`nunsafe_convert`对于``vector {u}`对于索引而不是uinerange {u}

Question

我注意到用索引向量而不是unitrange给出的索引处理索引时，我注意到了以下错误。

c = rand(10)
c1 = view(c, [1, 3])
c2 = view(c1, 2:2)
Base.unsafe_convert(Ptr{Float64}, c2)

现在返回

ERROR: conversion to pointer not defined for SubArray{Float64, 1, Vector{Float64}, Tuple{Vector{Int64}}, false}
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] unsafe_convert(#unused#::Type{Ptr{Float64}}, a::SubArray{Float64, 1, Vector{Float64}, Tuple{Vector{Int64}}, false})
   @ Base ./pointer.jl:67
 [3] top-level scope
   @ REPL[6]:1

Answer 1

它们都会给我带来相同的错误（即，对于不连续的视图，明确不允许使用指针 - 由false> false键入参数指示）：

julia> Base.unsafe_convert(Ptr{Float64}, c1)
ERROR: conversion to pointer not defined for SubArray{Float64, 1, Vector{Float64}, Tuple{Vector{Int64}}, false}
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] unsafe_convert(#unused#::Type{Ptr{Float64}}, a::SubArray{Float64, 1, Vector{Float64}, Tuple{Vector{Int64}}, false})
   @ Base ./pointer.jl:67
 [3] top-level scope
   @ REPL[8]:1

julia> Base.unsafe_convert(Ptr{Float64}, c2)
ERROR: conversion to pointer not defined for SubArray{Float64, 1, Vector{Float64}, Tuple{Vector{Int64}}, false}
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] unsafe_convert(#unused#::Type{Ptr{Float64}}, a::SubArray{Float64, 1, Vector{Float64}, Tuple{Vector{Int64}}, false})
   @ Base ./pointer.jl:67
 [3] top-level scope
   @ REPL[9]:1

这是有道理的 - 连续块并不是真正有意义的（有关选定索引的信息丢失了）。

但是，您可以为一个范围获得一个指针：

julia> Base.unsafe_convert(Ptr{Float64}, view(c, 2:2))
Ptr{Float64} @0x00007f1feaa58b88