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In C++ that makes sense as this can be NULL in some situations.
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My c++ is obviously rusty, what situations are those?
Today's lesson is brought to you by the word "niggardly". Remember kids, don't attribute to racism what can be explained by Scandinavian language roots.
-- Robert Royall
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Call a method passing a null object...
In Delphi, the Free method checks that it was called for an object that was really created. It's quite useful, since you can write "MyObject.Free" instead of "if Assigned(MyObject) then MyObject.Free;". A real time saver, and code is so much more readable.
-- Quidquid latine dictum sit, altum sonatur.
http://streambolics.flimbase.com
S. L.
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C++ lets you set the this object in the parameters instead of always using the object the method is called on?
Today's lesson is brought to you by the word "niggardly". Remember kids, don't attribute to racism what can be explained by Scandinavian language roots.
-- Robert Royall
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No, but if you write:
int foo ()
{
string *s = null;
return s->length();
}
the string::length function will be called with this==null. This could be used so that the null string would behave as the empty string.
-- Quidquid latine dictum sit, altum sonatur.
http://streambolics.flimbase.com
S. L.
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OK, that makes sense.
Today's lesson is brought to you by the word "niggardly". Remember kids, don't attribute to racism what can be explained by Scandinavian language roots.
-- Robert Royall
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Thereby obscuring the real problem, which is that you have a null object instance you're trying to use. Loads of fun tracking down something like that.
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William has already explained it neatly. Here is another example if you are interested.
class Dummy{
public:
void CheckNull(){
if(!this)
std::cout << "This is NULL!";
}
};
Dummy* dummy = 0;
dummy->CheckNull();You can see This is NULL message appearing in the above example. Above code is problematic and as per C++ standard, calls on an uninitialized pointer will lead into unexpected results.
dan neely wrote: C++ lets you set the this object in the parameters instead of always using the object the method is called on?
No. AFAIK, compiler compiles the CheckNull() method like
void Dummy_CheckNull(Dummy* this); and the Dummy instance we created will be passed to this method. Your code will work fine if you are not using the this. Any operation on this will fail as it is not initialized.
Hope it is clear now!
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would this be another example?
class NullCall {
public:
char PublicMethod(int x) {
return (char)x; }
};
NullCall::PublicMethod(12);
I.E. using the namespace/scope operator '::' ?
-Adam
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TheScientistIsDead wrote: would this be another example?
I don't think so. Your code produces error " error: cannot call member function ‘char NullCall::PublicMethod(int)’ without object". You can compile this by making the method as static but you can't use this inside a static method.
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When I saw that line written by one of our 'expert' all over the place;
i=i++;
It made me wonders if the compiler was smart enough to do only the increment or if it was really assigning i to itself and then doing the increment.
modified on Thursday, March 19, 2009 3:06 PM
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Kurdy Malloy wrote: It made me wonders if the compiler was smart enough to ...
We can only hope the compiler generates code compatible with the language specification; i++ is a post-increment operator, it increments after the main operation (=) got executed. It isn't a compiler's duty to guess at what the programmer intended, it should do so only when reporting error and warning messages ("Are you missing a ;").
Luc Pattyn [Forum Guidelines] [My Articles]
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It is also its duty to optimize the code if you tell it to, but this case is not optimized (I checked with the Reflector). Not even if i is not used (it should be automatically removed!). i isn't a property so removing an assignment of i to itself has no effect other than making the code faster.
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Make sure the project is set to build for release (actually check the optimize switch is on).
the MS compilers are somewhat lazy when there is the slightest doubt you are going to debug...
Furthermore they always have the ultimate excuse: the JIT compiler will take care of it!
[ADDED]I somehow replied to another version of your message;
also it gets interesting when i happens to be volatile...
[/ADDED]
Luc Pattyn [Forum Guidelines] [My Articles]
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It was in release mode of course otherwise it's only normal that it wouldn't do any optimizations
Ok didn't think of volatile - well then "any variable or field that is not volatile" can have an assignment to itself removed without any harm (unless you're doing scary reflection tricks..)
But the JIT compiler is a problem.. how can you see what code it produces?
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harold aptroot wrote: how can you see what code it produces?
Not sure. Some statements in one of the forums seemed to indicate it would be a feature of Visual Studio Pro or TS, I am using Express so I have no support for it. Maybe attaching another Studio to a running process works, haven't tried it yet with a managed app.
Luc Pattyn [Forum Guidelines] [My Articles]
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Hey,
The CLR ships with a tool called NGen. NGen compiles IL into native code so if you use this you will be able
to see what code is produced
Hope that helps
At university studying Software Engineering - if i say this line to girls i find they won't talk to me
Dan
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I don't like to think what would happen to "i = i++" if "i" is truely volatile. Would the post increment be actioned on an interrupt modified value? Now that would be a good bug to find!
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Luc Pattyn wrote: It isn't a compiler's duty to guess at what the programmer intended, it should do so only when reporting error and warning messages ("Are you missing a ;").
Waaay back in the good old days at University we were blessed / cursed with using a PL/C compiler. It was a student version of the full PL/I compiler and was guaranteed to always produce some sort of output regardless of how stupid your input was. This was a good thing as often you only got one chance a day (yes kids, that is one chance a day) to see if your program would compile and work. The results were often spectacular with pages of fanfold shooting out of the line printer.
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I do remember IBM mainframes batch processing Fortran jobs (actually Watfor/Watfiv), punched on Hollerith cards, read one by one by a noisy machine, compiled one by one (no intermediate storage), listed one by one on an even noisier line printer, with each student allotted 5 minutes of execution time, for a total of one hour a day; the remainder of the day the uni needed its mainframe for its own IT handling.
In 5 minutes the huge machine performed much less than a typical desktop would do today in 1 second. The good thing was the compile and link stages were not time-limited, and the compilers tried to maximize the number and meaningfulness of the messages it emitted.
Luc Pattyn [Forum Guidelines] [My Articles]
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You forgot to mention the keypunches never had a ribbon with ink so your deck of cards were completly blank making inserting and modifing the code next to impossible. (and we did it while walking uphill in the pouring rain).
My favorite was taking the compiler course (Dragon Book) using an IBM 1130 [^]running I think Fortran 66 -- only arithmatic IF for conditional processing. IF( expression ) 100, 200, 300 Fun.
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"Oh, I see by your variable name that you intended this to be an unsigned imaginary number. Level of certainty: 95%"
That's not actually what the output said, but that's an example of the kind of thing that I imagined the compiler saying that as it was processing the code. I do seem to recall it giving some kind of "level of certainty".
We are dyslexic of Borg. Refutance is systile. Your a$$ will be laminated!
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That's right. It was somewhat conversational with a hint of arrogant superiority. You wrote "this" (you bonehead). PL/C chooses to do "this". If smileys had been invented then I'm sure it would have followed with a :P
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In c++, VS 2005, the unoptimized code is actually kind of stupid
i = i++;
mov eax,dword ptr [i (40904Ch)]
mov dword ptr [i (40904Ch)],eax
mov ecx,dword ptr [i (40904Ch)]
add ecx,1
mov dword ptr [i (40904Ch)],ecx
It gets i, stores it in i, gets i, increments i, stores it in i. Not sure why it thinks it needs to do the first 2 steps.
When I turn on "optimized for speed" (not comfort like me) it generates the exact same (smart) code as
++i;
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the stoopit code is pretty normal since i=i++; falls apart in i=i; and i++; and that is what the assembly code does, without any optimization.
Luc Pattyn [Forum Guidelines] [My Articles]
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