# Please explain this error **URL:** \ **Category:** Help & Support\ **Created:** [April 1, 2022, 3:23am UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537 "2022-04-01T03:23:03Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![jzakiya](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/jzakiya/32/278_2.png) [@jzakiya](https://forum.crystal-lang.org/u/jzakiya)\ **Post date:** [April 1, 2022, 3:23am UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/1 "2022-04-01T03:23:03Z")
I was experimenting with some code. These compile. ```auto cnt = seg[..(kn - 1) >> s].map { |m| (~m).popcount }.sum cnt = seg[0..(kn - 1) >> s].map { |m| (~m).popcount }.sum cnt = seg[0i32..(kn - 1).to_i32 >> s].map { |m| (~m).popcount }.sum ``` But these don’t ```auto cnt = seg.to_unsafe[0..(kn - 1) >> s].map { |m| (~m).popcount }.sum cnt = seg.to_unsafe[0i32..(kn - 1).to_i32 >> s].map { |m| (~m).popcount }.sum ``` And gives this. error for both. ```auto ➜ crystal build twinprimes_ssozgisty.cr -Dpreview_mt --release Showing last frame. Use --error-trace for full trace. In /home/jzakiya/crystal/share/crystal/src/pointer.cr:117:11 117 | (self + offset).value ^ Error: no overload matches 'Pointer(UInt64)#+' with type Range(Int32, Int32) Overloads are: - Pointer(T)#+(offset : Int64) - Pointer(T)#+(other : Int) ``` ---
**Author:** ![yxhuvud](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/yxhuvud/32/27_2.png) [@yxhuvud](https://forum.crystal-lang.org/u/yxhuvud)\ **Post date:** [April 1, 2022, 6:09am UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/2 "2022-04-01T06:09:21Z")
It is hard to fully know as you don’t provide enough information (what is seg? what do you get if you add `--error-trace` when you compile?), but `seg.to_unsafe` likely returns a pointer, which define `[]` as ```auto def [](offset) (self + offset).value end ``` which would then trigger your error. What do you expect to happen if you add a range to a pointer? I’d expect the compilation error you are getting. ---
**Author:** ![hholst](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/hholst/32/1573_2.png) [@hholst](https://forum.crystal-lang.org/u/hholst)\ **Post date:** [April 1, 2022, 6:11am UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/3 "2022-04-01T06:11:58Z")
Well, a ranged pointer, i.e. an Array? I think the semantics makes sense, less so the operator overload. ---
**Author:** ![jzakiya](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/jzakiya/32/278_2.png) [@jzakiya](https://forum.crystal-lang.org/u/jzakiya)\ **Post date:** [April 1, 2022, 3:20pm UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/4 "2022-04-01T15:20:00Z")
`seg = Slice(UInt64).new(((ks - 1) >> s) + 1)` It seems odd the same line with bounds checking will compile but won’t compile when bounds checking turned off: `seg.to_unsafe[range]` Without knowing the motivations behind it, I would consider this a bug. ---
**Author:** ![asterite](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/asterite/32/60_2.png) [@asterite](https://forum.crystal-lang.org/u/asterite)\ **Post date:** [April 1, 2022, 4:13pm UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/5 "2022-04-01T16:13:07Z")
What do you expect the return type of asking a subrange of a pointer? That doesn’t make a lot of sense. With a pointer you can only point to a single place, not a range, exactly because the pointer has no bounds. In general I would avoid not doing bound checks. In all my tests avoiding this leads to the same performance, not a better performance. I don’t know why, maybe it’s branch prediction. ---
**Author:** ![jzakiya](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/jzakiya/32/278_2.png) [@jzakiya](https://forum.crystal-lang.org/u/jzakiya)\ **Post date:** [April 1, 2022, 5:34pm UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/6 "2022-04-01T17:34:19Z")
> [@asterite](#): > > What do you expect the return type of asking a subrange of a pointer? That doesn’t make a lot of sense. With a pointer you can only point to a single place, not a range, exactly because the pointer has no bounds. As you can see, I’m reading over a range in all cases. It compiles and runs fine as I initially show, but using `.to_unsafe` causes an error. As a user, this is surprising, because I would expect if it compiles with bounds checks it should compile with no bounds checks (I thought that’s what `.to_unsafe` does). I had|have no knowledge of the implementation details (pointers, etc), but I didn’t expect this behavior, as it worked everywhere else I used it. > [@asterite](#): > > In general I would avoid not doing bound checks. In all my tests avoiding this leads to the same performance, not a better performance. I don’t know why, maybe it’s branch prediction. I have code sections, particularly in inner loops, where doing read|writes using `.to_unsafe` produces substantial performance benefits. I normally don’t use it willy-nilly, but it DOES make a difference in certain places. (I first observed this in the Rust version of the same code when turning off bounds checking in it.) ---
**Author:** ![asterite](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/asterite/32/60_2.png) [@asterite](https://forum.crystal-lang.org/u/asterite)\ **Post date:** [April 1, 2022, 7:47pm UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/7 "2022-04-01T19:47:59Z")
To clarify, `to_unsafe` isn’t equivalent to “I want to do things without bound checks” Maybe that’s why your expectations don’t match what ends up happening. ---
**Author:** ![jzakiya](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/jzakiya/32/278_2.png) [@jzakiya](https://forum.crystal-lang.org/u/jzakiya)\ **Post date:** [April 1, 2022, 8:02pm UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/8 "2022-04-01T20:02:13Z")
OK, but I’m just trying to understand why the later code doesn’t compile. Doesn’t `to_unsafe` not do bounds checking? Isn’t that why it’s faster for the uses I cited? Or where, or for what other purposes, would you use it? Again, I’m just trying to understand what’s going on. ```auto def **to_unsafe** : [Pointer](https://crystal-lang.org/api/1.3.2/Pointer.html)(T) [#](https://crystal-lang.org/api/1.3.2/Array.html#to_unsafe%3APointer%28T%29-instance-method) Returns a pointer to the internal buffer where `self` 's elements are stored. This method is **unsafe** because it returns a pointer, and the pointed might eventually not be that of `self` if the array grows and its internal buffer is reallocated. ary = [1, 2, 3] ary.to_unsafe[0] # => 1 ``` ---
**Author:** ![asterite](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/asterite/32/60_2.png) [@asterite](https://forum.crystal-lang.org/u/asterite)\ **Post date:** [April 1, 2022, 8:23pm UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/9 "2022-04-01T20:23:24Z")
Right, `to_unsafe` returns a `Pointer`, as you mention. If you check the available methods of `Pointer` there’s no `#[](Range)` overload, simply because such overload doesn’t make sense. > Doesn’t `to_unsafe` not do bounds checking? No, `to_unsafe` returns a `Pointer`. > Isn’t that why it’s faster for the uses I cited? I don’t have a proof that it’s faster. > Or where, or for what other purposes, would you use it? I would never use it unless I need to interface with C. ---
**Author:** ![HertzDevil](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/hertzdevil/32/1023_2.png) [@HertzDevil](https://forum.crystal-lang.org/u/HertzDevil)\ **Post date:** [April 1, 2022, 8:43pm UTC](https://forum.crystal-lang.org/t/please-explain-this-error/4537/10 "2022-04-01T20:43:03Z")
Some methods in `Pointer` still have range semantics, e.g. `Pointer#map!` implements `Slice#map!` and not the other way round, which may be where the confusion comes from. I think we should drop those instance methods from `Pointer`.