Why allocate memory? (C++)

Question

I am trying to understand memory allocation in C++. A question that comes to my mind is why is it so necessary to allocate memory? And what happens if we use memory without allocating it? Also, I was shocked to see how careless C++ is on memory allocation. If gives free access to memory through arrays with no bounds checking.

int main()
{
int *p = new int[5];
p[1] = 3;
p[11118] = 9;
cout<<p[11118]<<'\n';
}

The above code works, outputs 9.

In what cases would assigning a value to a non allocated memory location be dangerous? What are the potential ill-effects? Is it possible that the memory location I am accessing has been allocated to some other program and assigning a value to it might cause that program to crash/behave in a very unexpected fashion?

Answer 1

The above code is Undefined Behaviour. It can work, work incorrectly, not work at all, crash, or order pizza through Microsoft Skype. Thou shalt not rely on undefined behavior.