foo.cc: In member function 'BigInt BigInt::operator+(const BigInt&) const':
foo.cc:31:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (count == 0 && i >= s.size() && i >= b.s.size()) break;
                               ~~^~~~~~~~~~~
foo.cc:31:50: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (count == 0 && i >= s.size() && i >= b.s.size()) break;
                                                ~~^~~~~~~~~~~~~
foo.cc:33:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (i < s.size()) x += s[i];
                 ~~^~~~~~~~~~
foo.cc:34:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (i < b.s.size()) x += b.s[i];
                 ~~^~~~~~~~~~~~
foo.cc: In member function 'BigInt BigInt::operator-(const BigInt&) const':
foo.cc:47:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (carry == 0 && i >= s.size() && i >= b.s.size()) break;
                               ~~^~~~~~~~~~~
foo.cc:47:50: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (carry == 0 && i >= s.size() && i >= b.s.size()) break;
                                                ~~^~~~~~~~~~~~~
foo.cc:50:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (i < s.size()) x += s[i]; else break;
                 ~~^~~~~~~~~~
foo.cc:51:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (i < b.s.size()) x -= b.s[i];
                 ~~^~~~~~~~~~~~
foo.cc: In function 'std::ostream& operator<<(std::ostream&, const BigInt&)':
foo.cc:68:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for(int j = 0; j < strlen(buf); j++) out << buf[j];
                            ~~^~~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:77:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d",&n);
     ~~~~~^~~~~~~~~