Chad Berchek schrieb:
Thaddy wrote:
It is a contract between the compiler and the programmer in which it
is expected that the string will not be modified inside a procedure,
function or method.
This is the crux of the controversy. I realized this when I was writing
the original post, but did not mention it explicitly because I thought
it would come up anyway.
The difference between a feature and a bug is the specifications. Here
the specifications are the documentation. I have not found any
documentation in either FPC or Delphi that there is some implicit
contract whereby the programmer promises not to modify other variables
which happen to refer to the same instance as a const parameter.
*Aliasing* is a common problem, not restricted to const parameters.
A subroutine may e.g. free a global object, and replace it by its
argument. Calling this subroutine with that global object results in an
invalid reference in the global variable.
To me, a const parameter means that
you cannot modify that parameter by pointing it to something else, nor
(in the case of strings and dynamic arrays) alter the contents by means
of said parameter. (Although, you can't really "alter" the contents of
the instance, as copy-on-write simply creates a whole new instance.)
That's what it means in other languages I've used, and nothing more.
In C "const" has several meanings, comparison with other languages is
not a good idea.
In OPL "const" means that the argument will not be altered inside the
subroutine, so that the compiler can apply some optimizations[1]. That's
all. The compiler *can* check for e.g. illegal writes to an const
parameter, but there is no such *obligation*, and it's known that the
compiler can be fooled by trickery, in any language.
[1] E.g. const record parameters can be passed by reference, instead of
passing a copy (by value) to the subroutine. Managed types can be passed
without refcounting and associated try-finally constructs. But there
exists no guarantee, which compiler will apply what optimization, under
which conditions.
I do not defend critical optimizations in an compiler. E.g. it's known
that managed types with a RefCount of zero should not be passed as const
parameters, *when* inside the subroutine the RefCount can be increased
and decreased again. Here the compiler may have a chance to report an
*possibly* inappropriate use of "const".
DoDi
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