foo.cc: In function ‘int main()’:
foo.cc:9:22: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<int>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    9 |         for(int i=0;i<a.size();i++)sum+=a[i];
      |                     ~^~~~~~~~~
foo.cc:7:20: warning: ‘i’ is used uninitialized [-Wuninitialized]
    7 |         for(int i;i<=xmax;i++)a.push_back(i);
      |                   ~^~~~~~
foo.cc:7:17: note: ‘i’ declared here
    7 |         for(int i;i<=xmax;i++)a.push_back(i);
      |                 ^