C++ Refresher

In this lesson, we'll get a quick C++ refresher.


Let’s quickly go over a few concepts, most of which you should already know if you have some experience with C++.

These will come handy and help you avoid common mistakes that beginners make.


You can always use a simple text editor and terminal to compile and run C++ programs.

I personally like using codeblocks. You can download the IDE for most of the platforms here.


Including libraries can quickly become a pain. You can always Google the required library for a built-in function or data type, but a great alternative is to use a single include statement that includes all libraries or to use a long list of include statements in your code.

#include <bits/stdc++.h>

If the above doesn’t work for you, try this fix.

Global and local variables

Here is how you declare global and local variables.

#include <iostream>
using namespace std;
int global = 10;
int main() {
int local = 5;
return 0;

Important: When you want to declare an array of say, a million integers, declaring it as local might not work depending on various factors, like memory issues for one.

But as a general rule, you will see most everyone declares large arrays that the program will need as global, thus creating the array in heap memory. I would suggest doing this.

Note: Declaring variables using the new operator or using STL structures like vectors allocates the memory to heap in this case, meaning it can be declared locally as well.


Data types int and long long int are 32-bit and 64-bit integers, respectively.

While doing arithmetic operations, always know the data type’s limit.

For example, int (signed) can store values a little over 2 billion, so squaring 5 million will definitely overflow for int but not for long long int.

int x = 5000000;
int y = x * x; //Overflow
long long int = (long long int)x * x; //This works, type casting to long long int

Pass by value and pass by reference

When passing arguments to a function, there are 2 ways of doing it.

  1. Pass by value: The arguments are copied when the function executes. So any changes to the arguments are not reflected back to the calling code.
  2. Pass by reference: The arguments are not copied, a reference is passed. Since the argument will point to the same memory address when passed by reference, any changes made are reflected back to the calling code

the and operator & is used to pass by reference.

#include <iostream>
using namespace std;
void by_value(int x) {
x *= 2;
void by_reference(int &x) {
x *= 2;
int main() {
int a1 = 5, a2 = 5;
cout << "Before: a1 = " << a1 << ", a2 = " << a2 << "\n";
cout << "After: a1 = " << a1 << ", a2 = " << a2 << "\n";
return 0;

In the next lesson, we’ll study some handy built-in C++ methods.