使用 `std::max_element` `error: void value not ignored as it should be`

Question

I was trying to get the element with largest size from a vector of elements, as succinct as possible. The original code looks like,

std::cout << ((*std::max_element(rooms.begin(), rooms.end(),
          [](auto a, auto b){a.size() < b.size();})).size()) << std::endl;

which g++ complains,

wall3.cpp: In function ‘int main()’:
wall3.cpp:150:47: error: ‘class __gnu_cxx::__normal_iterator<std::set<std::pair<long unsigned int, long unsigned int> >*, std::vector<std::set<std::pair<long unsigned int, long unsigned int> > > >’ has no member named ‘size’
  150 |     [](auto a, auto b){a.size() < b.size();}).size() << std::endl;
      |                                               ^~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:71,
                 from /usr/include/c++/9/bits/char_traits.h:39,
                 from /usr/include/c++/9/string:40,
                 from /usr/include/c++/9/bitset:47,
                 from wall3.cpp:1:
/usr/include/c++/9/bits/predefined_ops.h: In instantiation of ‘constexpr bool __gnu_cxx::__ops::_Iter_comp_iter<_Compare>::operator()(_Iterator1, _Iterator2) [with _Iterator1 = __gnu_cxx::__normal_iterator<std::set<std::pair<long unsigned int, long unsigned int> >*, std::vector<std::set<std::pair<long unsigned int, long unsigned int> > > >; _Iterator2 = __gnu_cxx::__normal_iterator<std::set<std::pair<long unsigned int, long unsigned int> >*, std::vector<std::set<std::pair<long unsigned int, long unsigned int> > > >; _Compare = main()::<lambda(auto:1, auto:2)>]’:
/usr/include/c++/9/bits/stl_algo.h:5692:12:   required from ‘constexpr _ForwardIterator std::__max_element(_ForwardIterator, _ForwardIterator, _Compare) [with _ForwardIterator = __gnu_cxx::__normal_iterator<std::set<std::pair<long unsigned int, long unsigned int> >*, std::vector<std::set<std::pair<long unsigned int, long unsigned int> > > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(auto:1, auto:2)> >]’
/usr/include/c++/9/bits/stl_algo.h:5743:43:   required from ‘constexpr _FIter std::max_element(_FIter, _FIter, _Compare) [with _FIter = __gnu_cxx::__normal_iterator<std::set<std::pair<long unsigned int, long unsigned int> >*, std::vector<std::set<std::pair<long unsigned int, long unsigned int> > > >; _Compare = main()::<lambda(auto:1, auto:2)>]’
wall3.cpp:150:45:   required from here
/usr/include/c++/9/bits/predefined_ops.h:143:18: error: void value not ignored as it ought to be
  143 |         { return bool(_M_comp(*__it1, *__it2)); }
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Answer 1

It turns out that I forgot to use return in the lambda expression. Adding return solved the problem.

Lesson: use a different compiler and turn on the strictest warning configurations to reveal the problem faster. The output from clang++ is far more explicit,

wall3.cpp:150:33: warning: relational comparison result unused [-Wunused-comparison]
                  [](auto a, auto b){a.size() < b.size();})).size()) << std::endl;
                                     ~~~~~~~~~^~~~~~~~~~