foo.cc: In function 'int main()':
foo.cc:17:6: warning: 'x3' may be used uninitialized in this function [-Wmaybe-uninitialized]
  int sum=x1+x2+x3;
      ^~~
foo.cc:18:21: warning: 'x2' may be used uninitialized in this function [-Wmaybe-uninitialized]
  cout<<x1<<" "<<x2<<" "<<x3<<" "<<sum<<endl;
                     ^~~
foo.cc:18:12: warning: 'x1' may be used uninitialized in this function [-Wmaybe-uninitialized]
  cout<<x1<<" "<<x2<<" "<<x3<<" "<<sum<<endl;
            ^~~

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