/in/foo.cc: In function 'int main()':
/in/foo.cc:16:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=1;i<b.size();i++) //预处理 
              ~^~~~~~~~~
/in/foo.cc:29:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.size();i++)
              ~^~~~~~~~~
/in/foo.cc:35:15: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
   if(b[n]=a[i])
               ^
/in/foo.cc:39:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(n==b.size())
      ~^~~~~~~~~~
/in/foo.cc:41:28: warning: format '%d' expects argument of type 'int', but argument 2 has type 'std::__cxx11::basic_string<char>::size_type {aka long unsigned int}' [-Wformat=]
    printf("%d",i-b.size()+1);
                            ^