foo.cc: In function 'int Substring(std::__cxx11::string)':
foo.cc:8:12: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
  char  *b= "5" ;
            ^~~
foo.cc:11:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<str.size();i++)
              ~^~~~~~~~~~~
foo.cc:12:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=1;j<=((str.substr(i)).size());j++)
               ~^~~~~~~~~~~~~~~~~~~~~~~~~~

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