[fpc-pascal] PChar & AnsiString

[Martin]

On 01/06/2010 11:23, spir ☣ wrote:
> What is the actual benefit of copy-on-write? I ask because of the following reasoning:
> * If a string is just used at several places, for example in output or into bigger strings, then there is no reason reason to copy it into a new variable.
> * If a programmer explicitely assigns an existing string to a new variable, the intent is precisely copy-semantics, to make them independent for further changes. If there is no change, there is also no reason for such an assignment.
> As a consequence, s2:=s1 will nearly always be followed by modification of either string, which will result on copy anyway, according to copy-on-write semantics. So, the initial gain at assignment time is soon lost. While the cost I imagine in terms of type complexity remains (every builtin modification method must ensure copying; no user-defined modification method should be possible without using builtin ones -- else copy-on-write is lost and consequences undefined).
>    
No, an assignment, or increase in ref count is not necessarily followed 
by a change:

Procedure Foo(a:String);
begin ... end;

Procedure Bar;
var s: string;
begin
   s:= SomeString;
   Foo(s);
end;

Now a copy of s has to be given to Foo, because Foo *may* changes the 
string. But Foo may also *not* change the string => so leave it till later.

Example 2:

Procedure Bar;
var a, b: string;
begin
   a:= SomeString;
   b:=a;
   if SomeCondition then
      b:=b+"...";
   if SomeOtherCondition then
      b:=b+"???";
end;

So b may never be changed at all.

Martin