post and pre increment | Newbie | | Join Date: Jan 2009
Posts: 2
| | |
what is difference between
1. y=++x++;
2. y=(++x)++;
3. y=++(x++);
|  | Expert | | Join Date: Mar 2007
Posts: 10,611
| | | re: post and pre increment
Quote:
Originally Posted by jyots  what is difference between
1. y=++x++;
2. y=(++x)++;
3. y=++(x++); Have you tried to compile your examples? If so, you would've noticed that none of them compile. A ++ operator (pre- and post) takes an lvalue as its operand and its result is an rvalue so you can't apply two of them in a row.
kind regards,
Jos
| | Newbie | | Join Date: Jan 2009
Posts: 2
| | | re: post and pre increment
y=(++x)++;
this statement is givin no problem... but rest of them are givin error..
| | Expert | | Join Date: Mar 2007
Posts: 10,611
| | | re: post and pre increment
Quote:
Originally Posted by jyots y=(++x)++;
this statement is givin no problem... but rest of them are givin error.. That expression shouldn't compile either; if your compiler accepts it throw your compiler away and install a decent one. This is what the Standard has to say about it: Quote:
Originally Posted by Standard 6.3.2.1 [#2] Except when it is the operand of the sizeof operator,
the unary & operator, the ++ operator, the -- operator, or
the left operand of the . operator or an assignment
operator, an lvalue that does not have array type is
converted to the value stored in the designated object (and
is no longer an lvalue). If the lvalue has qualified type,
the value has the unqualified version of the type of the
lvalue; otherwise, the value has the type of the lvalue. If
the lvalue has an incomplete type and does not have array kind regards,
Jos
|  | Expert | | Join Date: Aug 2007 Location: Belgium
Posts: 1,118
| | | re: post and pre increment
Same here on bcc32, got any suggestions on another (freeware) compiler to use?
| | Newbie | | Join Date: Dec 2006
Posts: 26
| | | re: post and pre increment
Download codeblocks it is free and update IDE and also come with mingw compiler
| | Expert | | Join Date: Aug 2007 Location: Belgium
Posts: 1,118
| | | re: post and pre increment
Tried it, with the same results. Are you sure about this Jos, because now you've already declared 2 quite well known compilers (as far as I know anything about it) to be not good.
| | Moderator | | Join Date: Mar 2007 Location: North Bend Washington USA
Posts: 5,366
| | | re: post and pre increment
This code: -
y=++x++; //ERROR
-
y=(++x)++; //OK
-
y=++(x++); //ERROR
-
The postfix increment has a higher precedence so it goes first. The result of an x++ is not an l_value. That means you can't use it implicitly as an assignment. For that reason this code is also no good:
However, y= y=(++x)++ is OK because the result of ++x is an l_value:
The reason for the difference is that the prefix operator actually changes the object whereas the postfix operator changes only a copy and that copy is not to be used as an l_value.
If your compiler rejects three of the examples, get a new compiler. Only the second example should compile.
| | Member | | Join Date: Aug 2008
Posts: 121
| | | re: post and pre increment
Quote:
Originally Posted by weaknessforcats This code:
-
y=++x++; //ERROR
-
y=(++x)++; //OK
-
y=++(x++); //ERROR
-
The postfix increment has a higher precedence so it goes first. The result of an x++ is not an l_value. That means you can't use it implicitly as an assignment. For that reason this code is also no good:
However, y= y=(++x)++ is OK because the result of ++x is an l_value:
The reason for the difference is that the prefix operator actually changes the object whereas the postfix operator changes only a copy and that copy is not to be used as an l_value.
If your compiler rejects three of the examples, get a new compiler. Only the second example should compile.
No this is not correct: - The result of a prefix incrementation operator is not an lvalue.
- Postfix incrementation does not affect a copy of the operand but the operand itself. The difference, from the prefix incrementation, is the timing that the value is noted. As far as postfix incrementation is concerned, the value gets noted first and then the operand is incremented (++) or decremented (--) by 1.
| | Expert | | Join Date: Aug 2007 Location: Belgium
Posts: 1,118
| | | re: post and pre increment
Quote:
Originally Posted by Tassos Souris No this is not correct: It compiles without a problem though.
But if you try: - int x = 0;
-
int y = 0;
-
++x = x;
-
++y = y+5;
-
cout << x << " " << y;
Output when compiled with bcc32: 1 5
Output when compiled with mingw: 1 6
So which one is right, or is neither of them?
| | Member | | Join Date: Aug 2008
Posts: 121
| | | re: post and pre increment
Neither of them is correct. As Jos said the result is not a lvalue.
Visual Studio gives (correctly) a compile error: -
error C2106: '=' : left operand must be l-value
-
For example, consider this: -
int x[ 2 ];
-
int *ptr = x;
-
This is not correct:
That is, because the 'result' of the assignment expression depends on the ++ operator. The zero (0) will be placed after the ++ has been 'resolved'. Of course, ++ does not produce a lvalue so a compile time error must be given by the compiler.
However, these are correct
Because the 'result' of the assignment "does not depend" on the ++ operator.
Let's examine the first case:
... the ++ does not produce a lvalue but then the * operator takes effect so everything is ok
... the value is noted first, * takes place and then the ++ is activated (which has no effect on the assignment).
So the problem is not where the ++ or -- are: on the left or on the right of the = operator. But, if the result that they do not produce a lvalue actually affects the assignment.
| | Needs Regular Fix | | Join Date: Jul 2008
Posts: 381
| | | re: post and pre increment
Quote:
Originally Posted by Tassos Souris Neither of them is correct. As Jos said the result is not a lvalue. Jos said "...Except when it is the operand of the sizeof operator,
the unary & operator, the ++ operator..."
And i can quote(retype, as it's secured) 5.3.2.1 of the 14822 standard, regarding prefix ++/-- operator:
"The value is the new value of the operand; it is an lvalue"
You can -
printf("%p\n", &a );
-
printf("%p\n", &++a );
And it should print the same.
| | Member | | Join Date: Aug 2008
Posts: 121
| | | re: post and pre increment
Quote:
Originally Posted by newb16 Jos said "...Except when it is the operand of the sizeof operator,
the unary & operator, the ++ operator..."
And i can quote(retype, as it's secured) 5.3.2.1 of the 14822 standard, regarding prefix ++/-- operator:
"The value is the new value of the operand; it is an lvalue"
You can -
printf("%p\n", &a );
-
printf("%p\n", &++a );
And it should print the same. correct result:
++ on a does not produce a lvalue so the use of &++a is illegal.
And this is of course logical. What does ++a do? It increments 'a' and returns back the new value.. why do you want to get the address of that new value?? this is both illegal and not logical...
And is exactly what Jos stated...
Common English indicate that the expression Except when means 'every other case except this'. When used other than as a operand to the sizeof() operator (that means in all cases; including our examples) it does not produce an lvalue.
This is legal: -
int y = 0;
-
size_t size = sizeof( ++y );
-
, and also, since the operand of the sizeof() operator is not evaluated if it is not a variable length array type, y remains 0.
| | Needs Regular Fix | | Join Date: Jul 2008
Posts: 381
| | | re: post and pre increment
Quote:
Originally Posted by Tassos Souris correct result:
++ on a does not produce a lvalue so the use of &++a is illegal. Compiler version, please. If it works contrary to what 5.3.2.1 of the 14822 standard says regarding prefix ++/-- operator:
"The value is the new value of the operand; it is an lvalue"
It works on old dos turbo c and on gcc 3.3.3 from cygwin. Quote:
And this is of course logical. What does ++a do? It increments 'a' and returns back the new value.. why do you want to get the address of that new value?? this is both illegal and not logical...
Not only new value, but the variable itself that holds the new value. This is not the case of postfix one, when the old value is returned and the variable itself holds the new value.
| | Needs Regular Fix | | Join Date: Jul 2008
Posts: 381
| | | re: post and pre increment
Quote:
Originally Posted by YarrOfDoom ++y = y+5;
So which one is right, or is neither of them? The result of this expression is undefined, because compiler can add 5 to old value, increment y in left part, and then overwrite it with 5, or increment first and then calculate 1+5 and then assign; It happns because the only 'sequence point' here is at the end of the expression.
| | Expert | | Join Date: Mar 2007
Posts: 10,611
| | | re: post and pre increment
There's a difference between C and C++ here; I assumed C was being used by the OP and quoted the C Standard text. In C none of the expressions compile because none of the (pre and post) increment operators result in an lvalue.
C++ however treats both operators differently: the preincrement ++ operator seems to leave an lvalue; the postincrement ++ operator doesn't. While I agree that my assumption could've been totally wrong (about the language being used), the OP should've said which one actually was used.
kind regards,
Jos
| | Member | | Join Date: Aug 2008
Posts: 121
| | | re: post and pre increment
Well, clearly there was a difference in us about the language.
I referred to C99.. and newb99 probably to a C++ Standard... jyots was probably referring to C++ too..
Since i am not familiar with C++ standards i assumed that (since it is told C++ has compatibility with C) it would be the same...
Just wrong of us not to clarify the language and look at both languages as we should do...
| | Expert | | Join Date: Mar 2007
Posts: 10,611
| | | re: post and pre increment
Quote:
Originally Posted by Tassos Souris Well, clearly there was a difference in us about the language.
I referred to C99.. and newb99 probably to a C++ Standard... jyots was probably referring to C++ too..
Since i am not familiar with C++ standards i assumed that (since it is told C++ has compatibility with C) it would be the same...
Just wrong of us not to clarify the language and look at both languages as we should do... Indeed; being a C person myself and *knowing* that none of the ++ nor -- operators yield an lvalue in any of the C versions (ranging from K&R1 up to the current version) I bluntly assumed that the OP was talking about C.
I heartly support splitting this forum in a C and C++ forum just to get rid of the confusion (we even get C# questions here because that also starts with a C ;-)
Assumptions are wrong of course but I fail to see what the benefit would be for the pre ++ operator yielding an lvalue: -
int y= 42, x= 54;
-
-
++y= x;
-
Great, what's the benefit of ++y being an lvalue? It ends up with the value 54 anyway, effectively annihilating the pre increment ...
kind regards,
Jos
edit: I failed to find a satisfactory explanation in Bjarne Stroustrups brick of a book ...
| | Needs Regular Fix | | Join Date: Jul 2008
Posts: 381
| | | re: post and pre increment
Quote:
Originally Posted by JosAH Great, what's the benefit of ++y being an lvalue? It ends up with the value 54 anyway, effectively annihilating the pre increment ... you can pass it by reference - void bar(int &x);
-
...
-
bar(++x); // is it different than x++;bar(x); ?
-
//fail: bar(x++);
-
|  | AdministratorVoR | | Join Date: Feb 2006 Location: South West UK
Posts: 6,165
| | | re: post and pre increment
This
y=(++x)++; //OK
may be valid syntax and compilable in C++ but if x is a basic type, say an int, it still falls fowl of modifying a variable twice between sequence points and thus produces undefined behaviour.
On the other hand if x is an object with an overloaded operator++ then rather than being an actual operator it is a function call and it is not undefined behaviour, as there is a sequence point "At a function return, after the return value is copied into the calling context."
Needless to say this sort of silly coding structure is best avoided.
| | Member | | Join Date: Aug 2008
Posts: 121
| | | re: post and pre increment
isn't C++ fully backwards compatible with C (as much as i know.. i am not a fun of C++)?
How can there exist such a difference?
| | Expert | | Join Date: Mar 2007
Posts: 10,611
| | | re: post and pre increment
Quote:
Originally Posted by Tassos Souris isn't C++ fully backwards compatible with C Nope; this is one of the peculiarities; there are more: quite a few of them handle widening conversions and the scope of variables. C and C++ are two different languages after all (although people may not realize this).
kind regards,
Jos
| | AdministratorVoR | | Join Date: Feb 2006 Location: South West UK
Posts: 6,165
| | | re: post and pre increment
No C and C++ are more like siblings than C++ a descendant of C. They have a large area of functionality that they both support but each language has things that are not part of the other Compatibility of C and C++ - Wikipedia, the free encyclopedia
Perhaps the easiest example of this is int *pi = malloc(sizeof *pi); which is completely valid C and in fact in C it would be best practice not to cast the output of malloc. This wont compile in C++ which doesn't allow assignment of non-equal pointers, it would require a cast.
The thing is people almost never compile C code as C++, most C++ compilers include a C compiler and when compiling a file with a .C extension compile it as C.
| | Expert | | Join Date: Mar 2007
Posts: 10,611
| | | re: post and pre increment
Quote:
Originally Posted by Banfa No C and C++ are more like siblings than C++ a descendant of C. I didn't say that; all I said was that C and C++ are two different languages.
kind regards,
Jos
| | AdministratorVoR | | Join Date: Feb 2006 Location: South West UK
Posts: 6,165
| | | re: post and pre increment
Quote:
Originally Posted by JosAH I didn't say that; I didn't say you did, that's my opinion, look at the post times.
| | Expert | | Join Date: Mar 2007
Posts: 10,611
| | | re: post and pre increment
Quote:
Originally Posted by Banfa I didn't say you did, that's my opinion, look at the post times. Well, you *could've* typed that all in two minutes ;-)
kind regards,
Jos (<--- slow typer of the year)
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