Actually an commission, but an important one.

https://www.freepascal.org/docs-html/current/ref/refsu67.html#x183-20700014.4.4

Specifying a parameter as Constant is giving the compiler a hint that

the contents of the parameter will not be changed by the called routine.

"by the called routine."
Should that not be "by any code (the caller, the called, or any nested callee, or any other thread, interrupt or hardware modifying memory)"? Or maybe "until the called routine has finished, and returned to the caller" ?

If the value is copied then nothing else can change it since it is only accessible to the routine itself ? (unless you're doing some really dirty tricks, obviously)

Michael.

In the below example "GlobalTxt := 'abc';" is indeed in the "called routine".
But insert a call to a subroutine doing the work, or start a thread....

procedure foo(const t: AnsiString);
begin
   GlobalTxt := 'abc';
   // do some mem alloc/dealloc work
   // do something with t, it still points to the location that the old
content was in. If that mem has been changed....
end;
var
   GlobalTxt: AnsiString;
begin
   GlobalTxt := getRandomTextWichIsNotAConstant;
   foo(GlobalTxt);
end;

PM Martin Frb wrote

https://www.freepascal.org/docs-html/current/ref/refsu67.html#x183-20700014.4.4 > Specifying a parameter as Constant is giving the compiler a hint that the contents of the parameter will not be changed by the called routine.

Actually it is a contract between the programmer and the compiler, telling the compiler that the value of sais constant parameter will not be altered in any way.
The compiler will enforce this as much as it can, but it is up to the programmer to not have other processes alter the value of that parameter, whilst that function is still running.

Any side effects of circumventing the compilers efforst to enforce this are at your own risks.

Well, that's what I meant with really dirty tricks.

I will probably have to rephrase the documentation to 'The compiler will not allow you to modify the parameter value by assigning something directly to it."

Michael.

That's what I meant with dirty tricks. There are many ways to do so.

Michael.

Michael Van Canneyt wrote

I will probably have to rephrase the documentation to 'The compiler will not allow you to modify the parameter value by assigning something directly to it."

Neither would the compiler allow (I hope) the const parameter to be passed as a var parameter.

Regards,

Adriaan van Os

PM Adriaan van Os wrote

Neither would the compiler allow (I hope) the const parameter to be passed as a var parameter.

It does not:

{$mode objfpc}
{$h+}
procedure foo(var x: integer);
begin
end;
procedure bar(const y: integer);
begin
  foo(y); //line 10
end;
begin
end.
test.pas(10,8) Error: Can't assign values to const variable

The first quote was probably a ref to the (current) case from the forum, that triggered this.

procedure Foo(const A: String; var B: string);
begin ... end;
var s: string;
Foo(s,s);

It is most likely (but not certain) that B will be modified. And with that A would be modified.

The tricky bits are:
- If A would no longer be accessed after B was modified, then that code will *currently* work.  Yet new optimizations could be added, that affect this (though really unlikely)
- could be a virtual method, and each version only accesses either A or B.
- it can not always be caught "Foo(s1,s2)" could have s1 and s2 have the same var (and with a ref count of 1, if both were "var param" to the caller).