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digitalmars.D.learn - immutable array in constructor

reply Jeff Thompson <nospam example.com> writes:
In the following code, I explicitly declare array as immutable. 
But it compiles with the error shown below in the comment. The 
array object is declared immutable, so how can the compiler say 
it is a mutable object? In summary, how to pass an immutable 
array to an immutable constructor?

class C {
   int i;
   this(immutable int[] array) immutable {
     i = array[0];
   }
}

void func() {
   immutable int[] array = [1];
   auto c = new C(array); // Error: immutable method C.this is not 
callable using a mutable object
}
Mar 17 2016
parent reply Anonymouse <asdf asdf.com> writes:
On Thursday, 17 March 2016 at 09:57:37 UTC, Jeff Thompson wrote:

 In the following code, I explicitly declare array as immutable. 
 But it compiles with the error shown below in the comment. The 
 array object is declared immutable, so how can the compiler say 
 it is a mutable object? In summary, how to pass an immutable 
 array to an immutable constructor?

 class C {
   int i;
   this(immutable int[] array) immutable {
     i = array[0];
   }
 }

 void func() {
   immutable int[] array = [1];
   auto c = new C(array); // Error: immutable method C.this is 
 not callable using a mutable object
 }


The error message isn't very good, but remove immutable from the 
constructor and it works.

   this(immutable int[] array) {
Mar 17 2016
parent reply Jeff Thompson <nospam example.com> writes:
On Thursday, 17 March 2016 at 10:04:53 UTC, Anonymouse wrote:

 On Thursday, 17 March 2016 at 09:57:37 UTC, Jeff Thompson wrote:

 In the following code, I explicitly declare array as 
 immutable. But it compiles with the error shown below in the 
 comment. The array object is declared immutable, so how can 
 the compiler say it is a mutable object? In summary, how to 
 pass an immutable array to an immutable constructor?

 class C {
   int i;
   this(immutable int[] array) immutable {
     i = array[0];
   }
 }

 void func() {
   immutable int[] array = [1];
   auto c = new C(array); // Error: immutable method C.this is 
 not callable using a mutable object
 }


 The error message isn't very good, but remove immutable from 
 the constructor and it works.

   this(immutable int[] array) {




This is a simplified example from a larger class I have where I 
need an immutable constructor. This is because I need to 
construct an object an pass it to other functions which take an 
immutable object. So, how to keep an immutable constructor?
Mar 17 2016
next sibling parent tsbockman <thomas.bockman gmail.com> writes:
On Thursday, 17 March 2016 at 10:11:43 UTC, Jeff Thompson wrote:

 This is a simplified example from a larger class I have where I 
 need an immutable constructor. This is because I need to 
 construct an object an pass it to other functions which take an 
 immutable object. So, how to keep an immutable constructor?


The "mutable object" the compiler is complaining about is the 
instance of C being constructed, not the array.

Change `new C(array)` to `new immutable(C)(array)` and it should 
work.
Mar 17 2016
prev sibling parent reply Rene Zwanenburg <renezwanenburg gmail.com> writes:
On Thursday, 17 March 2016 at 10:11:43 UTC, Jeff Thompson wrote:

 This is a simplified example from a larger class I have where I 
 need an immutable constructor. This is because I need to 
 construct an object an pass it to other functions which take an 
 immutable object. So, how to keep an immutable constructor?


In that case, new immutable C() should work I believe. Also, if 
you mark the constructor as pure, new C() should be implicitly 
convertible to an immutable C.
Mar 17 2016
next sibling parent tsbockman <thomas.bockman gmail.com> writes:
On Thursday, 17 March 2016 at 11:27:01 UTC, Rene Zwanenburg wrote:

 Also, if you mark the constructor as pure, new C() should be 
 implicitly convertible to an immutable C.


Ah! That's a good tip. Now I understand why I never have to say 
`new immutable(C)()` in my own code. (I am in the habit of 
marking everything I can as `pure`.)
Mar 17 2016
prev sibling parent reply Jeff Thompson <nospam example.com> writes:
On Thursday, 17 March 2016 at 11:27:01 UTC, Rene Zwanenburg wrote:

 On Thursday, 17 March 2016 at 10:11:43 UTC, Jeff Thompson wrote:

 This is a simplified example from a larger class I have where 
 I need an immutable constructor. This is because I need to 
 construct an object an pass it to other functions which take 
 an immutable object. So, how to keep an immutable constructor?


 In that case, new immutable C() should work I believe. Also, if 
 you mark the constructor as pure, new C() should be implicitly 
 convertible to an immutable C.


new immutable C() worked! Thanks for the insight.
Mar 17 2016
parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 03/17/2016 09:32 AM, Jeff Thompson wrote:

 On Thursday, 17 March 2016 at 11:27:01 UTC, Rene Zwanenburg wrote:

 On Thursday, 17 March 2016 at 10:11:43 UTC, Jeff Thompson wrote:

 This is a simplified example from a larger class I have where I need
 an immutable constructor. This is because I need to construct an
 object an pass it to other functions which take an immutable object.
 So, how to keep an immutable constructor?


 In that case, new immutable C() should work I believe. Also, if you
 mark the constructor as pure, new C() should be implicitly convertible
 to an immutable C.


 new immutable C() worked! Thanks for the insight.


In case it's useful to others, I have qualified constructors covered at 
the following link (without the compilation error that you've faced but 
still pointing out the fact that the mutable constructor may be called 
unintentionally):

 
http://ddili.org/ders/d.en/special_functions.html#ix_special_functions.qualifier,%20constructor

Ali
Mar 17 2016