digitalmars.D.learn - why won't byPair work with a const AA?

Matthew Gamble (19/19) Jul 29 2017 I have a class member function from which I'm trying to return a

=?UTF-8?Q?Ali_=c3=87ehreli?= (6/23) Jul 29 2017 I think it should work. I think a cast to unqualified is a safe

Matthew Gamble (8/41) Jul 29 2017 Thanks Ali,

=?UTF-8?Q?Ali_=c3=87ehreli?= (29/37) Jul 29 2017 As your question reveals, casting away const is not safe in general and

Steven Schveighoffer (11/32) Aug 01 2017 byPair must store a pointer to the data in the AA. If you mark the AA

H. S. Teoh via Digitalmars-d-learn (13/32) Aug 01 2017 [...]

Steven Schveighoffer (9/42) Aug 01 2017 It works, because the byKeyValue implementation is so... ugly.

H. S. Teoh via Digitalmars-d-learn (27/51) Aug 01 2017 [...]

Steven Schveighoffer (6/34) Aug 01 2017 No, you can't const the Slot, because if the value type is a pointer,

H. S. Teoh via Digitalmars-d-learn (25/59) Aug 01 2017 [...]

Steven Schveighoffer (9/69) Aug 02 2017 You can iterate a const AA, but if you want to iterate a non-const AA,

H. S. Teoh via Digitalmars-d-learn (20/31) Aug 02 2017 [...]

Steven Schveighoffer (13/44) Aug 02 2017 It's not currently legal, you can't have inout members of a struct. This...

H. S. Teoh via Digitalmars-d-learn (26/39) Aug 02 2017 [...]

Steven Schveighoffer (8/49) Aug 02 2017 I have ideas :) I just haven't fleshed them out enough to present the
Olivier FAURE (4/19) Aug 03 2017 I understand the general concept you're describing, but what

Steven Schveighoffer (44/47) Aug 03 2017 tail modifiers are modifiers that only apply to the "tail" of the type.

H. S. Teoh via Digitalmars-d-learn (47/59) Aug 02 2017 [...]

Matthew Gamble <gamblemj gmail.com> writes:

I have a class member function from which I'm trying to return a 
sorted array of key, value tuples stored in an associative array 
as a private member. The member function should be able to be 
marked const to prevent the AA from being modified. I have 
reduced the problem to the simple case below which won't compile 
with DMD v2.072.2.

import std.array;
import std.algorithm;

class A
{
	this() { aa = ["a":1, "b" : 2, "c" : 3]; }
	auto pairs()  property const { return 
aa.byPair.array.sort().release; }
private:
	int[string] aa;
}

If I remove const from the pairs function it compiles fine. I'm 
just not sure this is a behavior I want. Any help/recommendation 
would be appreciated.

Jul 29 2017

=?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:

On 07/29/2017 09:19 PM, Matthew Gamble wrote:
 I have a class member function from which I'm trying to return a sorted
 array of key, value tuples stored in an associative array as a private
 member. The member function should be able to be marked const to prevent
 the AA from being modified. I have reduced the problem to the simple
 case below which won't compile with DMD v2.072.2.

 import std.array;
 import std.algorithm;

 class A
 {
     this() { aa = ["a":1, "b" : 2, "c" : 3]; }
     auto pairs()  property const { return aa.byPair.array.sort().release; }
 private:
     int[string] aa;
 }

 If I remove const from the pairs function it compiles fine. I'm just not
 sure this is a behavior I want. Any help/recommendation would be
 appreciated.

I think it should work. I think a cast to unqualified is a safe 
workaround in this case:

     auto pairs()  property const { return 
(cast(int[string])aa).byPair.array.sort().release; }

Ali

Jul 29 2017

Matthew Gamble <gamblemj gmail.com> writes:

On Sunday, 30 July 2017 at 04:36:19 UTC, Ali Çehreli wrote:
 On 07/29/2017 09:19 PM, Matthew Gamble wrote:
 I have a class member function from which I'm trying to return 
 a sorted
 array of key, value tuples stored in an associative array as a 
 private
 member. The member function should be able to be marked const 
 to prevent
 the AA from being modified. I have reduced the problem to the 
 simple
 case below which won't compile with DMD v2.072.2.

 import std.array;
 import std.algorithm;

 class A
 {
     this() { aa = ["a":1, "b" : 2, "c" : 3]; }
     auto pairs()  property const { return 
 aa.byPair.array.sort().release; }
 private:
     int[string] aa;
 }

 If I remove const from the pairs function it compiles fine. 
 I'm just not
 sure this is a behavior I want. Any help/recommendation would 
 be
 appreciated.

 I think it should work. I think a cast to unqualified is a safe 
 workaround in this case:

     auto pairs()  property const { return 
 (cast(int[string])aa).byPair.array.sort().release; }

 Ali

Thanks Ali,

That works to solve the compile problem. Seems the data in aa was 
safe from modification without the const or casting, even if the 
values are reference types (i.e. ClassB[string]). Is that 
expected?

Best,
Matt

Jul 29 2017

=?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:

On 07/29/2017 10:15 PM, Matthew Gamble wrote:

 I think it should work. I think a cast to unqualified is a safe
 workaround in this case:

     auto pairs()  property const { return
 (cast(int[string])aa).byPair.array.sort().release; }


As your question reveals, casting away const is not safe in general and 
not for this code. :-/

 That works to solve the compile problem. Seems the data in aa was safe
 from modification without the const or casting, even if the values are
 reference types (i.e. ClassB[string]). Is that expected?

No, they are not safe as you can mutate values in the AA:

import std.stdio;
import std.array;
import std.algorithm;

class B {
     int i;
     void mutate() {
         ++i;
     }
}

class A
{
     this() { aa = ["a" : new B(), "b" : new B(), "c" : new B()]; }
     auto pairs()  property const { return 
(cast(B[string])aa).byPair.array.sort().release; }
private:
     B[string] aa;
}

void main() {
     auto a = new A();
     auto p = a.pairs();
     p.front[1].mutate();
     assert(p.front[1].i == 1);    // <-- Mutated :(
}

Of course, the problem is the same without const and the cast.

Ali

Jul 29 2017