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From: Anand Hariharan on 7 Apr 2008 22:48
On Tue, 08 Apr 2008 00:20:31 +0000, Hal Vaughan wrote:
(...)
> Does it work that way in C and C++? I didn't expect it to, but that
> code example implies that you can do:
>
> if (p)...
>
> and if p is defined it evaluates as true and if p isn't defined, it
> evaluates as false.
>

The word 'definition' has a very precise meaning in C and C++. If
'p' were not to be defined, the code would not compile.

There are, however, language rules that state when a particular
value of a type is to be evaluated as true and when as false.

If 'p' is a built-in type then the language allows you to write code
like above. Similar (but not identical) to Perl, a 'zero' value
implies false, anything else is true.

If 'p' is your own type, then you can write an operator so that a
line of code such as

if (p)

has defined semantics per the way you would like it to have.

--
ROT-13 email address to reply
From: Philip Potter on 8 Apr 2008 03:55
Jack Klein wrote:
> So in both C and C++, you can test the results of standard library
> functions like this:
>
> FILE *f = fopen("some_file_name", "r");
> void *v = malloc(SOME_NUMBER_OF_BYTES);
>
> Each of these functions returns a valid pointer on success, or NULL on
> failure. So normally you test them:
>
> if (f != NULL) // file opened OK
> if (v == NULL) // memory allocated OK

This should be != in both cases.

> ...and take appropriate action if you could not open the file or
> allocate memory.
>
> But in both C and C++, a null pointer compares equal to a constant
> expression with a value of 0, so you could just as easily write:
>
> if (f == 0) // file opened OK
> if (v == 0) // memory allocated OK

Here too, it should be != in both cases.
From: Francis Glassborow on 8 Apr 2008 04:15
Anand Hariharan wrote:
> On Tue, 08 Apr 2008 00:20:31 +0000, Hal Vaughan wrote:
> (...)
>> Does it work that way in C and C++? I didn't expect it to, but that
>> code example implies that you can do:
>>
>> if (p)...
>>
>> and if p is defined it evaluates as true and if p isn't defined, it
>> evaluates as false.
>>
>
> The word 'definition' has a very precise meaning in C and C++. If
> 'p' were not to be defined, the code would not compile.

Yes 'definition' has a very precise meaning but not quite what you
think. For the code to compile it only needs to have been declared
(names are declared, the entities they name are defined)

>
From: André Castelo on 8 Apr 2008 07:06
> > FILE *f = fopen("some_file_name", "r");
> > void *v = malloc(SOME_NUMBER_OF_BYTES);


Actually,

if (f) corresponds to if (f != 0).
From: Philip Potter on 8 Apr 2008 08:21
Andr� Castelo wrote:
>>> FILE *f = fopen("some_file_name", "r");
>>> void *v = malloc(SOME_NUMBER_OF_BYTES);
>
>
> Actually,
>
> if (f) corresponds to if (f != 0).

Please don't snip attribution lines. Although my threaded newsreader
makes it look like you replied to me, you only quoted Jack Klein in your
message.

Moreover, Jack himself said upthread:

=== begin quote ===
if (f == 0) // file opened OK
if (v == 0) // memory allocated OK

....and finally, evaluating a built-in type all by itself is equivalent
to comparing it to 0, so:

if (f) // f != 0, file opened OK
if (v) // v != 0, memory allocated OK
=== end quote ===

so it seems he's perfectly aware that if(f) is equivalent to if(f!=0).
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