Compilare: user.cpp: In function ‘void convertInfixToPostfix()’: user.cpp:38:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < infix.size(); ++i) { ~~^~~~~~~~~~~~~~ user.cpp:75:40: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] } while (!isOperator(infix[i]) && i < infix.size()); ~~^~~~~~~~~~~~~~ user.cpp: In function ‘void printPostfixExpressionResult()’: user.cpp:106:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < postfix.size(); ++i) { ~~^~~~~~~~~~~~~~~~ user.cpp: In function ‘long long int calculate(long long int, long long int, std::__cxx11::string)’: user.cpp:101:1: warning: control reaches end of non-void function [-Wreturn-type] } ^