, then the class is _not_ of the type that you cast to.  
> e.g.
>
> if(auto d = cast(Derived) b)
>  //do stuff with d

It depends on the usage.  If you want to see what the most derived type  
is, using typeid is best (for those old-schoolers, this used to be  
.classinfo).

If you want to *verify* that the given type is derived from some other  
type, using cast is best.

-Steve


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