[Inkscape-devel] reinterpret_cast unreliable on windows?

I have fixed this windows-only bug, but I cannot fully understand how:

https://bugs.launchpad.net/inkscape/+bug/169011

The problem was in this line:

g_snprintf( b, len, "%s:%s;", key, css.gcharp() );

and gcharp is defined as:

gchar const *gcharp() const { return reinterpret_cast<gchar const *>(ostr.str().c_str()); }

The problem is that sometimes, rarely on some Windows systems and more frequently on others, gcharp() returned empty string for non-empty ostr. I replaced it with simply:

g_snprintf( b, len, "%s:%s;", key, css.str().c_str() );

and the problem went away.

Can anyone explain how this could happen? Is reinterpret_cast unreliable? Maybe we should eliminate it everywhere?