Re: TClonesArray typecasting

Hi All,

On Tue, 2006-11-14 at 19:48 +0100, Rene Brun wrote:
> Replace

> (TH1F *) bigArr[chan]->Fill(j, value);
> by
> (TH1F *) bigArr[chan])->Fill(j, value);

This is old-style C-like casts, that is generally frowned upon in C++. Instead, you should use one of the 4 cast operators in C++:

        dynamic_cast<T>(o) 
        Returns 0 if o is of a class that doesn't doesn't inherit from
        T.  Is expensive. T must be a reference or pointer type. 
        
        static_cast<T>(o)
        Casts o to an object of type T.  T must be a reference or
        pointer type. 
        
        const_cast<T>(o)
        Adds or removes const and/or volatile qualification of object o
        of unqualified type T. 
        
        reinterpret_cast<T>(o)
        Cast to anything.  This is what is closest to the C-style cast.
        This cast should never really be used if at all avoidable. 
        

In your case, it's probably best to do

        static_cast<TH1*>(bigArr[chan])->Fill(j,value);
        

since you _know_ that the objects stored in bigArr are of class TH1 or derived classes.

When reading from a file, it's best to use dynamic_cast, as you may not know the type of the object returned. Suppose there's an object in the file named `o' and it's of class TH1F. If you do

        TH2* h = dynamic_cast<TH2*>(file->Get("o"))
        

You will get a null pointer back, since `o' is not of type TH2 or any of it's decedents. However, if you do one of

        TH2* h = static_cast<TH2*>(file->Get("o"))
        TH2* h = (TH2*)(file->Get("o"))
        

you will get a pointer back, but it will not be valid - i.e., the program will fail at runtime.

The basic type system of C++ is pretty rich, but by using C-style casts you are completely circumventing that.

Yours,

-- 
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