Error(s):

source_file.cpp: In function ‘int main()’:
source_file.cpp:28:90: error: no matching function for call to ‘find(std::vector<std::__cxx11::basic_string<char> >::iterator, std::vector<std::__cxx11::basic_string<char> >::iterator, __gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> > >::value_type&)’
     if ( find(allowedExtensions.begin(), allowedExtensions.end(), output[output.size()-1]) != allowedExtensions.end() )
                                                                                          ^
In file included from /usr/include/c++/5/bits/locale_facets.h:48:0,
                 from /usr/include/c++/5/bits/basic_ios.h:37,
                 from /usr/include/c++/5/ios:44,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from source_file.cpp:3:
/usr/include/c++/5/bits/streambuf_iterator.h:369:5: note: candidate: template<class _CharT2> typename __gnu_cxx::__enable_if<std::__is_char<_CharT2>::__value, std::istreambuf_iterator<_CharT> >::__type std::find(std::istreambuf_iterator<_CharT>, std::istreambuf_iterator<_CharT>, const _CharT2&)
     find(istreambuf_iterator<_CharT> __first,
     ^
/usr/include/c++/5/bits/streambuf_iterator.h:369:5: note:   template argument deduction/substitution failed:
source_file.cpp:28:90: note:   ‘__gnu_cxx::__normal_iterator<std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >’ is not derived from ‘std::istreambuf_iterator<_CharT>’
     if ( find(allowedExtensions.begin(), allowedExtensions.end(), output[output.size()-1]) != allowedExtensions.end() )
                                                                                          ^