Error(s):

C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\algorithm(1015): error C2678: binary '=': no operator found which takes a left-hand operand of type 'const std::basic_string<char,std::char_traits<char>,std::allocator<char>>' (or there is no acceptable conversion)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstring(1032): note: could be 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> &std::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator =(_Elem)'
        with
        [
            _Elem=char
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstring(1027): note: or       'std::basic_string<char,std::char_traits<char>,std::allocator<char>> &std::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator =(const _Elem *)'
        with
        [
            _Elem=char
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstring(1011): note: or       'std::basic_string<char,std::char_traits<char>,std::allocator<char>> &std::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator =(const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &)'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstring(972): note: or       'std::basic_string<char,std::char_traits<char>,std::allocator<char>> &std::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator =(std::initializer_list<_Elem>)'
        with
        [
            _Elem=char
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstring(914): note: or       'std::basic_string<char,std::char_traits<char>,std::allocator<char>> &std::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator =(std::basic_string<char,std::char_traits<char>,std::allocator<char>> &&) noexcept(<expr>)'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\algorithm(1015): note: while trying to match the argument list '(const std::basic_string<char,std::char_traits<char>,std::allocator<char>>, std::string)'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\algorithm(1027): note: see reference to function template instantiation '_OutIt std::_Transform<_Iter,_OutIt,std::string(__cdecl *)(const std::string &)>(_InIt,_InIt,_OutIt,_Fn1)' being compiled
        with
        [
            _OutIt=std::_Tree_const_iterator<std::_Tree_val<std::_Tree_simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char>>>>>,
            _Iter=std::istream_iterator<std::string,char,std::char_traits<char>,ptrdiff_t>,
            _InIt=std::istream_iterator<std::string,char,std::char_traits<char>,ptrdiff_t>,
            _Fn1=std::string (__cdecl *)(const std::string &)
        ]
source_file.cpp(56): note: see reference to function template instantiation '_OutIt std::transform<std::istream_iterator<std::string,char,std::char_traits<char>,ptrdiff_t>,std::_Tree_const_iterator<std::_Tree_val<std::_Tree_simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char>>>>>,std::string(__cdecl *)(const std::string &)>(_InIt,_InIt,_OutIt,_Fn1)' being compiled
        with
        [
            _OutIt=std::_Tree_const_iterator<std::_Tree_val<std::_Tree_simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char>>>>>,
            _InIt=std::istream_iterator<std::string,char,std::char_traits<char>,ptrdiff_t>,
            _Fn1=std::string (__cdecl *)(const std::string &)
        ]
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x64