Visual Studio: "str() is not a member of std::ostringstream"

Question

I have a compiler error with Visual Studio 2013 (while Xcode 6.2 compiles) which I can't make any sense of:

following example code is an abstract, reduced excerpt from a format conversion:

#include<sstream>
void main(...){
    (std::ostringstream()<<0).str();
}

while following version compiles:

#include<sstream>
void main(...){
    (std::ostringstream()).str();
}

context:

std::string result=(std::ostringstream()<<value).str(); 

what do I miss here? Thanks!

Answer 1

The error message is error C2039: 'str': is not a member of 'std::basic_ostream<char,std::char_traits<char>>'. It's complaining that str is missing in std::basic_ostream<char,std::char_traits<char>> aka std::ostream, not std::ostringstream.

std::ostringstream's operator<< still returns std::ostream& (it inherits these operators from std::ostream), which has no str() member.

Clang/libc++ (which is what Xcode uses) implements an extension in its rvalue stream insertion operator that

1. Makes it a better match for rvalue streams compared to the inherited member operator<<s, and
2. returns a reference to the stream as is, without conversion to std::ostream&.

Together this made your code compile in Xcode.

To call .str(), you can manually static_cast (std::ostringstream()<<value) back to std::ostringstream& (use either std::ostringstream && or const std::ostringstream& for libc++ compatibility, since its rvalue stream insertion operator returns an rvalue reference to the stream).