What is the need to specify "std" prefix?

Question

I'm a beginner in programming and I often see many programs using the prefix std if they are using any std functions like std::cout, std::cin, etc. I was wondering what is it's purpose ? Is it just a way of good programming or is there more to it ? Does it make any difference for the compiler or is it readability or what ? Thanks.

Answer 1

The STL types and functions are defined in the namespace named std. The std:: prefix is used to use the types without fully including the std namespace.

Option 1 (use the prefix)

#include <iostream>

void Example() {
  std::cout << "Hello World" << std::endl;
}

Option #2 (use the namespace)

#include <iostream>
using namespace std;

void Example() {
  cout << "Hello World" << endl;
}

Option #3 (use types individually)

#include <iostream>
using std::cout;
using std::endl;

void Example() {
    cout << "Hello World" << endl;
}

Note: There are other implications to including an entire C++ namespace (option #2) other than not having to prefix every type / method with std:: (especially if done within a header) file. Many C++ programmers avoid this practice and prefer #1 or #3.