Re: [boost] Input archive loads seem to corrupt sharedcharacterstd::strings

From: Hervé Brönnimann (hervebronnimann_at_[hidden])
Date: 2007-12-04 04:59:17

I'd like to go back to Chuck's original posting:

> Inspection of the code reveals the following is happening (both in  
> 1.34 and HEAD):
> text_iarchive_impl<Archive>::load(std::string &s)
> {
>     std::size_t size;
>     * this->This() >> size;
>     // skip separating space
>     is.get();
>     // borland de-allocator fixup
>     #if BOOST_WORKAROUND(_RWSTD_VER, BOOST_TESTED_AT(20101))
>     if(NULL != s.data())
>     #endif
>         s.resize(size);
>     is.read(const_cast<char *>(s.data()), size); // <== This looks  
> suspect
> }

Inspection of the g++ string implementation (latest 4.1.x) shows that  
resize() will *almost* always create a new rep and thus a new data()  
for the string, if it is shared.  The only exception is when the  
requested size == s.size().  Chuck, was this the bug that hit you?

Seems to me, since you are going to rewrite your string anyway, you  
might as well clear() it first, then resize(size).  That'll take care  
of creating a new representation.  The problem with that is that it  
will write over the string twice, first filling it with 0s then with  
the read data.  But think about it:  if the string is shared, you  
need to construct a new representation with the requested size  
anyway, and it will have to be filled with 0s.

Still, suppose you wanted to avoid going over your string twice, what  
you'd need is to create a string with an unitialized buffer and start  
writing into it directly. There's only one way I can think of to  
achieve that: have an input iterator from your "is" stream, such as  
std::istream_iterator<char>(is), and use  s.assign(is_begin, is_end),  
assuming you can define and end that will stop reading after size  
chars (don't even know how to do that with istream_iterator, and  
there is no assign(int, InputIterator) method in string, sadly).  In  
any case, however, you then lose the benefit of the single is.read()  
call, because the iterator is going to do ++iter and read character  
by character.  If your stream is buffered, there shouldn't much of an  
impact, but then you have a double copy from stream buffer to string  
buffer anyway, which is akin to going over your string twice.

In short, I don't think you can avoid the cost of going over your  
string twice, so I can't see anything better than:

text_iarchive_impl<Archive>::load(std::string &s)
{
     std::size_t size;
     * this->This() >> size;
     // skip separating space
     is.get();
     // borland de-allocator fixup
     #if BOOST_WORKAROUND(_RWSTD_VER, BOOST_TESTED_AT(20101))
     if(NULL != s.data())
     #endif
+  s.clear();
      s.resize(size);
     is.read(const_cast<char *>(s.data()), size); // <== This looks  
suspect
}

--
Hervé Brönnimann
hervebronnimann_at_[hidden]