c++ - std::vector : cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'

Question

I encountered a confusing error message when trying to do something as simple as

std::cout << std::vector<int>{1,2,3};

which says

 cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'
 int main() {  std::cout << std::vector<int>{1,2,3}; }

(tested using gcc-4.8.1 with -std=c++11)

SO has similar questions like Overloading operator<<: cannot bind lvalue to ‘std::basic_ostream<char>&&’, which is about some user defined class with nested classes. There is also a work around the accepted answer to that question.

But I don't know whether this applies to std::vector. Can someone explain why this error happens to std::vector, and how to interpret it?

Thanks

Answer 1

Template-related error messages can be confusing at times. The problem is that the standard library does not define an overload of operator << for inserting std::vector (or any other container, for that matter) into a std::ostream. So the compiler fails to find a suitable overload for operator <<, and reports this failure as best as it's able (which is unfortunately not too good/readable in your case).

If you want to stream an entire container, you can use std::ostream_iterator for that:

auto v = std::vector<int>{1, 2, 3};
std::copy(begin(v), end(v), std::ostream_iterator<int>(std::cout, " "));

---

As for why you're getting precisely this cryptic error, it helps to analyse the full error message:

prog.cpp: In function ‘int main()’:
prog.cpp:13:37: error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’
  std::cout << std::vector<int>{1,2,3};
                                     ^
In file included from /usr/include/c++/4.8/iostream:39:0,
                 from prog.cpp:3:
/usr/include/c++/4.8/ostream:602:5: error:   initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = std::vector<int>]’
     operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
     ^

There is apparently a template overload of operator<< which takes a lhs argument of type std::ostream&& and a rhs argument of the templated type; it exists to allow insertion into temporary streams. Since it's a template, it becomes the best match for the expression in your code. However, std::cout is an lvalue, so it cannot bind to std::ostream&&. Hence the error.