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digitalmars.D.learn - Assoc. Array and struct with immutable member

reply Dgame <r.schuett.1987 gmail.com> writes:
How am I supposed to insert a struct with immutable members into 
an assoc. array?

Reduced example:
----
struct A {
     immutable string name;
}

A[string] as;
as["a"] = A("a"); // Does not work
----
Apr 15 2018
next sibling parent reply Alex <sascha.orlov gmail.com> writes:
On Sunday, 15 April 2018 at 17:59:01 UTC, Dgame wrote:

 How am I supposed to insert a struct with immutable members 
 into an assoc. array?

 Reduced example:
 ----
 struct A {
     immutable string name;
 }

 A[string] as;
 as["a"] = A("a"); // Does not work
 ----


Via a static this() it would work. But I guess, this is not what 
you are looking for, is it?

´´´
static this()
{
	as["a"] = A("a");
}

void main()
{	
	assert(as["a"].name == "a");
}


struct A {
     immutable string name;
}

A[string] as;
´´´


It also works, if you hide the AA in a class and all the 
insertions into the constructor.


´´´
/*
static this()
{
	as["a"] = A("a");
}
*/
void main()
{	
	auto c = new Container(A("a"));
	assert(c.as["a"].name == "a");
}

struct A {
     immutable string name;
}

class Container
{
	A[string] as;
	this(A a)
	{
		as[a.name] = a;
	}
}
´´´
Apr 15 2018
parent reply bauss <jj_1337 live.dk> writes:
On Sunday, 15 April 2018 at 18:51:52 UTC, Alex wrote:

 On Sunday, 15 April 2018 at 17:59:01 UTC, Dgame wrote:

 How am I supposed to insert a struct with immutable members 
 into an assoc. array?

 Reduced example:
 ----
 struct A {
     immutable string name;
 }

 A[string] as;
 as["a"] = A("a"); // Does not work
 ----


 Via a static this() it would work. But I guess, this is not 
 what you are looking for, is it?

 ´´´
 static this()
 {
 	as["a"] = A("a");
 }

 void main()
 {	
 	assert(as["a"].name == "a");
 }


 struct A {
     immutable string name;
 }

 A[string] as;
 ´´´


 It also works, if you hide the AA in a class and all the 
 insertions into the constructor.


 ´´´
 /*
 static this()
 {
 	as["a"] = A("a");
 }
 */
 void main()
 {	
 	auto c = new Container(A("a"));
 	assert(c.as["a"].name == "a");
 }

 struct A {
     immutable string name;
 }

 class Container
 {
 	A[string] as;
 	this(A a)
 	{
 		as[a.name] = a;
 	}
 }
 ´´´


Even though it works in static this, then it still looks like a 
bug if you ask me, because the associative array itself isn't 
immutable.
Apr 17 2018
parent Alex <sascha.orlov gmail.com> writes:
On Tuesday, 17 April 2018 at 11:07:55 UTC, bauss wrote:

 Even though it works in static this, then it still looks like a 
 bug if you ask me, because the associative array itself isn't 
 immutable.


Yeah... I'm not sure, if this behavior is wanted, too...

If you argue, that an absent field in in AA is not created, then 
it is a bug.
But if you argue that after creation of the AA everything is 
created, then adding another field is equal to modifying this 
very field, and that would be correctly forbidden.

So... no clue where a hint in the docu is hidden.
Apr 17 2018
prev sibling parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Sunday, April 15, 2018 17:59:01 Dgame via Digitalmars-d-learn wrote:

 How am I supposed to insert a struct with immutable members into
 an assoc. array?

 Reduced example:
 ----
 struct A {
      immutable string name;
 }

 A[string] as;
 as["a"] = A("a"); // Does not work
 ----


I would point out that in general, having const or immutable fields in a
struct is a terrible idea. It causes all kinds of problems because stuff
like assignment doesn't work. If you want the field to be read-only, you'll
have far fewer problems if you simply use a function to access it instead of
making the member public. e.g.

struct A
{
public:

     property string name() { return _name; }

private:

    string _name;
}

The problem you're having here is just one example of the list of things
that don't work if you have a struct with immutable members. It looks like
the AA probably is doing something like initializing the entry with A.init
and then assigning it the new value, and that's not going to work if the
struct has immutable members - which is exactly what the error message says.

- Jonathan M Davis
Apr 17 2018
parent Dgame <r.schuett.1987 gmail.com> writes:
On Tuesday, 17 April 2018 at 11:38:17 UTC, Jonathan M Davis wrote:

 On Sunday, April 15, 2018 17:59:01 Dgame via 
 Digitalmars-d-learn wrote:

 How am I supposed to insert a struct with immutable members 
 into an assoc. array?

 Reduced example:
 ----
 struct A {
      immutable string name;
 }

 A[string] as;
 as["a"] = A("a"); // Does not work
 ----


 I would point out that in general, having const or immutable 
 fields in a struct is a terrible idea. It causes all kinds of 
 problems because stuff like assignment doesn't work. If you 
 want the field to be read-only, you'll have far fewer problems 
 if you simply use a function to access it instead of making the 
 member public. e.g.

 struct A
 {
 public:

      property string name() { return _name; }

 private:

     string _name;
 }

 The problem you're having here is just one example of the list 
 of things that don't work if you have a struct with immutable 
 members. It looks like the AA probably is doing something like 
 initializing the entry with A.init and then assigning it the 
 new value, and that's not going to work if the struct has 
 immutable members - which is exactly what the error message 
 says.

 - Jonathan M Davis


That's how I solved it. But it is troublesome and annoying 
because it increases the amount of manually work I have to do.
Apr 17 2018