rgba.ptr[0] vs rgba[0]

Adam D. Ruppe destructionator at gmail.com
Mon Nov 23 17:39:09 UTC 2020


On Monday, 23 November 2020 at 17:34:27 UTC, visitor wrote:
> Hi all,
>
> I would like to know why in the code below, rgba.ptr[0] is used 
> instead of rgba[0] and allowing the method to be @safe

The .ptr[0] skips bounds checking.

Since this example is static length with a constant index it 
shouldn't matter anyway; the compiler can see it is obviously in 
bounds and skip it too.

But if there's any runtime value there's a bounds check with 
`foo[0]` and that can be surprisingly expensive in certain 
situations. So `foo.ptr[0]` skipping that can give a nice 
performance boost.

Just without bounds checking the code is obviously trusting the 
programmer... hence @trusted is required instead of safe.


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