It just came to my mind that for C programmers I have to add the
following remark:
it is not easy to pass arrays (vectors) by value in C, in contrast to -
for example - Pascal.
Because in C the name of a vector is equivalent to the address of the
element of its element zero,
this means that, if you pass a vector in C, you pass THE ADDRESS of the
first element of the vector,
which in fact implies call by reference.
So, to get a "real" call by value for a vector in C, you have to enclose
the vector in a struct definition,
because structs are "really" passed by value.
Something like this:
typedef struct
{
int x [100];
}
int_vector;
void call_by_value_example (int_vector v)
{
}
int main (void)
{
int_vector a;
call_by_value_example (a); // a is copied during call
}
In Pascal, there is no need to do such "struct embedding"; Pascal arrays
will be passed by value,
if you specify the array name as a parameter (and if the function
prototype specifies call by value;
in Pascal this is controlled by the VAR keyword ... with VAR: call by
reference; without VAR: call by value).
And: Pascal doesn't have a rule similar to C, that the array name is the
address of the first element;
an array is an array in Pascal and nothing else.
I mixed this up when I talked about copies of large structures during
call by value (in C).
(I'm the maintainer of New Stanford Pascal, BTW - New Stanford Pascal
has a CONST keyword
on parameters, too, which means: copy and call by reference, much the
same as dummy arguments
in PL/1 or - as I learned recently - CALL BY CONTENT in COBOL).
Kind regards
Bernd
Am 02.04.2023 um 16:41 schrieb Bernd Oppolzer:
Am 02.04.2023 um 16:29 schrieb Seymour J Metz:
Regardless of the implementation, call by reference is about more
than efficiency; sometimes a subroutine is required to alter one of
its parameters, and call by value doesn't allow that.
That's well known;
most C textbooks tell that call by value is impractical for large
structures or arrays
because of the copy involved; that's why for performance reasons often
pointers are passed,
although no modification of the parameters passed is desired or required.
That's what I referred to.
Kind regards
Bernd
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