bool user_says_yes(){int c;bool initial_response=true;do{if(initial_response)cout<<"(y,n)?"<<flush;else cout<<"Respond with either y or n: "<<flush;do{c=cin.get();}while(c=='\n' || c==' ' || c=='\t');initial_response=false;}while(c!='y' && c!='Y' && c!='n' && c!='N');return (c=='y' || c=='Y');}flush是干嘛用的？\t是干嘛用的？