When I have myarray.length, why isn't that considered an lvalue, and as a result, why cannot I get a pointer to it?

It seems kinda dumb, I understand it cannot be changed manually, but surely you should be able to get a const(type)* from it?

On Monday, 7 April 2014 at 19:13:31 UTC, Jeroen Bollen wrote:
> When I have myarray.length, why isn't that considered an lvalue, and as a result, why cannot I get a pointer to it?
>
> It seems kinda dumb, I understand it cannot be changed manually, but surely you should be able to get a const(type)* from it?

Well type would be ulong, so const(ulong)*

On Monday, 7 April 2014 at 19:23:45 UTC, Jeroen Bollen wrote:
> On Monday, 7 April 2014 at 19:13:31 UTC, Jeroen Bollen wrote:
>> When I have myarray.length, why isn't that considered an lvalue, and as a result, why cannot I get a pointer to it?
>>
>> It seems kinda dumb, I understand it cannot be changed manually, but surely you should be able to get a const(type)* from it?
>
> Well type would be ulong, so const(ulong)*

Array length is size_t which is uint on x86 and ulong on x86_64.

On Monday, 7 April 2014 at 19:56:38 UTC, MrSmith wrote:
> On Monday, 7 April 2014 at 19:23:45 UTC, Jeroen Bollen wrote:
>> On Monday, 7 April 2014 at 19:13:31 UTC, Jeroen Bollen wrote:
>>> When I have myarray.length, why isn't that considered an lvalue, and as a result, why cannot I get a pointer to it?
>>>
>>> It seems kinda dumb, I understand it cannot be changed manually, but surely you should be able to get a const(type)* from it?
>>
>> Well type would be ulong, so const(ulong)*
>
> Array length is size_t which is uint on x86 and ulong on x86_64.

Alright, but why can't you get a pointer to it?

On Mon, 07 Apr 2014 19:13:30 +0000, Jeroen Bollen wrote:

> When I have myarray.length, why isn't that considered an lvalue, and as a result, why cannot I get a pointer to it?
> 
> It seems kinda dumb, I understand it cannot be changed manually, but surely you should be able to get a const(type)* from it?

I believe `length` is implemented as a property function, though I can't find the source for the life of me.

On Monday, 7 April 2014 at 20:12:48 UTC, Justin Whear wrote:
> On Mon, 07 Apr 2014 19:13:30 +0000, Jeroen Bollen wrote:
>
>> When I have myarray.length, why isn't that considered an lvalue,
>> and as a result, why cannot I get a pointer to it?
>> 
>> It seems kinda dumb, I understand it cannot be changed manually,
>> but surely you should be able to get a const(type)* from it?
>
> I believe `length` is implemented as a property function, though I can't
> find the source for the life of me.

Then you still should be able to get a pointer of the result. I was also considering it to be a @property function, as indeed that'd explain this behaviour, but it still doesn't make sense to not allow it. After all, length is just a variable.

On Mon, 07 Apr 2014 20:14:13 +0000, Jeroen Bollen wrote:

> On Monday, 7 April 2014 at 20:12:48 UTC, Justin Whear wrote:
>> On Mon, 07 Apr 2014 19:13:30 +0000, Jeroen Bollen wrote:
>>
>>> When I have myarray.length, why isn't that considered an lvalue, and as a result, why cannot I get a pointer to it?
>>> 
>>> It seems kinda dumb, I understand it cannot be changed manually, but surely you should be able to get a const(type)* from it?
>>
>> I believe `length` is implemented as a property function, though I can't find the source for the life of me.
> 
> Then you still should be able to get a pointer of the result. I was also considering it to be a @property function, as indeed that'd explain this behaviour, but it still doesn't make sense to not allow it. After all, length is just a variable.

Unless the length function returns by ref, the result is an rvalue and you won't be able to take a reference.

On Mon, 07 Apr 2014 16:12:48 -0400, Justin Whear <justin@economicmodeling.com> wrote:

> On Mon, 07 Apr 2014 19:13:30 +0000, Jeroen Bollen wrote:
>
>> When I have myarray.length, why isn't that considered an lvalue,
>> and as a result, why cannot I get a pointer to it?
>>
>> It seems kinda dumb, I understand it cannot be changed manually,
>> but surely you should be able to get a const(type)* from it?
>
> I believe `length` is implemented as a property function, though I can't
> find the source for the life of me.

It's a compiler builtin property. Reading simply returns the value from inside the array (does not map to a property). Setting calls one of these two depending on the array type:

https://github.com/D-Programming-Language/druntime/blob/master/src/rt/lifetime.d#L1282
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/lifetime.d#L1460

The reason you can't get a pointer to it is that it's not a simple variable.

Note, unlike C++, D does not allow you to get a const reference or pointer to an Rvalue.

-Steve

On Monday, 7 April 2014 at 19:58:21 UTC, Jeroen Bollen wrote:
> On Monday, 7 April 2014 at 19:56:38 UTC, MrSmith wrote:
>> On Monday, 7 April 2014 at 19:23:45 UTC, Jeroen Bollen wrote:
>>> On Monday, 7 April 2014 at 19:13:31 UTC, Jeroen Bollen wrote:
>>>> When I have myarray.length, why isn't that considered an lvalue, and as a result, why cannot I get a pointer to it?
>>>>
>>>> It seems kinda dumb, I understand it cannot be changed manually, but surely you should be able to get a const(type)* from it?
>>>
>>> Well type would be ulong, so const(ulong)*
>>
>> Array length is size_t which is uint on x86 and ulong on x86_64.
>
> Alright, but why can't you get a pointer to it?

I've tried, but no luck here
http://dpaste.dzfl.pl/be526902ef4f

On 4/7/14, 1:30 PM, MrSmith wrote:
> On Monday, 7 April 2014 at 19:58:21 UTC, Jeroen Bollen wrote:
>> On Monday, 7 April 2014 at 19:56:38 UTC, MrSmith wrote:
>>> On Monday, 7 April 2014 at 19:23:45 UTC, Jeroen Bollen wrote:
>>>> On Monday, 7 April 2014 at 19:13:31 UTC, Jeroen Bollen wrote:
>>>>> When I have myarray.length, why isn't that considered an lvalue,
>>>>> and as a result, why cannot I get a pointer to it?
>>>>>
>>>>> It seems kinda dumb, I understand it cannot be changed manually,
>>>>> but surely you should be able to get a const(type)* from it?
>>>>
>>>> Well type would be ulong, so const(ulong)*
>>>
>>> Array length is size_t which is uint on x86 and ulong on x86_64.
>>
>> Alright, but why can't you get a pointer to it?
>
> I've tried, but no luck here
> http://dpaste.dzfl.pl/be526902ef4f

This stuff is best moved to the learn forum. -- Andrei