why won't byPair work with a const AA?

[Ali Çehreli via Digitalmars-d-learn]

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