foo.cc: In function 'std::string cf(std::string)':
foo.cc:15:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   15 |   for(int i=0;i<=a.size();i++)
      |               ~^~~~~~~~~~
foo.cc:27:14: warning: comparison of integer expressions of different signedness: 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   27 |   if(a.size()>len)
      |      ~~~~~~~~^~~~
foo.cc:32:14: warning: comparison of integer expressions of different signedness: 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   32 |   if(a.size()==len)
      |      ~~~~~~~~^~~~~
foo.cc: In function 'int main()':
foo.cc:124:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
  124 |   for(int i=0;i<s.size();i++)
      |               ~^~~~~~~~~

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