Posted by Bob Stearns on 11/18/87 11:44

Roman Ziak wrote:
> Tim Martin wrote:
>
>>Kimmo Laine wrote:
>>
>>>"Emil" <emjot_wytnij_to_@podczta.onet.pl> wrote in message
>>>news:e15ag1$3ee$1@news.onet.pl...
>>>
>>>>Is there any hope that new versions of PHP
>>>>will support macros similar to C or C++?
>>
>>>What's the actual difference between a function and a macro? How would
>>>use of macros differ from functions?
>>>
>>>Let's pretend there is a way of defining a macro in php...
>>>define ("MAX($a,$b)", "(($a<$b)?$b:$a)");
>>>
>>>vs.
>>>
>>>function max( $a, $b ) {
>>> return $a < $b ? $b : $a;
>>>}
>>
>>The difference is that macros are applied as text substitution before
>>parsing is carried out. This means that macros can carry out
>>substitutions on the code that would not in itself be a valid language
>>construct.
>
>
> Beside possibility to create funny constructs, the difference between
> function and macro both implementing same code is, that macro will
> expand the code inline but function will create a call. Macro give the
> benefit of faster execution for the penalty of more code. Function gives
> the benefit of smaller code for the penalty of slower execution.
>
In addition, you get the current context. When you have to do something
complex two or more times in code, either your function has the
parameter list from hell or it contains a (set of) global statement from
hell, either of which lead to maintenance nightmares and coding errors,
especially since php is not (strongly) type checked.
> in C/C++ macros can also beter optimise with surrounding code, although
> function usually does not ake any assumtions about register content,
> will not fold constants (e.g. x=5+MAX(3,6) will fold into x=11, but not
> so with the function call).
>
> In PHP every function call has to switch context, initialise local
> variables, which in C is done with few instructions (on x86 is either
> ENTER or PUSH EBP, MOV EBP, ESP, ADD ESP,frame), but in PHP carries
> little more everhead.
>
>
>>To the OP: What are you trying to achieve with macros? With a
>>combination of pass-by-reference, soft references and eval() you can
>>achieve many of the things that C macros are able to achieve, with
>>better syntax and less potential for misuse.
>
>
> I agree, PHP is a text processor itself. Just to make a point here is an
> example of incomplete macro expansion implementation in PHP, which would
> be almost impossible in C:
>
> function ExpandMacros(path,macros)
> {
> text = get_file_content(path);
>
> // expand'em - skipped
>
> put_file_content(path.'.expanded',
> 'define(\'EXPANDED\', 1);\n\n' . text);
> }
>
> @include __FILE__.'.expanded"
>
> if(!EXPANDED)
> {
> ExpandMacros(__FILE__, array(/* macros definitions */) )
> return;
> }
>
> For brewity reason, I did not go into the details of expanding macros.
>
The m4 macro processor, for instance, would do the job.
> Roman

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