Re: Casting the return value of ...

bart <bc@freeuk.com> writes:

On 30/03/2024 09:32, Tim Rentsch wrote:
>

bart <bc@freeuk.com> writes:
>

I was aware of the double conversion but KT used 'a cast' so I
wondered if there was a single cast that could be used.

>
There is not, if it's important that it work reliably across
different compilers and different platforms.
>

It is odd however that function and object pointers can be
considered so different that even an explicit conversion
between them is deemed to be meaningless.

>
Function pointers and object pointers don't have to be the same
size, or use the same form of representation.  The C standard
allows implementations where code and data live in completely
separate memories.  In such cases there is no sensible way to
convert between the two kinds of pointers, because the two kinds
of addresses have no relationship to each other.

>
Suppose a object pointer is 32 bits, and a function pointer is a
32-byte descriptor.
>
An implementation could choose to present a function pointer as a
32-bit object pointer, which points to the full 32-byte descriptor in
data memory.
>
The simplest way of doing that is to have, for each function (or each
one whose address is taken), a fixed corresponding descriptor in data
memory.  So here function and object pointers can be exactly the same
size, and can both refer to data memory, as far as the programmer is
concerned.
>
Dereferencing such a function pointer, to call the function, will
involve an extra bit of indirection.  It would need something extra
anyway to deal with those 32 bytes.

Two problems.  One, even if the proposed scheme is workable in some
cases that doesn't mean it will be in all cases.  Two, it imposes
what may be a significant cost but offers essentially no benefit.
The only useful thing that can be done with a converted function
pointer is cast it to an appropriate function pointer type so that
the function can be called.  If someone wants to have values and
variables that can hold both object pointers and function pointers
it is easy enough to do that by using a union:

    typedef union { void *pv; void (*pf)(); } VorF;

with no hidden implementation machinery needed.  There is no good
reason to gussy up the language or have implementations jump
through hoops when the needed capability is already present in
the language as it is now (and has been for more than 30 years).