View Compilation Error

0_0_2571717_24919.cpp: In function `int main()':
0_0_2571717_24919.cpp:188: error: non-lvalue in unary `&'
0_0_2571717_24919.cpp:188: error: no match for call to `(std::plus<_Tp>) (std::string&, std::string&, std::string&)'
GCC/bin/../lib/gcc/mingw32/3.4.2/../../../../include/c++/3.4.2/bits/stl_function.h:138: note: candidates are: _Tp std::plus<_Tp>::operator()(const _Tp&, const _Tp&) const