50 lines
1.7 KiB
Markdown
50 lines
1.7 KiB
Markdown
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% Insert and Remove
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Something *not* provided but slice is `insert` and `remove`, so let's do those next.
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Insert needs to shift all the elements at the target index to the right by one.
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To do this we need to use `ptr::copy`, which is our version of C's `memmove`.
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This copies some chunk of memory from one location to another, correctly handling
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the case where the source and destination overlap (which will definitely happen
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here).
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If we insert at index `i`, we want to shift the `[i .. len]` to `[i+1 .. len+1]`
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using the *old* len.
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```rust
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pub fn insert(&mut self, index: usize, elem: T) {
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// Note: `<=` because it's valid to insert after everything
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// which would be equivalent to push.
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assert!(index <= self.len, "index out of bounds");
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if self.cap == self.len { self.grow(); }
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unsafe {
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if index < self.len {
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// ptr::copy(src, dest, len): "copy from source to dest len elems"
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ptr::copy(self.ptr.offset(index as isize),
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self.ptr.offset(index as isize + 1),
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len - index);
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}
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ptr::write(self.ptr.offset(index as isize), elem);
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self.len += 1;
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}
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}
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```
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Remove behaves in the opposite manner. We need to shift all the elements from
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`[i+1 .. len + 1]` to `[i .. len]` using the *new* len.
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```rust
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pub fn remove(&mut self, index: usize) -> T {
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// Note: `<` because it's *not* valid to remove after everything
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assert!(index < self.len, "index out of bounds");
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unsafe {
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self.len -= 1;
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let result = ptr::read(self.ptr.offset(index as isize));
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ptr::copy(self.ptr.offset(index as isize + 1),
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self.ptr.offset(index as isize),
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len - index);
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result
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}
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}
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```
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