istringstream.rdbuf with custom streambuf does not compile

[杨岑]

Thanks for the iss.istream::rdbuf tip. I never knew C++ syntax has
evolved so much.

在 2016/5/6 23:12, Martin Sebor 写道:
> 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
>