Re: enum and operator++

From:

Richard Herring <junk@[127.0.0.1]>

Newsgroups:

comp.lang.c++

Date:

Wed, 20 Jan 2010 16:47:03 +0000

Message-ID:

<8EEXsYkHOzVLFwJ2@baesystems.com>

In message
<87d63a4b-5ff5-402a-ba52-4c9e69814254@34g2000yqp.googlegroups.com>,
Saeed Amrollahi <amrollahi.saeed@gmail.com> writes

On Jan 20, 6:38?pm, REH <spamj...@stny.rr.com> wrote:

On Jan 20, 10:07?am, Saeed Amrollahi <amrollahi.sa...@gmail.com>
wrote:

Dear all
I am struggling with a problem that's simple per se.
I have an enum and a prefix ++ operator:
enum Dir { North = 1, East = 2, South = 3, West = 4, Max_ = 5 }; ?//
clockwise

Dir operator++(Dir d)
{
? Dir dd;
? switch (d) {
? case North:
? ? dd = East;
? ? break;
? case East:
? ? dd = South;
? ? break;
? case South:
? ? dd = West;
? ? break;
? case West: ?// wrap
? ? dd = North;
? ? break;
? };

? return dd;

}

void f()
{
? ?for (Dir d = North; d < Max_; ++d)
? ? // do something

}

The problem is: in the above loop the ++ doesn't work,
indeed the loop is infinite. If you replace // do something with
output statement, it prints 1 forever.

Of course it's forever. Your comment says it all: West wrapped back to
North. You never hit Max.

Incidentally, you can write your operator much, much simpler than
using a case statement.

Hi

You are right. My code had two problems. Yu found one of them
and you found the wraping one. I removed the wrap logic in switch
statement.

BTW, why I have to write
d = ++d;
rather than
++d; ?

Because your operator++, despite its name, doesn't modify its argument,
and doesn't return a reference to it. That's very confusing.

You should probably have started by writing
Dir & operator++(Dir & d)
{
// code to "increment" d
return d;
}

--
Richard Herring