On 05/06/2016 06:40 AM, 杨春雷 wrote:
I have the following simple test code (t.cxx):
#include <streambuf>
#include <sstream>
using namespace std;
class FileBuf: public streambuf
{
};
int main(int argc, char** argv)
{
FileBuf fbuf;
istringstream iss;
// The following line does not compile!
iss.rdbuf(&fbuf);
return 0;
}
When compiling the above code using g++ (version 5.3.1 20160406 (Red Hat 5.3.1-6)), I got very strange errors:
$ g++ -o t t.cxx
t.cxx: In function ‘int main(int, char**)’:
t.cxx:14:17: error: no matching function for call to ‘std::__cxx11::basic_istringstream<char>::rdbuf(FileBuf*)’
iss.rdbuf(&fbuf);
^
In file included from t.cxx:2:0:
/usr/include/c++/5.3.1/sstream:472:7: note: candidate: std::__cxx11::basic_istringstream<_CharT, _Traits, _Alloc>::__stringbuf_type* std::__cxx11::basic_istringstream<_CharT, _Traits, _Alloc>::rdbuf() const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_istringstream<_CharT, _Traits, _Alloc>::__stringbuf_type = std::__cxx11::basic_stringbuf<char>]
rdbuf() const
^
/usr/include/c++/5.3.1/sstream:472:7: note: candidate expects 0 arguments, 1 provided
However, I have to a (unnecessary) cast to get it working, like this:
((istream&)iss).rdbuf(&fbuf);
To write the code without a cast it's possible to qualify the name
rdbuf with the name of the class where to look up its name:
iss.istream::rdbuf (&fbuf);
But there an important subtlety to these overloads of rdbuf that's
worth mentioning. istringstream::rdbuf() always returns a pointer
to the the same stream buffer object (the member of the stream
object). basic_ios::rdbuf() returns a pointer the basic_ios
subobject was last associated with by calling the one argument
overload of basic_ios::rdbuf(basic_streambuf*) with. If in the
example above one were to also call rdbuf() to obtain the streambuf
pointer. To retrieve the basic_ios streambuf pointer, call
istream::rdbuf() (e.g., iss.istream::rdbuf()).
Martin
