Re: array of reference?
* terminator:
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, 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.
First consider the case where the function doesn't use iref. In that
case the argument can be optimized away.
Second, consider the case where f doesn't assign to iref or take its
address. In that case, the compiler is allowed to just pass the int
value. And sizeof(int) can be much less than sizeof(int*).
Third, consider the case where in all calls of f, iref is a local
variable of the calling function, at the same offset in the stack frame.
In that case, again, the argument passing can be optimized away. Not
that I know of any C++ implementation that does that.
Fourth, consider the case of a segmented architecture where all calls of
f are passed a reference to a variable in the currently established
default data segment of the caller. In that case, an offset suffices,
no need for a full segment selector + offset pointer. That optimization
would screw up any processing of "..." arguments depending on the
assumption that the reference is the size of a pointer, if C++ allowed
the second argument to va_start to be a reference (it's currently UB).
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.
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.
Cheers, & hth.,
- Alf
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