error: `cout' was not declared in this scope

Linux下C++使用GCC编译出错:

程序：
#include<iostream>
int main()
{
cout << "Hello World!" << endl;

return 0;
}

编译出错：
$ g++ s.cpp -o s.out
s.cpp: In function `int main(int, char**)':

s.cpp:12: error: `cout' was not declared in this scope
s.cpp:12: error: `endl' was not declared in this scope

原因：
C++ 1998 要求cout and endl被调用使用'std::cout'和'std::endl'格式,或using namespace std;

修改后：
#include<iostream>
int main()

{

std::cout << "Hello World!" << std::endl;

return 0;

}

或
#include<iostream>
using namespace std;
int main(int argc, char *argv[])
{

cout << "hello world" << endl;

return 0;

}

编译通过。