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typeid() after const/immutable qualifiers
Aug 19, 2010
Andrej Mitrovic
Aug 19, 2010
Andrej Mitrovic
Aug 19, 2010
bearophile
Aug 19, 2010
Andrej Mitrovic
```In TDPL, page 289 & 299, there's this code (I've modified the names slightly) and explanation:

struct A
{
const(int[]) c;
immutable(int[]) i;
}

import std.stdio;

unittest
{
const(A) const_a;
immutable(A) immu_b;
}

A short version of the explanation:

"if qualifiers would apply blindly, the types would be:

const_a.c == const(const(int[]))
const_a.i == const(immutable(int[]))

immu_b.c == immutable(const(int[])).
immu_b.i == immutable(immutable(int[])).

When two qualifiers are identical, they are collapsed into one, otherwise const(immutable(T)) and immutable(const(T)) are both collapsed into immutable(T)"

From my interpretation, that would mean the types are now:

const_a.c == const(int[])
const_a.i == immutable(int[])

immu_b.c == immutable(int[])
immu_b.i == immutable(int[])

Am I correct so far?

Well first of all, DMD doesn't actually print it out simple qualifiers when arrays are used, for example:

const(int[]) x;
writeln(typeid(x));

Writes: const(const(int)[])

Which is fine, both x and it's contents are const so it's the correct output.

The second thing which I'm actually puzzled by, is why I'm getting typeid() return the same qualifiers as defined in the struct. Here's some simplified code with using basic types, not arrays:

struct A
{
const(int) c;
immutable(int) i;
}

import std.stdio;

unittest
{
const(A) const_a;
immutable(A) immu_b;

writeln("const_a.c == ", typeid(const_a.c));
writeln("const_a.i == ", typeid(const_a.i));

writeln("immu_b.c == ", typeid(immu_b.c));
writeln("immu_b.i == ", typeid(immu_b.i));
}

void main()
{

}

Writes:
const_a.c == const(int)
const_a.i == immutable(int)

immu_b.c == const(int)
immu_b.i == immutable(int)

const_a.c == const(int)
const_a.i == immutable(int)

immu_b.c == immutable(int)  // immu_b.c is now immutable
immu_b.i == immutable(int)

AFAIK immutable propagates to all fields of the struct, so const c should be an immutable now?
```
```Btw, should I skip trying to use inout at all for now? I've read some posts saying that it's awfully broken, and the example of inout in TDPL doesn't work..

Andrej Mitrovic Wrote:

> snip
```
```Andrej Mitrovic:
> Well first of all, DMD doesn't actually print it out simple qualifiers when arrays are used, for example:

I have an open bug report on this.

Bye,
bearophile
```
```Good to hear. I was almost going to open an enhancement request. :)

bearophile Wrote:

> Andrej Mitrovic:
> > Well first of all, DMD doesn't actually print it out simple qualifiers when arrays are used, for example:
>
> I have an open bug report on this.
>
> Bye,
> bearophile

```
```On Thu, 19 Aug 2010 12:47:45 -0400, Andrej Mitrovic <andrej.mitrovich@gmail.com> wrote:

> Btw, should I skip trying to use inout at all for now? I've read some posts saying that it's awfully broken, and the example of inout in TDPL doesn't work..

Yes.  I have expressed the cases that inout should deal with in bug http://d.puremagic.com/issues/show_bug.cgi?id=3748

-Steve
```