why can't we have array of references.
Dec 4 '07
31  3236 
On Dec 6, 6:47 pm, Joe Greer <jgr...@doublet ake.comwrote:
terminator <farid.mehr...@ gmail.comwrote in news:b2d73c27-cfd5-42ee-ae2d-
2b5790802...@s3 6g2000prg.googl egroups.com:
On Dec 5, 12:32 am, "Alf P. Steinbach" <al...@start.no wrote:
* siddhu:
why can't we have array of references.
A reference is not an object, it has no size.
it has a size usually equal to that of a pointer in case of usage an
argument to a none inlined function.
No, references don't even have to have a physical representation.
Analogous to typedef, a reference only introduces a new name for an object.
It turns out that usually memory for an address is allocated for reference
parameters and reference object members, but it would be a pretty poor
compiler that allocated anything at all for a local reference.
joe
I said in case of usage as **an argument to a none inlined function**.
On Dec 6, 12:41 am, "Alf P. Steinbach" <al...@start.no wrote:
* terminator:
On Dec 5, 12:32 am, "Alf P. Steinbach" <al...@start.no wrote:
* siddhu:
>why can't we have array of references.
A reference is not an object, it has no size.
it has a size usually equal to that of a pointer in case of usage an
argument to a none inlined function.
Sorry, that's incorrect. Or not even wrong. If you want to make an
argument, cough up chapter and verse from the standard.
I do not get it:
void f(int& iref);
can you explain how 'iref ' is passed to 'f' if it is not inlined?
The simplest way is to implement it as a pointer ,so stack register
will be decreased sizeof(int*) for passing 'iref'.
regards,
FM.
On Dec 6, 12:35 am, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
terminator wrote:
On Dec 5, 12:32 am, "Alf P. Steinbach" <al...@start.no wrote:
* siddhu:
>why can't we have array of references.
A reference is not an object, it has no size.
it has a size usually equal to that of a pointer in case of usage an
argument to a none inlined function.
And when the size isn't equal to that of a pointer, what is it?
Nothing,A pseudo C++ instruction, not even some gaseus.when not an
argument to out of line functions it is resolved at compile time.
regards,
FM.
terminator <fa***********@ gmail.comwrote in
news:e7******** *************** ***********@e10 g2000prf.google groups.com:
On Dec 6, 6:47 pm, Joe Greer <jgr...@doublet ake.comwrote:
>terminator <farid.mehr...@ gmail.comwrote in
news:b2d73c2 7-cfd5-42ee-ae2d-
2b5790802...@s 36g2000prg.goog legroups.com:
On Dec 5, 12:32 am, "Alf P. Steinbach" <al...@start.no wrote:
* siddhu:
why can't we have array of references.
>A reference is not an object, it has no size.
it has a size usually equal to that of a pointer in case of usage
an argument to a none inlined function.
No, references don't even have to have a physical representation.
Analogous to typedef, a reference only introduces a new name for an
object. It turns out that usually memory for an address is allocated
for reference parameters and reference object members, but it would
be a pretty poor compiler that allocated anything at all for a local
reference.
joe
I said in case of usage as **an argument to a none inlined function**.
Sadly (or is it gladly? Depends upon the topic and POV.) you don't
really have any control over that. Some of today's modern compilers can
inline in the linker stage. Some classes with reference member
variables can also be unrolled and inlined during compilation.
Therefore, it's hard to make any flat statement about the storage of
references other than they don't require any.
joe
Alf P. Steinbach wrote:
>it has a size usually equal to that of a pointer in case of usage an
argument to a none inlined function.
Sorry, that's incorrect. Or not even wrong. If you want to make an
argument, cough up chapter and verse from the standard.
He said "usually", which I understand to mean "in practice with
current compilers and architectures".
On Dec 6, 5:52 pm, terminator <farid.mehr...@ gmail.comwrote:
On Dec 6, 12:41 am, "Alf P. Steinbach" <al...@start.no wrote:
* terminator:
On Dec 5, 12:32 am, "Alf P. Steinbach" <al...@start.no wrote:
>* siddhu:
>>why can't we have array of references.
>A reference is not an object, it has no size.
it has a size usually equal to that of a pointer in case
of usage an argument to a none inlined function.
Sorry, that's incorrect. Or not even wrong. If you want to
make an argument, cough up chapter and verse from the
standard.
I do not get it:
void f(int& iref);
can you explain how 'iref ' is passed to 'f' if it is not
inlined? The simplest way is to implement it as a pointer ,so
stack register will be decreased sizeof(int*) for passing
'iref'.
First, inlining or not is irrelevant. The standard specifically
states that inlining has no effect on a function, so it can't
possibly the size of an argument to the function can't possibly
depend on that. Second, how the compiler passes arguments with
reference type is it's business, not mine. My compiler
certainly doesn't pass them on the stack; it puts them in a
register. A 64 bit register, even when I'm compiling in 32 bit
mode (where pointers are 32 bits). And on my compiler, the
actual memory space used by a reference in a structure will
depend on what is in front of and behind the reference. (The
same is true of pointers, of course.)
What is relevant is that the compiler says that sizeof( char& )
== 1, and sizeof( T& ) == sizeof( T ), regardless of the size of
a pointer. A reference doesn't have size. At least not of its
own. A reference is an other name for a variable or a value,
and its "size" is the size of whatever it names.
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
On Dec 7, 4:06 am, "Alf P. Steinbach" <al...@start.no wrote:
* terminator:
I do not get it:
void f(int& iref);
can you explain how 'iref ' is passed to 'f' if it is not inlined?
The simplest way is to implement it as a pointer ,so stack register
will be decreased sizeof(int*) for passing 'iref'.
First, note that there may not be a stack register.
The C++ abstract machine implicitly defines a logical stack, a
LIFO queue, for automatic variables and function arguments,
but that does not mean that it depends on a stack register at
the hardware level or hardware support for a call stack.
That said, using a hardware stack is the most common and today in
practice the only implementation of the abstract machine's stack.
Sort of. In practice, except for Intel architecture (which is
notoriously poor in registers), I don't know of any case where
arguments are passed on the stack. They're passed in registers.
On a modern Sun Sparc (v9 or later), those registers are always
64 bits: 8 bytes. So a reference will be passed in 8 bytes. As
will a char, a short, an int, a long or a long long, all
pointers (even in 32 bit mode, where a pointer is normally only
4 bytes), and all enums. (None of which has anything to do with
the size of the type in the C++ sense, but I'm sure you know
that already.)
Also, in addition to a stack register, the Sparc has a register
stack. When you call a function, it is shifted down 192 bytes,
regardless of the number and types of the arguments. Does this
mean that if I call a function with one reference argument, the
size of the reference is 192, but if I call it with 2, the size
is 96?
[...]
Common to all these cases is that because references do not
have size at the abstract machine level, and are not objects
at the abstract machine level, the compiler is free to
optimize or do anything.
Note that unless you actually output the value of a pointer,
that's fairly true for pointers as well. All the actual machine
has to do is ensure that the observable behavior is the same as
one possible instantiation of the abstract machine.
All the compiler needs to do -- barring use of
compiler-specific language extensions -- is to make sure the
requirements of the abstract machine are fulfilled, treating
the reference as an alias.
Just to add to the confusion: according to the standard,
references do have size, since sizeof( T& ) is a perfectly
legal, well defined instruction. And a reference to a pointer
does have the size of a pointer. Just as a reference to an int
has the size of an int, and a reference to a double has the size
of a double. And on my machine, a reference to an int[1000000]
has a size of 8000000---good thing pointers aren't that big.
Also, C++ guarantees that a[i] is the same as *(a + i), and that
if a has type T[] (or T*), (char*)a + sizeof(T)*i == &a[i]. Now
explain what that last expression does where T is a reference.
(I think new T[n] would also be a bit of a problem if T could
have a reference type. Or even simply new T---which guarantees
that operator new( sizeof( T ) ) will be called.)
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
On Dec 6, 5:48 pm, terminator <farid.mehr...@ gmail.comwrote:
On Dec 6, 6:47 pm, Joe Greer <jgr...@doublet ake.comwrote:
terminator <farid.mehr...@ gmail.comwrote in news:b2d73c27-cfd5-42ee-ae2d-
2b5790802...@s3 6g2000prg.googl egroups.com:
On Dec 5, 12:32 am, "Alf P. Steinbach" <al...@start.no wrote:
>* siddhu:
why can't we have array of references.
>A reference is not an object, it has no size.
it has a size usually equal to that of a pointer in case of usage an
argument to a none inlined function.
No, references don't even have to have a physical
representation. Analogous to typedef, a reference only
introduces a new name for an object. It turns out that
usually memory for an address is allocated for reference
parameters and reference object members, but it would be a
pretty poor compiler that allocated anything at all for a
local reference.
I said in case of usage as **an argument to a none inlined
function**.
Which is a pretty meaningless statement. With Sun CC, on a
Sparc, when I call something like f(int&), the compiler
decrements the stack pointer by 192. What does this tell us
about the size of a reference, or the size of the return value,
or anything else?
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
On Dec 7, 3:00 pm, "Alf P. Steinbach" <al...@start.no wrote:
* James Kanze:
On Dec 7, 4:06 am, "Alf P. Steinbach" <al...@start.no wrote:
That said, using a hardware stack is the most common and today in
practice the only implementation of the abstract machine's stack.
Sort of. In practice, except for Intel architecture (which is
notoriously poor in registers), I don't know of any case where
arguments are passed on the stack.
Recursion.
Even then, the *arguments* are passed in registers. (You can't
have the API of a function call depending on whether the
function is recursive or not.) Naturally, the function will
take whatever steps are necessary to avoid overwriting the value
(if it still needs it, of course) before placing the argument of
the next call in the register. On a Sparc, of course, nothing
is necessary; each function automatically gets a (partially) new
set of registers.
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
On Dec 6, 6:47 pm, Joe Greer <jgr...@doublet ake.comwrote:
terminator <farid.mehr...@ gmail.comwrote in news:b2d73c27-cfd5-42ee-ae2d-
2b5790802...@s3 6g2000prg.googl egroups.com:
On Dec 5, 12:32 am, "Alf P. Steinbach" <al...@start.no wrote:
* siddhu:
why can't we have array of references.
A reference is not an object, it has no size.
it has a size usually equal to that of a pointer in case of usage an
argument to a none inlined function.
No, references don't even have to have a physical representation.
Analogous to typedef, a reference only introduces a new name for an object.
It turns out that usually memory for an address is allocated for reference
parameters and reference object members, but it would be a pretty poor
compiler that allocated anything at all for a local reference.
joe
that is just what I wrote.maybe I am so bad a composer.
regards,
FM. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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