Re: Error with basic string

[Sebastian Huber <sebastian-huber@web.de>]

Hello!

On Thursday 23 January 2003 19:31, Rupert Wood wrote:
> Ajay Bansal wrote:
> > char *func(string a, int n)
> > {
> > 	a.resize(n);
> > 	return a.begin();
> > }
>
> a.begin() is an iterator. You'll have to dereference it to get the
> character and then re-reference that for the pointer, i.e. (bracketed
> for clarity)
>
>     char *func(string a, int n)
>     {
>         a.resize(n);
>         return &(*(a.begin()));
>     }

You should use 'return const_cast<char*>( a.data())' or 'a.c_str()'. But you 
have to keep in mind, that every non-constant method of std::string may make 
the data invalid.

> *However*
>
>     1) you're not passing your string by reference; what you're
>        telling it to do is to take a copy of string a, resize the
>        copy to n characters and then return a pointer to the first
>        character in the copy - i.e. a pointer to memory owned by a
>        dead object
>
>     2) (I'm not an STL guru so take this with a pinch of salt)
>        I don't think there's a requirement that the string be
>        stored continuously in memory, except for a read-only copy
>        between c_str() the next non-const operation. It looks like
>        you want func to allocate you an n-byte continuous character
>        buffer in a basic_string and, whilst some implementations will
>        give you this, I don't think you can assume all will. (But you
>        probably know this - the comment about changing the return
>        type?) I can't tell you if G++'s STL plays along.
>
> Rup.