order of evaluation

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2 total messages Started by Bernd Anhaeupl Mon, 22 Jan 1996 16:39

#3090

Author: Bernd Anhaeupl
Date: Mon, 22 Jan 1996 16:39

50 lines
2460 bytes

In various places the DWP specifies, that the order of evaluation of
some expressions is unspecified. 

Some questions:

1.) Does this mean, that for example in the following function call

	f(a(...),b(....),c(....));

    a(...), b(...), c(...) may even be evaluated in parallel in a
    multitasking environment? 

    1a) Is a statement like the one above valid C++, if a(), b()
        and c() are using and modifying the same global variables
        --- What is the exact meaning of "sequence point" in section 5
        paragraph 4 of the april DWP in this context?

	      static global_variable;

	      int a(){.....; global_variable++; .....}
	      int b(){.....; global_variable++; .....}
	      int c(){.....; global_variable++; .....}

	Please think of the consequences. Even such basic operations
        like free store management (User defined operator new,...!)
        will access and modify global data (structures).

2.) Is it legal for a compiler to reorder the evaluation of
    the operator new and the constructor calls of some new expressions.
    For example, is it legal for a compiler to evaluate

	f(new a(new b()))             or    f2(new a, new b)   (see also question 1)
 
                       in the following order:

	tmp1=a::new(sizeof(a));             tmp1=a::new(sizeof(a))
	tmp2=b::new(sizeof(b));             tmp2=b::new(sizeof(b))
	tmp2.b();                           tmp1.a()
	tmp1.a(tmp2);                       tmp2.b() 
	f(tmp1);                            f2(tmp1,tmp2);

-- 
Bernd Anhaeupl			Tel.:  +49 9131 857787
LATE - Uni Erlangen			
Cauerstr. 7			Email: anhaeupl@late.e-technik.uni-erlangen.de
91058 Erlangen

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Re: order of evaluation

#3088

Author: Steve Clamage
Date: Mon, 22 Jan 1996 17:22

43 lines
2332 bytes

In article aul@rznews.rrze.uni-erlangen.de, anhaeupl@late.e-technik.uni-erlangen.de (Bernd Anhaeupl) writes:
>In various places the DWP specifies, that the order of evaluation of
>some expressions is unspecified. 

>1.) Does this mean, that for example in the following function call
>
>	f(a(...),b(....),c(....));
>
>    a(...), b(...), c(...) may even be evaluated in parallel in a
>    multitasking environment? 

No. Section 1.8 says function calls may not be interleaved. In your
example, "a", "b", and "c" may be called in any order, but whichever
one is called first must complete before the second one is called.
(The C++ language definition does not address parallelizing of program
execution, but we can talk about interleaving parts of expressions.)

>2.) Is it legal for a compiler to reorder the evaluation of
>    the operator new and the constructor calls of some new expressions.
>    For example, is it legal for a compiler to evaluate

>	f(new a(new b()))             or    f2(new a, new b)   (see also question 1)
> 
>                       in the following order:

>	tmp1=a::new(sizeof(a));             tmp1=a::new(sizeof(a))
>	tmp2=b::new(sizeof(b));             tmp2=b::new(sizeof(b))
>	tmp2.b();                           tmp1.a()
>	tmp1.a(tmp2);                       tmp2.b() 
>	f(tmp1);                            f2(tmp1,tmp2);

I believe the reordering is allowed, because I don't think there is
any sequence point that prevents it. 

---
Steve Clamage, stephen.clamage@eng.sun.com

[ comp.std.c++ is moderated.  Submission address: std-c++@ncar.ucar.edu.
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