Re: assignment in if statement

From:

Alec Ross <alec@arlross.demon.co.uk>

Newsgroups:

comp.lang.c++.moderated

Date:

30 Apr 2006 13:07:00 -0400

Message-ID:

<phdukTOt+MVEFwFD@arlross.demon.co.uk>

In message <1146245313.177998.97930@j33g2000cwa.googlegroups.com>,
cole.harvey@yahoo.com writes

Hi all,

I am trying to minimize dynamic_cast by doing the following:

How is the code below minimising the use of dynamic_cast? I.e. what
other options that use it more have you rejected? (E.g. is the ordering
of your "if ... " tests putting the most frequent cases first?)

foo* myFoo = NULL;
bar* myBar = NULL;
blah* myBlah = NULL;

// for each pointer in a list: pointer
myFoo = NULL;

                 // these initializations are not needed
                 // your dynamic_casts will set up the pointers to 0
                 // or non-zero when needed

myBar = NULL;
myBlah = NULL;

if(myFoo = dynamic_cast<foo*>(pointer) && myFoo != NULL) {
  // Do it
}
else if(myBar = dynamic_cast<bar*>(pointer) && myBar != NULL) {
  // Do it
}
else if(myBlah = dynamic_cast<blah*>(pointer) && myBlah != NULL) {
  // Do it
}

// end loop

So will the compiler do something smart and still call dynamic_cast on
each pointer? And will myFoo = dynamic_cast<foo*>(pointer) return
true, or do I need to use OR (||) instead of AND (&&)? Any other
comment are welcom.

Thanks.

Assuming that there is no inheritance relationship between foo, bar, and
blah, perhaps what you want is:

// loop start

if ( dynamic_cast<foo*>(pointer) || dynamic_cast<bar*>(pointer) ||
dynamic_cast<blah*>(pointer) ) // line wrapped by mailer
{
     // Do it
}

// loop end

or, if you need the pointers in the loop body, then:

foo *myFoo;
bar *myBar;
blah *myBlah;
// loop start

if ( myFoo = dynamic_cast<foo*> (pointer)
      || myBar = dynamic_cast<bar*> (pointer)
      || myBlah = dynamic_cast<blah*>(pointer)
    )
{
     // Do it
}

// loop end

(The example code above uses the so-called "short-circuit" evaluation of
expressions using the "||" operator.)

If there is a public inheritance relationship between any of foo, bar,
and blah, then use of nested "if ... else" and ordering the tests base
--> derived might be faster (since if the pointer does not point to a
base object, neither does it point to a derived one). However, if a
derived object is by far the most common case, then putting this test
first could minimise the dynamic_cast invocations.
--
Alec Ross

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