Basic Syntax & I/O

Let's kick things off by familiarising ourselves with the basic structure of a C++ program.

Anatomy of a C++ Program

Recall the source code you previously saw:

main.cpp

#include <iostream>

int main() {
    std::cout << "Hello, world!";
    return 0;
}

Breaking it down:

Code	What it does
#include <iostream>	Brings in the standard I/O library so that we can use things like std::cout
int main() {	The entry point of every C++ program. int indicates the return type of the function main is the function name.
std::cout << "Hel...	Prints the text to the console. 👉 You can remember << as pushing the text into the output stream (i.e std::cout in this case)
return 0;	Recall that int is the return type of the main function. Hence, at the end of our main function, we should[1] return an int. You can return anything, really. But usually returning 0 indicates our program has successfully executed.

Try augmenting the Hello, world! string to something else like your name!

_{[1] Your code will still compile even if you don't return anything from the main function. This is because C++ implicilty returns 0 at the end of main(). For more info, refer here.}

Newlines

If you try printing multiple times, you will notice that the output gets concatenated on the same line. You can fix this by adding a newline using std::endl .

Code	Output
main.cpp #include <iostream> int main() { std::cout << "benn"; std::cout << "tan"; std::cout << "jia"; return 0; }
main.cpp #include <iostream> int main() { std::cout << "benn" << std::endl; std::cout << "tan" << std::endl; std::cout << "jia"; return 0; }

Input

It's not quite fun if we hardcode everything into program. Let's augment our program to accept user input externally.

To read input from the user, we can use std::cin >>

main.cpp

#include <iostream>

int main() {
    int x;
    std::cin >> x;
    std::cout << "You entered: " << x << std::endl;
}

Note that the direction of the arrows (>>) for std::cin is now reversed compared to <<.

👉 You can remember as data flowing into x.

Namespaces

If you find prefixing std:: before the function calls cumbersome, you can omit it with using namespace std

main.cpp

#include <iostream>
using namespace std;
int main() {
    int x;
    cin >> x;
    cout << "You entered: " << x << endl;
}

However, this is generally discouraged in larger projects as it does lead to naming conflicts.

👉 It's usually best practice to be explicit and use std::

Exercise

Build a program that takes in and prints the result of the summation of two numbers.