Re: array of reference?

James Kanze <>
Fri, 7 Dec 2007 05:51:17 -0800 (PST)
On Dec 7, 4:06 am, "Alf P. Steinbach" <> 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)
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