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Why I can't modify a const(char[])[N]?
Dec 29
monkyyy
December 29
Gravatar of Renato AthaydesPosted by Renato AthaydesReply
Renato Athaydes
Gravatar of Renato Athaydes


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I must be misunderstanding how const types work.

I expected this to be accepted by the compiler:

void foo(const(char[]) c) {
  const(char[])[2] d;
  d[0] = c;
}

But it isn't:

Error: cannot modify `const` expression `d[0]`

I thought that it would fail only if the const applied to the whole type, as in const(char[][2]) and that the type in the example meant only elements are const but not the array itself?!

However, this kind of thing works with slices:

void foo(const(char[]) c) {
  const(char[])[] d;
  d ~= c;
}

Why is this ok, but not the previous one?

If I use const(char[][]) instead, then it does fail as I had expected... so I can't see where is the hole in my understanding.

December 29
Gravatar of monkyyyPosted by monkyyy
in reply to Renato Athaydes		Reply
monkyyy
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Posted in reply to Renato Athaydes


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On Sunday, 29 December 2024 at 21:58:40 UTC, Renato Athaydes wrote:

>

I expected this to be accepted by the compiler:

void foo(const(char[]) c) {
  const(char[])[2] d;
  d[0] = c;
}

void foo(const(char)[] c){
  const(char)[][2] d;
  d[0] = c;
}

AND

void foo(const(char[]) c){
  const(char)[][2] d;
  d[0] = c;
}

AND

void foo(const(char[]) c) {
  const(char)[][2] temp;
  temp[0]=c;
  const(char[])[2] d=temp;
}

compile

>

Why is this ok, but not the previous one?

I dont use const if I can help it so idk details; but slices are reference types theres probably some theory about how the length is spookily making promises about the data.

Maybe slices are the worse of both worlds of being reference and value types, according to extreme safetyism?

December 29
Gravatar of Ali ÇehreliPosted by Ali Çehreli
in reply to Renato Athaydes		Reply
Ali Çehreli
Gravatar of Ali Çehreli
Posted in reply to Renato Athaydes


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On 12/29/24 1:58 PM, Renato Athaydes wrote:
> I must be misunderstanding how const types work.
>
> I expected this to be accepted by the compiler:
>
> ```
> void foo(const(char[]) c) {
>    const(char[])[2] d;
>    d[0] = c;
> }
> ```
>
> But it isn't:
>
> ```
> Error: cannot modify `const` expression `d[0]`
> ```

It's the same issue with the following code:

    const(char[]) a;
    a = c;

The only difference in your example is the fact that my variable 'a' happens to be array element for you. They are both const(char[]) and cannot be modified.

> I thought that it would fail only if the `const` applied to the whole
> type,

That would be true but it's "turtles all the way down": everthing that is accessed through a 'const' expression would be 'const'.

> as in `const(char[][2])` and that the type in the example meant
> only elements are `const` but not the array itself?!

That's true. But the problem you are having is that you are trying to change an element that is 'const'.

> However, this kind of thing works with slices:
>
> ```
> void foo(const(char[]) c) {
>    const(char[])[] d;
>    d ~= c;
> }
> ```
>
> Why is this ok, but not the previous one?

Because now you are not changing any element; you are changing the array, which is not const. The following will still fail with your new example:

  d[0] = c;

> If I use `const(char[][])` instead, then it does fail as I had
> expected... so I can't see where is the hole in my understanding.

Ali