On Sat, 13 Jun 2015, Martin Frb wrote:
"conts" param
http://www.freepascal.org/docs-html/ref/refsu63.html#x166-17600014.4.4
explicitly document that ref counting is not done. (But I have no const in my
example)
But "out" param have no such documentation. They also do *not* state in the
doc that:
1) the ref count is decremented
2) May be decremented in a way that leaves other (ref counted) parameters in
a zombie state (pointing to freed memory)
In fact the doc states
The difference of out parameters and parameters by reference is very small:
the former gives the compiler more information about what happens to the
arguments when passed to the procedure: it knows that the variable does not
have to be initialized prior to the call. The following example illustrates
this:
Yet "var" param also do not decrease the reference, so the statement is
wrong/incomplete.
Well. First off, just so we can be clear. Things can be tested rather easily:
araminta: >cat trf.pp
{$mode objfpc}{$h+}
uses sysutils;
procedure DoByValue(S : String);
begin
Writeln('By value : ',StringRefCount(S));
end;
procedure DoConst(const S : String);
begin
Writeln('As Const : ',StringRefCount(S));
end;
procedure DoVar(var S : String);
begin
Writeln('By ref : ',StringRefCount(S));
end;
procedure DoOut(out S : String);
begin
Writeln('Out : ',StringRefCount(S));
end;
Var
S1,S2 : String;
begin
S1:=IntToStr(123)+'abc';
S2:=S1;
Writeln('Initial ref count : ',StringRefCount(S2));
DoByValue(S2);
DoConst(S2);
DoVar(S2);
DoOut(S2);
end.
araminta: >cat trf.pp
araminta: >./trf
Initial ref count : 2
By value : 3
As Const : 2
By ref : 2
Out : 0
Note the 0 !
Secondly, the section about 'out' parameters is very old; there were not nearly
so much managed types at the time.
I will update it, I was just documenting the new string types anyway. The above
will nicely illustrate the point.
Michael.
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