foo.cc: In function 'int main()':
foo.cc:9:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    9 |  for(int i=0; i<a.size(); i++) a[i] = toupper(a[i]);
      |               ~^~~~~~~~~
foo.cc:10:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   10 |  for(int i=0; i<b.size(); i++) b[i] = toupper(b[i]);
      |               ~^~~~~~~~~

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