rust/vec-push-pop.md

% Push and Pop

Alright. We can initialize. We can allocate. Let's actually implement some functionality! Let's start with push. All it needs to do is check if we're full to grow, unconditionally write to the next index, and then increment our length.

To do the write we have to be careful not to evaluate the memory we want to write to. At worst, it's truly uninitialized memory from the allocator. At best it's the bits of some old value we popped off. Either way, we can't just index to the memory and dereference it, because that will evaluate the memory as a valid instance of T. Worse, foo[idx] = x will try to call drop on the old value of foo[idx]!

The correct way to do this is with ptr::write, which just blindly overwrites the target address with the bits of the value we provide. No evaluation involved.

For push, if the old len (before push was called) is 0, then we want to write to the 0th index. So we should offset by the old len.

pub fn push(&mut self, elem: T) {
    if self.len == self.cap { self.grow(); }

    unsafe {
        ptr::write(self.ptr.offset(self.len as isize), elem);
    }

    // Can't fail, we'll OOM first.
    self.len += 1;
}

Easy! How about pop? Although this time the index we want to access is initialized, Rust won't just let us dereference the location of memory to move the value out, because that would leave the memory uninitialized! For this we need ptr::read, which just copies out the bits from the target address and intrprets it as a value of type T. This will leave the memory at this address logically uninitialized, even though there is in fact a perfectly good instance of T there.

For pop, if the old len is 1, we want to read out of the 0th index. So we should offset by the new len.

pub fn pop(&mut self) -> Option<T> {
    if self.len == 0 {
        None
    } else {
        self.len -= 1;
        unsafe {
            Some(ptr::read(self.ptr.offset(self.len as isize)))
        }
    }
}