Re: Problem with "warning: depre cated conversion from string constant to âchar* â"

[Axel Freyn <axel-freyn at gmx dot de>]

Hi Jens,
On Tue, Oct 12, 2010 at 05:42:32PM +0200, Jens Rehsack wrote:
> 2010/10/12 Ian Lance Taylor <iant@google.com>:
> > Jens Rehsack <rehsack@googlemail.com> writes:
> >
> >> I currently write some c++ code using libConfuse
> >> (http://savannah.nongnu.org/projects/confuse/).
> >> During initialisation of the required cfg_opt_t structures I get a
> >> (very) lot of warnings like:
> >> c2.cpp:11: warning: deprecated conversion from string constant to âchar*'
> >>
> >> I attached a small example to demonstrate the problem. It's not
> >> trivial - changing the "name" member
> >> of struct demo being const, free failes (and is invalid, because
> >> there're demo_opt instances with
> >> modifyable names).
> >>
> >> Any suggestions welcome.
> >> 1) Killing the warning with -Wno-... is not an option
> >> 2) Remove the finding using const_cast<char *>() is the same as (1)
> >> 3) initializing the struct using strdup is neither reasonable
> >
> > I'm not sure what you want us to suggest. ÂYou say that the names are
> > modifiable.
> 
> Where do I say that? I apologize saying this (must be happened because
> of my not so good english).
I think, what Ian wanted to say: The problem is in your code -- your
code is NOT clean C++. What the code does, is to create non-constant
pointers of type "char*" pointing to string-constants. That IS a problem
in the code (The C++-Standard states "An ordinary string literal has
type âarray of n const charâ" (Â2.13.4.1) -- and you can't have a
non-constant pointer pointing to a constant array!).
> 
> > ÂString constants are not modifiable. ÂIf the strings are
> > indeed modified, and you pass in a string constant, then your program is
> > going to crash when the library tries to modify the strings. ÂThe gcc
> > warning is helpfully pointing that out.
> 
> Correct. And if the strings are not modifiable? (I thought I provided an
> example which shows, that they neither are assumed being modifiable
> nor being modified).
Well, your code does not modify the string, that's right. However, it
creates a non-constant pointer of type "char *" -- and that assums that
the string would be modifiable (later on, the compiler will never be
able to detect it when you try to change a character in the string!)
Also the fact that you declare "static const struct demo_opt foo" as
beeing const does not change this problem: you can use the pointers in
"foo" in order to get write-access to the string constants:
If you write somewhere in your main-function "foo[1].name[3] = 'c';" the
compiler won't detect an error -- however, you have undefined behaviour
as you try to change a string constant
> 
> > This is not a gcc issue. ÂIt's a C++ language issue. ÂIf you need a
> > modifiable string, then you must not use a string constant.
> 
> How can this being expressed without declaring dozen of helper
> variables?
I think, you can't. Maybe you could define two different structures
"demo_opt_const" and "demo_opt" and then use "demo_opt_const" as static
and pass it to "demo_init", and use in the struct "demo" the class
"demo_opt", where you define:
struct demo_opt {
    char *name;
    int flags;
};
struct demo_opt_const {
    const char *name;
    int flags;
};

That should work, I think. However, of course I have no idea how much
you would have to change in your final code.

Axel