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digitalmars.D.learn - Trying to understand a simple piece of code: dmd barray

reply Dibyendu Majumdar <mobile majumdar.org.uk> writes:
Hi,

I am trying to understand 
https://github.com/dlang/dmd/blob/master/src/dmd/backend/barray.d.

Two questions:

1. What does this mean and why is it needed?

line 95: alias array this;

2. The struct has no property called length - but this is 
referenced. Where does this come from?

Thank you

Regards
Mar 07 2020
parent reply drug <drug2004 bk.ru> writes:
07.03.2020 15:05, Dibyendu Majumdar пишет:

 Hi,
 
 I am trying to understand 
 https://github.com/dlang/dmd/blob/master/src/dmd/backend/barray.d.
 
 Two questions:
 
 1. What does this mean and why is it needed?
 
 line 95: alias array this;
 


This means that `array` can be used instead of `this`

 2. The struct has no property called length - but this is referenced. 
 Where does this come from?


it comes from `array` being alias for `this`, see above

 
 Thank you
 
 Regards
Mar 07 2020
next sibling parent Steven Schveighoffer <schveiguy gmail.com> writes:
On 3/7/20 7:26 AM, drug wrote:

 07.03.2020 15:05, Dibyendu Majumdar пишет:

 Hi,

 I am trying to understand 
 https://github.com/dlang/dmd/blob/master/src/dmd/backend/barray.d.

 Two questions:

 1. What does this mean and why is it needed?

 line 95: alias array this;


 This means that `array` can be used instead of `this`


To expand on this, the compiler basically substitutes the symbol aliased 
here for the item itself if all other members are a compiler error.

So for example, if you have:

barr.length;

and barr has no member length, it tries:

barr.array.length

-Steve
Mar 07 2020
prev sibling parent reply Dibyendu Majumdar <mobile majumdar.org.uk> writes:
On Saturday, 7 March 2020 at 12:26:32 UTC, drug wrote:


 I am trying to understand 
 https://github.com/dlang/dmd/blob/master/src/dmd/backend/barray.d.
 
 Two questions:
 
 1. What does this mean and why is it needed?
 
 line 95: alias array this;
 


 This means that `array` can be used instead of `this`


Hmm should not that change the meaning of this throughout the 
struct? is this good practice?

Thank you
Mar 07 2020
next sibling parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 3/7/20 8:06 AM, Dibyendu Majumdar wrote:

 On Saturday, 7 March 2020 at 12:26:32 UTC, drug wrote:
 

 I am trying to understand 
 https://github.com/dlang/dmd/blob/master/src/dmd/backend/barray.d.

 Two questions:

 1. What does this mean and why is it needed?

 line 95: alias array this;


 This means that `array` can be used instead of `this`


 
 Hmm should not that change the meaning of this throughout the struct? is 
 this good practice?


No, it's simply a fallback. If the symbol doesn't work with this.symbol, 
try this.array.symbol.

It's D's version of implicit conversion.

You can make the alias this a no-arg function and it will try calling 
that function.

-Steve
Mar 07 2020
parent Dibyendu Majumdar <d.majumdar gmail.com> writes:
On Saturday, 7 March 2020 at 14:33:29 UTC, Steven Schveighoffer 
wrote:

 It's D's version of implicit conversion.

 You can make the alias this a no-arg function and it will try 
 calling that function.


Okay thank you.
Mar 07 2020
prev sibling parent Ferhat =?UTF-8?B?S3VydHVsbXXFnw==?= <aferust gmail.com> writes:
On Saturday, 7 March 2020 at 13:06:39 UTC, Dibyendu Majumdar 
wrote:

 On Saturday, 7 March 2020 at 12:26:32 UTC, drug wrote:


 I am trying to understand 
 https://github.com/dlang/dmd/blob/master/src/dmd/backend/barray.d.
 
 Two questions:
 
 1. What does this mean and why is it needed?
 
 line 95: alias array this;
 


 This means that `array` can be used instead of `this`


 Hmm should not that change the meaning of this throughout the 
 struct? is this good practice?

 Thank you


This is also a way of imitating inheritance in c-ish code without 
classes.
Mar 07 2020