On 11/28/05, Mick Hollins <mick@hollins.id.au> wrote:

The following code causes a runtime assertion in VC 8.0:

     std::string l_param;
     std::string l_str = (boost::format("here is an empty string: %1%") % l_param).str();

It passes an empty string argument to boost::format.
The assert is inside the implementation of basic_string::append()
and boils down to the fact that the implementation of boost::format ends up calling
basic_string::append(const char *, size_type) with a null pointer, which causes
basic_string::append to assert.

This assertion seems to only happen with /MDd (e.g. debug runtime), but I'd say it qualifies as a bug in Boost.Format.

And here's the offending code from boost::io::detail::mk_str (feed_args.hpp):

        typedef typename std::basic_string<Ch,Tr,Alloc>::size_type size_type;
        res.resize(0);
        if(w<=0 || static_cast<size_type>(w) <=size) {
            // no need to pad.
            res.reserve(size + !!prefix_space);
            if(prefix_space)
              res.append(1, prefix_space);
=>          res.append(beg, size);
        }

I think the simple and obvious fix is to just check for size != 0 before calling res.append, but I'm not familiar with this code.