解决问题 “error: `cout' was not declared in this scope”

code is:
#include <iostream>

int main(int argc, char *argv[])
{
    cout << "hello world" << endl;
}

when compile the erro info is below:

test@HuiT43 ~/sutdy $ g++ scat.cpp  -o scat
scat.cpp: In function `int main(int, char**)':
scat.cpp:12: error: `cout' was not declared in this scope
scat.cpp:12: error: `endl' was not declared in this scope

the reason is :

This is becuase C++ 1998 requires cout and endl be called 'std::cout' and 'std::endl', or that a proper using directives such as 'using namespace std;' be used.

#include <iostream>

int main(int argc, char *argv[])
{
    std::cout << "hello world" << std::endl;
}

or

#include <iostream>

using namespace std;

int main(int argc, char *argv[])
{
     cout << "hello world" << endl;
}