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Document how to cast / recast / change type of pointer inside Ptr docs #58641

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Document how to cast / recast / change type of pointer inside Ptr docs #58641

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If someone hacks on pointers they likely want to know that
freemin7 Jun 4, 2025
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strip access white space
freemin7 Jun 4, 2025
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Document how to cast / recast / change type of pointer inside Ptr docs
freemin7 Jun 4, 2025
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Elaborate on role of T in pointer type
freemin7 Jun 6, 2025
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Grammar tweak
freemin7 Jun 6, 2025
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remove excess space
freemin7 Jun 6, 2025
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modify warning header
freemin7 Jun 6, 2025
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10 changes: 9 additions & 1 deletion base/pointer.jl
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Expand Up @@ -3,8 +3,16 @@
"""
Ptr{T}

A memory address referring to data of type `T`. However, there is no guarantee that the
A memory address referring to data of type `T`. However, there is no guarantee that the
memory is actually valid, or that it actually represents data of the specified type.
Some operations like [`unsafe_load`](@ref) are only supported when T is an [`isbitstype`](@ref)
while [`unsafe_wrap`](@ref) also supports abstract types.
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unsafe_wrap is a good see also, but it is not an alternative to Ptr in many cases so this is not appropriate as a "while" clause.

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It is not meant as an alternative to pointers. unsafe_wrap takes a pointer as an argument and if it is a Ptr{T} where T is an abstract type it will still worked, while unsafe_load will throw an error if given such a pointer.

Use [`reinterpret`](@ref) to get a pointer with the same address but a different type.

!!! warning
When T is a [`Union`](@ref) type, the pointer does not contain the necessary information
to reconstruct what elements in the array had which type. Such pointers can't be use to
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@LilithHafner LilithHafner Jun 6, 2025 •
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Pointers are not just for arrays. For example, you can get a pointer to a field in an mutable struct.

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s/immutable/mutable I assume

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@freemin7 freemin7 Jun 6, 2025 •
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"what data type it pointed to." would that be better?

retrieve any Julia objects.
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It's still possible to retrieve an object, you just have to know what type you're looking for.

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Well then you would manually de-unionize the pointer and it no longer the "same" pointer.

I will think of a different phrasing.

"""
Ptr

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