foo.cc: In function 'int main()':
foo.cc:10:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(i=0;i<t.length();i++) m[t[i]]=1;
          ~^~~~~~~~~~~
foo.cc:11:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(i=0;i<s.length();i++) if(m[s[i]]==0) cout<<s[i];
          ~^~~~~~~~~~~