foo.cc: In function 'void Test()':
foo.cc:10:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   10 |  for (int i = 0; i < a.length(); i++)
      |                  ~~^~~~~~~~~~~~
foo.cc:15:18: 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 (j = 0; j < a1.length(); j++)
      |                ~~^~~~~~~~~~~~~
foo.cc:20:10: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   20 |    if (j == a1.length())
      |        ~~^~~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务