alias this - am I using it wrong?

Aug 25, 2021

Johann Lermer

Aug 25, 2021

jfondren

Aug 25, 2021

Aug 25, 2021

Aug 25, 2021

Aug 25, 2021

01 struct Test_Struct {long t;} 02 03 class Alias_Class 04 { 05 Test_Struct ts; 06 alias ts this; 07 } 08 09 class Test_Class 10 { 11 Alias_Class ac; 12 } 13 14 void main () 15 { 16 auto ac = new Alias_Class; 17 Test_Struct ts = ac; // compiles 18 ac = ts; // compiles as well - why? 19 20 auto tc = new Test_Class; 21 ts = tc.ac; // compiles 22 tc.ac = ts; // again this compiles, but seg faults 23 }

14 void main () 15 { 16 auto ac = new Alias_Class; 17 Test_Struct ts = ac; // compiles 18 ac = ts; // compiles as well - why? 19 20 auto tc = new Test_Class; 21 ts = tc.ac; // compiles 22 tc.ac = ts; // again this compiles, but seg faults 23 }

On Wednesday, 25 August 2021 at 12:11:01 UTC, Johann Lermer wrote: > I have a little problem understanding alias this. I always thought, that alias this only makes implicit conversions from the aliased object to this. What it does is if "a SOMETHING b" doesn't compile, it instead tries "a.alias_this_member SOMETHING b" instead, or "a SOMETHING b.alias_this_member" instead if that's on the other side. The object with alias this must already exist though, so constructors are an exception (though another object's constructor can trigger some existing object's alias this when used as a param to that). The "SOMETHING" there can be operators like + or = or a .member. Only if both fail to compile do you actually get an error. > 17 Test_Struct ts = ac; // compiles So what really happens here is the compiler sees ts = ac; fails to compile, so it is rewritten into "ts = ac.ts;" > 18 ac = ts; // compiles as well - why? So ac = ts fails, meaning it rewrites into `ac.ts = ts;` > 20 auto tc = new Test_Class; > 21 ts = tc.ac; // compiles So here it is rewritten into `ts = tc.ac.ts`. > 22 tc.ac = ts; // again this compiles, but seg faults And now tc.ac.ts = ts; is the rewrite since the plain one didn't compile, thus accessing the null member. Note too that alias this can be to a function, in which case the rewrite will call the function. Implicit conversion isn't really what alias this is about. It kinda works (though note with a alias this struct it can be passed by value and thus copy, again the compiler just does the rewrite). It is just giving transparent access to a member.

August 25, 2021

Re: alias this - am I using it wrong?

Posted by FeepingCreature
in reply to Johann Lermer

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FeepingCreature

Posted in reply to Johann Lermer

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On Wednesday, 25 August 2021 at 12:11:01 UTC, Johann Lermer wrote:

Hi all,

I have a little problem understanding alias this. I always thought, that alias this only makes implicit conversions from the aliased object to this. Then, why do lines 18 and 22 compile in the code below? And, btw, line 22 crashes with a segmentation fault.

01 struct Test_Struct {long t;}
02
03 class Alias_Class
04 {
05     Test_Struct ts;
06     alias ts this;
07 }
08
09 class Test_Class
10 {
11     Alias_Class ac;
12 }
13
14 void main ()
15 {
16     auto ac = new Alias_Class;
17     Test_Struct ts = ac;  // compiles
18     ac = ts;              // compiles as well - why?
19
20     auto tc = new Test_Class;
21     ts = tc.ac;           // compiles
22     tc.ac = ts;           // again this compiles, but seg faults
23 }

Johann

ts is a field. You can assign to a field. So when the field is aliased to this, you can assign to the field through a class reference.

You can disable this behavior by creating a getter in Alias_Class, then aliasing it to this:

class Alias_Class
{
    Test_Struct ts;
    Test_Struct getter() { return ts; }
    alias getter this;
}

On Wednesday, 25 August 2021 at 12:23:06 UTC, Adam D Ruppe wrote: ... Thanks - that explains in all. On Wednesday, 25 August 2021 at 12:23:32 UTC, FeepingCreature wrote: > class Alias_Class > { > Test_Struct ts; > Test_Struct getter() { return ts; } > alias getter this; > } Good idea, that should solve my current problem. The reason for having an alias this was that I had a C data type (cairo_surface_t) that I transferred into a class and I'm using the alias so that I don't need to rewrite the whole application at once. Unfortunately I overlooked some code that resulted in a seg fault - and I was lost at understanding why.

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