Operator<< for std::string - c++

Question

I define the operator<< for std::string objects:
std::string & operator<< (std::string & left, std::string & right){
return left += right;
}
Then I use it:
std::string t1("t1"), t2("t2"), t3;
t3 = t2 << t1;
And get from a compiler:
t.cpp: In function 'int main()':
t.cpp:44:28: error: no matching function for call to 'operator<<(std::string&, std::string&)'
t.cpp:44:28: note: candidates are:
In file included from d:\mingw\bin\../lib/gcc/mingw32/4.7.2/include/c++/iostream:40:0,
from connection.h:10,
from login.cpp:1:
d:\mingw\bin\../lib/gcc/mingw32/4.7.2/include/c++/ostream:600:5: note: template<class _CharT, class _Traits, class _Tp> std::basic_ostream<_CharT
, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&)
d:\mingw\bin\../lib/gcc/mingw32/4.7.2/include/c++/ostream:600:5: note: template argument deduction/substitution failed:
t.cpp:44:28: note: 'std::string {aka std::basic_string<char>}' is not derived from 'std::basic_ostream<_CharT, _Traits>'
In file included from d:\mingw\bin\../lib/gcc/mingw32/4.7.2/include/c++/iostream:40:0,
Why talks it about ostream and does not talk about string? I.e. why it does not take in account my definition of the operator<< ?
Thanks.
Update. For those who is able just to say "why do you create operator<< for strings?" and is not able to say any helpful things:
std::string & operator<< (std::string & left, const int num){
return left += std::to_string(num);
}
std::string t3;
t3 << 3 << 5;
std::cout << t3 << std::endl;
And log:
t.cpp: In function 'int main()':
t.cpp:45:12: error: no match for 'operator<<' in 't3 << 3'
t.cpp:45:12: note: candidates are:
In file included from d:\mingw\bin\../lib/gcc/mingw32/4.7.2/include/c++/iostream:40:0,
from connection.h:10,
from login.cpp:1:
d:\mingw\bin\../lib/gcc/mingw32/4.7.2/include/c++/ostream:600:5: note: template<class _CharT, class _Traits, class _Tp> std::basic_ostream<_CharT
, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&)
d:\mingw\bin\../lib/gcc/mingw32/4.7.2/include/c++/ostream:600:5: note: template argument deduction/substitution failed:
t.cpp:45:12: note: 'std::string {aka std::basic_string<char>}' is not derived from 'std::basic_ostream<_CharT, _Traits>'

Answer 1

It does work:
#include <iostream>
#include <string>
std::string & operator<< (std::string & left, std::string & right){
return left += right;
}
int main()
{
std::string t1("t1"), t2("t2"), t3;
t3 = t2 << t1;
std::cout << t3;
}
Output: t2t1 [GCC 4.8.1]
The compiler output, as you say yourself, indicates that your operator overload is not even visible. You must not have your declaration in the right place.
This is not really a good idea, anyway: you will simply confuse the heck out of anybody who reads your code. Strings are not streams.