预处理指令using namespace std

要使用cin和cout，必须使用预处理指令

#include<iostream>

如果没有using namespace std，标识符应为std::cin和std::cout

#include<iostream>#include<stdio.h>using namespace std;int main(){cout << "Enter two numbers:" << endl;int v1, v2;cin >> v1 >> v2;cout << "The sum of" << v1 << "and" << v2<< "is" << v1 + v2 <<endl;system("pause");return 0;}

或者

#include<iostream>  #include<stdio.h>  int main()  {      std::cout << "Enter two numbers:" << std::endl;      int v1, v2;      std::cin >> v1 >> v2;      std::cout << "The sum of" << v1 << "and" << v2          << "is" << v1 + v2 << std::endl;      system("pause");      return 0;  }