Array index out of bounds

[kuushie118]

What is array index out-of-bound? Does C++ checks for array indices within bound?

[Ganado]

An array is a series of elements, each accessed by an index of that array.

If I say
int arr[5] = { 20, 30, 40, 50, 60 };,
arr has 5 elements in it, accessed through arr[0] to arr[4].

The first element, 20, is access through arr[0].
The second element, 30, is accessed through arr[1].
The third element, 40, is accessed through arr[2].
The fourth element, 50, is accessed through arr[3].
The last (5th) element, 60, is accessed through arr[4].

If an index is "out of bounds", it means that it would be attempting to access an array element that doesn't exist.
For example,

int arr[5] = { 20, 30, 40, 50, 60 }; // declare an array of length 5.
arr[5] = 70; // 5 here is out of bounds, because valid array indices only go from 0 to 4
arr[-1] = 42; // -1 here is out of bounds, because arrays start at index 0. 

If you were to attempt to access an out-of-bounds index, you invoke undefined behavior and your program is wrong.

______________________________________

The best way to check for out-of-bounds array indices is to not let it happen in the first place. Design clear logic in your loops so that it never happens.
But we're only human. Standard library provides std::vector.
std::vector has the option to throw an exception when an out-of-bounds index is used, through the at member function. It also has the standard [index] method of accessing elements, which doesn't do bounds checking.

For example,

#include <iostream>
#include <vector>

int main() {
	std::vector<int> vec = { 3, 1, 4, 1, 5 };
	
	vec[0] = 4; // arr is now: { 4, 1, 4, 1, 5 }
	
	//vec[5] = 9; // Undefined behavior: index ouf of bounds
	             // (No bounds checking was done here)
	try
	{
		vec.at(5) = 9; // use at() function to access elemebnt
	}
	catch (...)
	{
		std::cout << "Out-of-bounds exception captured!\n";
	}
	
	
	return 0;
}

[kuushie118]

Thank you so much :D

[jonnin]

vectors have a lot of good stuff going for them.
But c++ arrays, if using a standard enforced compile, MUST have a known size at compile time (cannot be a variable, only a compile time known value like enum, const int, literal number, etc.) You should always know the size of your array and never run out of bounds on it because of this. So while you should favor vectors in general, you will sometimes inherit code that has arrays, or have to deal with a C file in your compilation, or the like, so you need to know the basics. Typically the size is an enum or const somewhere with a name that you can use when looping to keep it safe. If you use arrays for whatever reason, use this idea and you will avoid most out of bounds issues. The other issues can happen on vectors too -- the ones where you computed the index and goofed it up, but vectors give you the tools to check this while arrays have fewer options for handling bugs.

c++ is a high performance language. It does NOT do unnecessary work behind the scenes in general, you have to MAKE it do that if you want it. So if you want bounds checking, you need code to do that. you can go out of bounds on a vector too, by the way.
vector <int> duh = {1,2,3};
cout << duh[23]; //out of bounds. crash, most likely.

its not that vector is checking the bounds or 'safer' when you mess up. Its that it has its own managed size variable so messing up should be harder, if you are writing your code carefully.