/in/foo.cc: In function 'double evalexpression(const string&)':
/in/foo.cc:35:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     while (idx < expression.size()) {
            ~~~~^~~~~~~~~~~~~~~~~~~
/in/foo.cc:42:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             while (idx < expression.size() && (isdigit(expression[idx]) || expression[idx] == '.')) {
                    ~~~~^~~~~~~~~~~~~~~~~~~
/in/foo.cc: In function 'double calculate(double, double, char)':
/in/foo.cc:28:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^