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?
Why is it neccessary to allocate memory? Because that way, you mark the memory as yours. Nobody else can use it. It also verifies that there is in fact memory available. If your system only has 1000 bytes of memory, just picking byte 1500 to store some data at is a bad idea.
What happens if we use memory without allocating it? Nobody knows. The address you write you might not exist. A different process might have already started using it, so you overwrite their data. The memory could be protected; in the former case, for instance, the operating system may notice that you are accessing memory another process has laid claim to, and stop you. You might own that region of memory, but a different part of the program is using it for some reason, and you've overwritten your own data.
Free access to memory through arrays with no bounds checking. That code does not work... it functions as expected, at the moment, but that is not the same thing. Formally, that is undefined behavior, so the compiler can emit code to do anything it wants.
In what cases would assigning value to a non allocated memory location would be dangerous? I gave some examples above. It is also possible to break your stack. When you call a function, the address the function should return to is stored. If you overwrite that value through careless memory access, then when you leave that function, who knows where you'll end up? Maybe the person exploiting your program... a common exploit is to load executable code into some part of memory, then use a bug in an existing program to run it. Once, on an embedded device I was working on, I had a fencepost error that resulted in my function returning into the middle of another instruction elsewhere. That should have crashed my chip, but as luck would have it the second half of that instruction was itself a valid instruction. The sequence of code that ended up running caused the device to gain sentience, and eventually finished the project we were working on itself. Now, it just plays WoW in my basement. Thus is the horror of undefined behavior.