digitalmars.D.learn - alias fails to compile

Arun Chandrasekaran (17/17) Apr 22 2019 What am I doing wrong here?

Stefan Koch (6/23) Apr 22 2019 You are aliasing B.init.bb;
Alex (45/70) Apr 22 2019 aa is a "static" semantic.

Arun Chandrasekaran (56/130) Apr 22 2019 I see. This is a simplified use case in /usr/include/net/if.h on

Adam D. Ruppe (2/2) Apr 22 2019 see this other thread for possible solutions too

aliak (20/37) Apr 22 2019 You can get the behaviour you want with opDispatch, and

Arun Chandrasekaran (2/44) Apr 22 2019 Works perfect for a single member! Not otherwise.

Arun Chandrasekaran <aruncxy gmail.com> writes:

What am I doing wrong here?

struct A
{
     union B
     {
         int bb;
     }
     B b;
     alias aa = B.bb;
}

void main()
{
     A a = A();
     // a.b.bb = 4; // works
     a.aa = 4; // fails
}


https://run.dlang.io/is/kXaVy2

Apr 22 2019

Stefan Koch <uplink.coder googlemail.com> writes:

On Monday, 22 April 2019 at 08:02:06 UTC, Arun Chandrasekaran 
wrote:
 What am I doing wrong here?

 struct A
 {
     union B
     {
         int bb;
     }
     B b;
     alias aa = B.bb;
 }

 void main()
 {
     A a = A();
     // a.b.bb = 4; // works
     a.aa = 4; // fails
 }


 https://run.dlang.io/is/kXaVy2

You are aliasing B.init.bb;
You should try aliasing b.bb;

though I am not sure in that'll work since alias is really only 
meant for symbols.

Apr 22 2019

Alex <AJ gmail.com> writes:

On Monday, 22 April 2019 at 08:02:06 UTC, Arun Chandrasekaran 
wrote:
 What am I doing wrong here?

 struct A
 {
     union B
     {
         int bb;
     }
     B b;
     alias aa = b.bb;
 }

 void main()
 {
     A a = A();
     // a.b.bb = 4; // works
     a.aa = 4; // fails
 }


 https://run.dlang.io/is/kXaVy2

aa is a "static" semantic.

your statement a.aa does not get translated in to a.b.bb like you 
think it does.

You are doing nothing any different than A.aa = 4.

which, when you realize this you'll understand the "need this" 
error.


It's as if you are doing

 struct A
 {
     union B
     {
         int bb;
     }
     B b;
 }

      alias aa = A.b.bb;

aa = 4;

which is like trying to say

A.b.bb = 4;


In D we can access "static" stuff using the object and so this 
can make things look like the mean something they are not.

If you could magically pass the this to it using something like 
UFCS then it could work.

import std.stdio;
struct A
{
     union B
     {
         int bb;
     }
     B b;
     alias aa = (ref typeof(this) a) { return &a.b; };
}

void main()
{
     A a = A();
     a.b.bb = 4;
     a.aa(a).bb = 5;
     writeln(a.aa(a).bb);
}

But here a.aa is not aa(a) and UFCS does not work so one must 
first access the alias and then pass the this.

Basically you are not going to get it to work. Just not how the 
semantics works.

It would be nice if a.aa(a) could reduce to a.aa as is UFCS was 
smart enough to realize it could pass it to the alias as this, 
but it doesn't.

else one might could do

alias aa = this.b.bb;

and

a.aa = 4;

Apr 22 2019

Arun Chandrasekaran <aruncxy gmail.com> writes:

On Monday, 22 April 2019 at 14:07:11 UTC, Alex wrote:
 On Monday, 22 April 2019 at 08:02:06 UTC, Arun Chandrasekaran 
 wrote:
 What am I doing wrong here?

 struct A
 {
     union B
     {
         int bb;
     }
     B b;
     alias aa = b.bb;
 }

 void main()
 {
     A a = A();
     // a.b.bb = 4; // works
     a.aa = 4; // fails
 }


 https://run.dlang.io/is/kXaVy2

 aa is a "static" semantic.

 your statement a.aa does not get translated in to a.b.bb like 
 you think it does.

 You are doing nothing any different than A.aa = 4.

 which, when you realize this you'll understand the "need this" 
 error.


 It's as if you are doing

 struct A
 {
     union B
     {
         int bb;
     }
     B b;
 }

      alias aa = A.b.bb;

 aa = 4;

 which is like trying to say

 A.b.bb = 4;


 In D we can access "static" stuff using the object and so this 
 can make things look like the mean something they are not.

 If you could magically pass the this to it using something like 
 UFCS then it could work.

 import std.stdio;
 struct A
 {
     union B
     {
         int bb;
     }
     B b;
     alias aa = (ref typeof(this) a) { return &a.b; };
 }

 void main()
 {
     A a = A();
     a.b.bb = 4;
     a.aa(a).bb = 5;
     writeln(a.aa(a).bb);
 }

 But here a.aa is not aa(a) and UFCS does not work so one must 
 first access the alias and then pass the this.

 Basically you are not going to get it to work. Just not how the 
 semantics works.

 It would be nice if a.aa(a) could reduce to a.aa as is UFCS was 
 smart enough to realize it could pass it to the alias as this, 
 but it doesn't.

 else one might could do

 alias aa = this.b.bb;

 and

 a.aa = 4;

I see. This is a simplified use case in /usr/include/net/if.h on 
Linux, where struct ifreq has something like this:

struct ifreq
   {


     union
       {
     char ifrn_name[IFNAMSIZ];   /* Interface name, e.g. "en0".  */
       } ifr_ifrn;

     union
       {
     struct sockaddr ifru_addr;
     struct sockaddr ifru_dstaddr;
     struct sockaddr ifru_broadaddr;
     struct sockaddr ifru_netmask;
     struct sockaddr ifru_hwaddr;
     short int ifru_flags;
     int ifru_ivalue;
     int ifru_mtu;
     struct ifmap ifru_map;
     char ifru_slave[IFNAMSIZ];  /* Just fits the size */
     char ifru_newname[IFNAMSIZ];
     __caddr_t ifru_data;
       } ifr_ifru;
   };




p-p lnk */

address    */

mask   */







index      */

   */




_IOT(_IOTS(char),IFNAMSIZ,_IOTS(short),1,0,0)

_IOT(_IOTS(char),IFNAMSIZ,_IOTS(int),1,0,0)


Macro magic has been doing the job and luckily there is no 
ifr_name, etc defined in the user code. Looks like there is no 
way but to name the elements of this struct as ifr_name, etc?

Apr 22 2019

Adam D. Ruppe <destructionator gmail.com> writes:

see this other thread for possible solutions too

https://forum.dlang.org/post/anasidaotexgfkyiqmlj forum.dlang.org

Apr 22 2019

aliak <something something.com> writes:

On Monday, 22 April 2019 at 08:02:06 UTC, Arun Chandrasekaran 
wrote:
 What am I doing wrong here?

 struct A
 {
     union B
     {
         int bb;
     }
     B b;
     alias aa = B.bb;
 }

 void main()
 {
     A a = A();
     // a.b.bb = 4; // works
     a.aa = 4; // fails
 }


 https://run.dlang.io/is/kXaVy2

You can get the behaviour you want with opDispatch, and 
generalize it with a mixin template:

mixin template AliasMember(string aliasName, string memberName) {
     ref opDispatch(string name)() if (name == aliasName) {
         return mixin(memberName);
     }
}

struct A {
     union B {
         int bb;
     }
     B b;
     mixin AliasMember!("aa", "b.bb");
}

void main() {
     A a = A();
     a.aa = 4;
}

Apr 22 2019

Arun Chandrasekaran <aruncxy gmail.com> writes:

On Monday, 22 April 2019 at 19:57:11 UTC, aliak wrote:
 On Monday, 22 April 2019 at 08:02:06 UTC, Arun Chandrasekaran 
 wrote:
 What am I doing wrong here?

 struct A
 {
     union B
     {
         int bb;
     }
     B b;
     alias aa = B.bb;
 }

 void main()
 {
     A a = A();
     // a.b.bb = 4; // works
     a.aa = 4; // fails
 }


 https://run.dlang.io/is/kXaVy2

 You can get the behaviour you want with opDispatch, and 
 generalize it with a mixin template:

 mixin template AliasMember(string aliasName, string memberName) 
 {
     ref opDispatch(string name)() if (name == aliasName) {
         return mixin(memberName);
     }
 }

 struct A {
     union B {
         int bb;
     }
     B b;
     mixin AliasMember!("aa", "b.bb");
 }

 void main() {
     A a = A();
     a.aa = 4;
 }

Works perfect for a single member! Not otherwise.

Apr 22 2019

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digitalmars.D.learn - alias fails to compile