Const, invariant, strings and "constant data need never be copied"

[Stewart Gordon]

"Walter Bright" <newshound1 at digitalmars.com> wrote in message 
news:fgguis$1hsk$1 at digitalmars.com...
> Stewart Gordon wrote:
>> Note that the text is never copied after it is read in, except by 
>> std.string.replace if it actually makes any change.  In D 2.x, it's 
>> necessary to change one line, to something like
>>
>>    text = text.idup.replace("\r\n", "\n").replace("\r", "\n").dup;
>>
>> which consequently adds two copy operations.
>
> Well, no. Take a look at the source to std.string.replace(). It does not 
> modify the input in place - it returns the input if there are no changes, 
> if there are changes, it returns a *copy*.

That's basically what I said.

> Second, text should be declared as a string, so you do not need either of 
> the dup's.  Two copies are made, just as with the 1.0 version, in that 
> line of code.
>
> You will need a third copy to do the loop which modifies the string in 
> place.

In the first of these two paragraphs, you suggest omitting the final .dup, 
and then in the next, you effectively tell me to put it back in.  Therein 
lies my point - the "third copy" ought not to be necessary for my ad hoc 
code.  (I know I could cast away the invariant, but that's a rather down and 
dirty trick.)

> I feel that, with strings, the advantages of invariant strings outweigh 
> the disadvantages.

When it comes to string manipulation functions, giving the programmer the 
choice, with my proposal of const-transparency, would AISI bring even more 
advantages and alleviate some of the disadvantages.

> Note that one can still do the modify-in-place D 1.0 code, and do it very 
> fast, by putting the tests for \r inside the loop rather than as separate 
> loops. The D 1.0 version isn't what you'd write if you wanted speed, 
> anyway.

Yes, that's another way to do it....

Stewart.

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