边界检查

宏 @boundscheck(…) 把代码块标记为要执行边界检查。但当这些代码块被被宏 @inbounds(…) 标记的代码包裹时，它们可能会被编译器移除。仅当@boundscheck(…) 代码块被调用函数包裹时，编译器会移除它们。比如你可能这样写的 sum 方法：

使用自定义的类数组类型 MyArray，我们有：

当 getindex 被 sum 包裹时，对 checkbounds(A,i) 的调用会被忽略。如果存在多层包裹，最多只有一个 @boundscheck 被忽略。这个规则用来防止将来代码被改变时潜在的多余忽略。

There may be certain scenarios where for code-organization reasons you want more than one layer between the @inbounds and @boundscheck declarations. For instance, the default getindex methods have the chain calls getindex(IndexStyle(A), A, i) calls _getindex(::IndexLinear, A, i).

The overall hierarchy is:

so checkindex checks a single dimension. All of these functions, including the unexported checkbounds_indices have docstrings accessible with ? .

If you have to customize bounds checking for a specific array type, you should specialize checkbounds(Bool, A, I…). However, in most cases you should be able to rely on checkbounds_indices as long as you supply useful axes for your array type.

If you have novel index types, first consider specializing checkindex, which handles a single index for a particular dimension of an array. If you have a custom multidimensional index type (similar to CartesianIndex), then you may have to consider specializing checkbounds_indices.

Note this hierarchy has been designed to reduce the likelihood of method ambiguities. We try to make checkbounds the place to specialize on array type, and try to avoid specializations on index types; conversely, is intended to be specialized only on index type (especially, the last argument).