重新编写下面的代码，使之使用using声明，而不是using编译指令。
#include <iostream>
using namespace std;
int main()
{
double x;
cout << "Enter value: ";
while (! (cin >> x) )
{
cout << "Bad input. Please enter a number: ";
cin.clear();
while (cin.get() != '\n')
continue;
}
cout << "Value = " << x << endl;
return 0;
} 
#include <iostream> int main() { using std::cin; using std::cout; using std::endl; double x; cout << "Enter value: "; while (! (cin >> x) ) { cout << "Bad input. Please enter a number: "; cin.clear(); while (cin.get() != '\n') continue; } cout << "Value = " << x << endl; return 0; }