foo.cc: In function 'int main()':
foo.cc:2:34: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    2 | #define _for(i,a,b) for(int i=a;i<b;i++)
......
   10 |         _for(i,0,str.length())
      |              ~~~~~~~~~~~~~~~~     
foo.cc:10:9: note: in expansion of macro '_for'
   10 |         _for(i,0,str.length())
      |         ^~~~

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