post and pre increment

[JosAH]

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

[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...

[JosAH]

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 ...

[newb16]

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++);

[Banfa]

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.

[Tassos Souris]

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?

[JosAH]

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

[Banfa]

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.

[JosAH]

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

[Banfa]

I didn't say that;

I didn't say you did, that's my opinion, look at the post times.

[JosAH]

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)