foo.cc: In function 'std::string add(std::string, std::string)':
foo.cc:38:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   38 |                 if(i<a.size()){sum=sum+a[i]-'0';}
      |                    ~^~~~~~~~~
foo.cc:39:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   39 |                 if(i<b.size()){sum=sum+b[i]-'0';}
      |                    ~^~~~~~~~~

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