Understanding namespace std

The line using namespace std; is a convenience feature that lets us use things from the standard library without typing std:: before them. Let's see what happens when we remove it:

1#include <iostream>
2using namespace std; 
3
4int main() {
5  // won't compile without std::
6  cout << "Hello World!\n";  
7}

1error: 'cout' was not declared in this scope

To fix this, we need to specify that we're using cout from the std namespace:

1#include <iostream>
2
3int main() {
4  //  works fine with std:: prefix
5  std::cout << "Hello World!";  
6}

1Hello World!

Namespaces help prevent naming conflicts. Imagine two different libraries both have a function called print():

1#include <iostream>
2
3namespace printerLib {
4void print() {
5  std::cout << "Printing document...\n";
6}
7}
8
9namespace graphicsLib {
10void print() {
11  std::cout << "Drawing to screen...\n";
12}
13}
14
15int main() {
16  // we know exactly which print() to use
17  printerLib::print();
18
19  // no confusion about which one we mean
20  graphicsLib::print();
21}

1Printing document...
2Drawing to screen...

While using namespace std; is fine for learning and small programs, many developers prefer to explicitly write std:: because:

• It makes it clear where things come from
• It prevents potential naming conflicts in larger programs
• It's considered more professional style

You can also be selective about what you import:

1#include <iostream>
2
3using std::cout;  // only import cout 
4
5int main() {
6  // cout works without std::
7  cout << "Hello World!";
8
9  // endl still needs std::
10  cout << std::endl;
11}

For this course, using using namespace std; is perfectly fine - just be aware that you'll see both styles in real-world code.