context-free grammar
Mafi
mafi at example.org
Sat Mar 5 07:53:29 PST 2011
Am 05.03.2011 16:44, schrieb Peter Alexander:
> On 5/03/11 1:39 PM, Mafi wrote:
>> Am 05.03.2011 13:10, schrieb Peter Alexander:
>>> How do you think array assignments in C++ work?
>>>
>>> a[i] = x;
>>>
>>> a[i] is just *(a + i), i.e. the evaluation of an expression that yields
>>> a temporary, which in this case is an lvalue. Same applies to all other
>>> operator[] overloads.
>>
>> No, the temporary in this case is not an lvalue. It's an adress whose
>> value is an lvalue.
>
> (a + i) is an address (type T*)
>
> *(a + i) is a lvalue reference (type T&)
>
I know. I meant the temporary itself (ie a + i) is not an lvalue; it
lies around in some register which should the coder should not
explicitly write to. The dereferencing only changed what to do with the
result. There's no computation behind derefencing.
>> The results of operator[] is a reference not a normal value. A reference
>> is an adress which is always implicitly derefenced when used.
>> In D we use opIndexAssign anyways.
>
> A reference is not an address. A reference is a synonym for another
> object. If that other object is an lvalue then the reference is also an
> lvalue.
>
A reference is nothing else than a pointer which the compiler handles
diferent at compile time. Look
/++++++ main.d +++++++++/
import std.stdio;
//extern(C) to avoid mangling
extern(C) void refTest(ref int x);
void main() {
int a = 5;
refTest(a);
a = 7;
refTest(a);
a = 42;
refTest(a);
writeln("END");
}
/+++++++ test.d ++++++++/
import std.stdio;
extern(C) void refTest(int* x) {
writefln(" x = %s, *x = %s", x, *x);
}
/+++++++ compile +++++++/
dmd -c test.d
dmd main.d test.obj
./main
/+++++ output ++++++++/
x = 12FE44, *x = 5
x = 12FE44, *x = 7
x = 12FE44, *x = 42
END
Tested with dmd 2.051 on Win7.
Look reference = pointer + implicite derefence
>.......
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