Let's break up the following code to understand it better:
Example
#include <iostream>
using namespace std;
int main() {
cout << "Hello World!";
return 0;
}Example explained
Line 1: #include <iostream> is a header file library that lets us work with input and output objects, such as cout (used in line 5). Header files add functionality to C++ programs.
Line 2: using namespace std means that we can use names for objects and variables from the standard library.
Don't worry if you don't understand how #include <iostream> and using namespace std works. Just think of it as something that (almost) always appears in your program.
Line 3: A blank line. C++ ignores white space. But we use it to make the code more readable.
Line 4: Another thing that always appear in a C++ program is int main() . This is called a function . Any code inside its curly brackets {} will be executed.
Line 5: cout (pronounced "see-out") is an object used together with the insertion operator ( << ) to output/print text. In our example, it will output "Hello World!".
Note
C++ is case-sensitive: "cout" and "Cout" has different meaning.
Note
Every C++ statement ends with a semicolon ; .
Note
The body of int main() could also been written as: int main () { cout << "Hello World! "; return 0; }
Remember
The compiler ignores white spaces. However, multiple lines makes the code more readable.
Line 6: return 0; ends the main function.
Line 7: Do not forget to add the closing curly bracket } to actually end the main function.
Omitting Namespace
You might see some C++ programs that run without the standard namespace line. The using namespace std; statement can be omitted, and replaced with the std keyword followed by the :: operator, for some objects (like std::cout in the example below):
Example
#include <iostream>
int main() {
std::cout << "Hello World!";
return 0;
}Both approaches are valid in C++. Using std:: makes it clear where names come from and avoids name conflicts in larger programs.
Which One Do We Use?
In this tutorial, we will mostly use using namespace std;
This keeps the code shorter and easier to read while learning the basics of C++. It helps beginners focus on understanding the language itself, instead of repeatedly writing std:: in every example.
As you write larger or more advanced programs, you may prefer using std:: explicitly. You will see both styles in real-world C++ code.