Re: [Plplot-devel] Can't eliminate legend string problem in Ada example 17

[Maurice L. <mj...@br...>]

On Thursday, August 14, 2008 at 12:56:45 (-0500) Maurice LeBrun writes:
 > 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’

Whoa, tripped up in my own example.  This is a syntax error because
"1" is a const char *, and foo[0] is a char.  This is what I meant
to write:

    char *foo = "0123"; 
    foo[0] = '0'; 

Now gcc doesn't complain, since you're assigning char to char.  But what
happens when I run the program?

$ ./tst
Segmentation fault (core dumped)

since I was trying to write into unmodifiable memory.

 > 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.

Forget this part, I was mistaken.

-- 
Maurice LeBrun