Exercise 2-10

Explain each of the uses of std:: in the following program:

int main ()
{
    int k = 0;
    while (k != 10)   // invariant: we have written k asterisks so far
    {
        using std::cout;
        cout << "*";
        ++k;
    }
    std::cout << std::endl;   // std:: is required here
    return 0;
}

Solution

Back to basics – what are namespace, names within a namespace, standard library, and iostream header? (refer to page 3 / chapter 0.5 of the book).

• std:: corresponds to a namespace called std.
• A namespace (e.g. std) is a collection of related names (e.g. cout, cin, endl). The standard library uses std to contain all the names that it defines.
• The iostream standard header defines the names (e.g. cout, cin, endl)

i.e. the iostream standard header defines the names cout, cin, endl, and we refer to these names as std::cout, std::cin, std::endl.

Now, let’s explain the uses of std:: in the program.

Within the while loop, we have using std::cout statement to declare that the name cout (residing in namespace std) is made available to the program. i.e. within the while loop scope whenever we have cout, the program “knows” that we are referring to std::cout – the cout of namespace std. The usage of std::cout statement has the life span of only within the while loop scope, as defined by the two curly braces { }. The while loop uses the std::cout to write asterisks on the console window.

Immediately after the while loop scope, we have std::cout – i.e. we need to explicitly specify that we are referring to the cout of the namespace std (and not other namespaces). std::cout is used here to write to the console window.

Lastly, we have the std::endl. Again, we need to explicitly specify that we are referring to the endl of the namespace std (and not other namespaces). std::endl is used for ending the currently line (and start a new line).

Putting this all together, the program does this:

• Write 10 asterisks to the console output window.
• End the current line and start a new line.

To run the program however, we must add the iostream header at the top which defines what the like of std::cout, std::endl, etc.

#include <iostream>

Result

**********

Reference

Koenig, Andrew & Moo, Barbara E., Accelerated C++, Addison-Wesley, 2000