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digitalmars.D.learn - inout after function

reply Dave Jones <dave jones.com> writes:
What does the "inout" after front() do here...


 property ref inout(T) front() inout
{
     assert(_data.refCountedStore.isInitialized);
     return _data._payload[0];
}

Cant seem to find an explanation in the docs or forums :(
Nov 25 2017
next sibling parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 11/25/2017 01:51 PM, Dave Jones wrote:

 What does the "inout" after front() do here...


  property ref inout(T) front() inout
 {
      assert(_data.refCountedStore.isInitialized);
      return _data._payload[0];
 }

 Cant seem to find an explanation in the docs or forums :(


It's for member functions. Without it, and if you needed, you would have 
to write separate functions for mutable, const, and immutable objects of 
that type.

For example, the following function works for all three qualifications. 
It won't compile if you remove that inout:

struct S {
     int i;

      property ref inout(int) front() inout {
         return i;
     }
}

void main() {
     auto m = S(1);
     auto c = const(S)(2);
     static assert(is(typeof(m.front) == int));
     static assert(is(typeof(c.front) == const(int)));
}

Ali
Nov 25 2017
parent reply Dave Jones <dave jones.com> writes:
On Saturday, 25 November 2017 at 21:59:54 UTC, Ali Çehreli wrote:

 On 11/25/2017 01:51 PM, Dave Jones wrote:

 What does the "inout" after front() do here...


  property ref inout(T) front() inout
 {
      assert(_data.refCountedStore.isInitialized);
      return _data._payload[0];
 }

 Cant seem to find an explanation in the docs or forums :(


 It's for member functions. Without it, and if you needed, you 
 would have to write separate functions for mutable, const, and 
 immutable objects of that type.

 For example, the following function works for all three 
 qualifications. It won't compile if you remove that inout:

 struct S {
     int i;

      property ref inout(int) front() inout {
         return i;
     }
 }

 void main() {
     auto m = S(1);
     auto c = const(S)(2);
     static assert(is(typeof(m.front) == int));
     static assert(is(typeof(c.front) == const(int)));
 }

 Ali


So it makes it a const/immutable/mutable method depending on 
whether the instance it is called on is const/immutable/mutable?

So


      property ref inout(int) front() inout {
         return i++;
     }


Would fail if you called it on an immutable instance of S.
Nov 25 2017
parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Sunday, 26 November 2017 at 01:35:01 UTC, Dave Jones wrote:

 So it makes it a const/immutable/mutable method depending on 
 whether the instance it is called on is const/immutable/mutable?


On the outside, yes.


 So


      property ref inout(int) front() inout {
         return i++;
     }


 Would fail if you called it on an immutable instance of S.


That wouldn't compile in any case: on the inside of the function, 
inout == const (this is the only way the one function can be used 
for all three). The inout propagation is just seen at the call 
site.
Nov 25 2017
parent Dave Jones <dave jones.com> writes:
On Sunday, 26 November 2017 at 04:51:08 UTC, Adam D. Ruppe wrote:

 On Sunday, 26 November 2017 at 01:35:01 UTC, Dave Jones wrote:

 So it makes it a const/immutable/mutable method depending on 
 whether the instance it is called on is 
 const/immutable/mutable?


 On the outside, yes.


 So


      property ref inout(int) front() inout {
         return i++;
     }


 Would fail if you called it on an immutable instance of S.


 That wouldn't compile in any case: on the inside of the 
 function, inout == const (this is the only way the one function 
 can be used for all three). The inout propagation is just seen 
 at the call site.


Ahh ok, makes sense now.
Nov 26 2017
prev sibling next sibling parent Adam D. Ruppe <destructionator gmail.com> writes:
On Saturday, 25 November 2017 at 21:51:41 UTC, Dave Jones wrote:

 What does the "inout" after front() do here...


Applies the `inout` modifier to the hidden `this` variable inside 
the function.

https://dlang.org/spec/function.html#inout-functions

It basically makes it const inside the function, but on the 
outside, it matches whatever the constness was of the object it 
is called on.
Nov 25 2017
prev sibling parent Guillaume Piolat <first.last gmail.com> writes:
On Saturday, 25 November 2017 at 21:51:41 UTC, Dave Jones wrote:

 What does the "inout" after front() do here...


  property ref inout(T) front() inout
 {
     assert(_data.refCountedStore.isInitialized);
     return _data._payload[0];
 }

 Cant seem to find an explanation in the docs or forums :(


https://p0nce.github.io/d-idioms/#Knowing-inout-inside-out
Nov 26 2017