const?? When and why? This is ugly!

[Sergey Gromov]

Sat, 07 Mar 2009 15:19:50 -0800, Andrei Alexandrescu wrote:

> To recap, if an API takes a string and all you have a char[], DO NOT 
> CAST IT. Call .idup - better safe than sorry. The API may evolve and 
> store a reference for later. Case in point: the up-and-coming 
> std.stdio.File constructor initially was:
> 
> this(in char[] filename);
> 
> Later on I decided to save the filename for error message reporting and 
> the such. Now I had two choices:
> 
> (1) Leave the signature unchanged and issue an idup:
> 
> this.filename = to!string(filename); // issues an idup
> 
> (2) Change the signature to
> 
> this(string filename);
> 
> Now all client code that DID pass a string in the first place (the vast 
> majority) was safe _and_ efficient. The minority of client code was that 
> that had a char[] or a const(char)[] at hand. That code did not compile, 
> so it had to insert a to!string on the caller side.
> 
> As has been copiously shown in other languages, the need for 
> character-level mutable string is rather rare. So most of the time you 
> will not traffic in char[], but instead you'll have a immutable(char)[] 
> to start with. This further erodes the legitimacy of your concern.

My file names are constructed most of the time.  And most of the time
they are simple char[]s.

It is not obvious that File should store the file name.  It's not
strictly necessary.  It's an *implementation detail.*  Now you expose
this implementation detail through the class interface, and you do this
without any good reason.  You save a 150 byte allocation per file.
Nice.

I can understand when a hash takes an immutable key.  It's in the hash's
contract.  Various lazy functions could take immutable input to
guarantee correct lazy execution.  But I think that overall use of
immutable types should be rare and thoroughly thought-out.  They should
be used only when it's absolutely, provably necessary.  That's why I
think aliasing string as immutable is a mistake.  It felt wrong when I
discovered D a year ago, and it feels wrong now.