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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: How do I write directly to a memory address?
Date: Mon, 07 Feb 2011 07:38:47 +0100
Organization: Tidorum Ltd
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Date: 2011-02-07T07:38:47+01:00
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Shark8 wrote:
> On Feb 6, 2:27 pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
>> On Feb 6, 9:01 pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
>> wrote:
>>
>>> Immutable scalar arguments still ire mutable within C and C++ subprograms.
>> Scalar arguments are passed by copy, so whether they are immutable or
>> mutable within the function is independent on their origin (and the
>> original is not affected by this in any way).
>>
>> Unless, of course, you have meant something else.
> 
> The scalar-argument that is passed when dealing with strings/arrays
> [in C & C++] is that of the address, IIRC. While that address is
> itself an "immutable scalar" the data pointed to is not so constrained
> and so may be 'mutated' thereby enabling a perceived-immutable to be
> altered. {The newer standards may [or may not] have addressed this
> issue: I seem to recall a "pointer to constant" idea which would
> [I believe] behave in the expected manner if it was passed as the
> parameter.}

 From the C99 standard, section 6.7.5.1, "Pointer declarators":

   <quote>
   EXAMPLE The following pair of declarations demonstrates
   the difference between a "variable pointer to a constant
   value" and a "constant pointer to a variable value".

       const int *ptr_to_constant;
       int *const constant_ptr;

   The contents of any object pointed to by ptr_to_constant
   shall not be modified through that pointer, but
   ptr_to_constant itself may be changed to point to another
   object. Similarly, the contents of the int pointed to by
   constant_ptr may be modified, but constant_ptr itself shall
   always point to the same location.
   <end quote>

In the first case, the protection is not removed by pointer arithmetic 
or array indexing, so ptr_to_constant+8 is also a pointer to a constant 
int. The constraints can be removed by "casting away" the "constant" 
quality.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .