On Thursday, August 14, 2008 at 10:18:32 (-0700) Alan W. Irwin writes:
> On 2008-08-14 09:58-0500 Maurice LeBrun wrote:
>
> > For C & C++, a const pointer means the memory it points to cannot be
> > altered.
> > The pointer can. So for example
> >
> > const char *foo = "bar";
> >
> > (or later reassignment) is perfectly legit.
>
> After years of dabbling at C, this pointer stuff still makes my head spin so
> could you expand a bit more, Maurice? What actually happens with the above
> statement? After the declaration of foo as a pointer to a const char is foo
> initialized as a pointer to the first character in the const "bar" string?
Correct.
> Or would a better way to look at it be that all character strings like "bar"
> can be interpreted as pointers so the above is simply a pointer assignment?
This too is correct. All string literals in a program are stored in the
executable, as a `strings <program>` will demonstrate. The proper way to
refer to these is via a const char* since they are considered fixed.
If you want a char* string that's modifiable, you need to define a character
buffer and copy a string into it. Whenever unsure, just construct a simple
test and try it (I keep several handy for that purpose). For example, what
if you assign a literal to a char* and then try to modify it?
char *foo = "0";
foo[0] = "1";
then gcc complains:
tst.cc: In function ‘int main(int, char**)’:
tst.cc:15: error: invalid conversion from ‘const char*’ to ‘char’
because apparently it's smart enough to know that even tho foo is defined as
char*, it's actually pointing to a const char*. I'm no language lawyer, but
it would seem that this syntax is supported for legacy reasons.
> Is this a good example of the later reassignment you referred to?
>
> *foo = "whatever";
>
> or would it be
>
> foo = "whatever";
> ?
The latter. *foo gives you what foo is pointing at. E.g.
char *foo = "0123";
char single = *foo;
or
char single = foo[0];
then single == '0'. It helps to remember that *foo and foo[0] mean exactly
the same thing.
> The first seems consistent with the mnemonic for the declaration, but I am
> virtually positive the latter is correct instead since *foo would refer to
> the first read-only character in "bar" which would make no sense for an
> assignment.
Right, since you are assigning to a pointer. If you assign to a char, you're
ok.
> Thanks in advance for trying to deal with my confusion on these issues. I
> guess the start of my confusion all these years has been what I consider to
> be a rather illogical mnemonic for pointer declarations using the
> dereferencing operator. I guess I just got off on the wrong foot with C
> pointers and have never recovered. :-)
:) It's a little wierd coming from a Fortran environment, for sure. But good
stuff to know.
--
Maurice LeBrun
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