Push! to reshape-d vector unshares data with parent array

Internals & Design
9

This does seem like an intentional design choice because reshapeing Arrays does not use the wrapper ReshapedArray. However, consider these counterexamples to the two points of your understanding:

  • The shared data error is not thrown if you made views or forced a ReshapedArray, even with the much more disruptive insert!. The flags.how protection might be limited to Arrays.
julia> x = zeros(2); y = reshape(x, 1, 2)
1×2 Matrix{Float64}:
 0.0  0.0

julia> push!(x, 7.0)
ERROR: cannot resize array with shared data

julia> x = zeros(2); y = view(x, 1:1)
1-element view(::Vector{Float64}, 1:1) with eltype Float64:
 0.0

julia> push!(x, 7.0); insert!(x, 1, 1.0); y # y is corrupted!
1-element view(::Vector{Float64}, 1:1) with eltype Float64:
 1.0

julia> x = zeros(2); y = Base.ReshapedArray(x, (1,2), ())
1×2 reshape(::Vector{Float64}, 1, 2) with eltype Float64:
 0.0  0.0

julia> push!(x, 7.0); insert!(x, 1, 1.0); y # y is corrupted!
1×2 reshape(::Vector{Float64}, 1, 2) with eltype Float64:
 1.0  0.0
  • When you reshape an Array to the same shape, you just reuse the instance, the ultimate sharing. Of course, push! does not reallocate. It could be a hassle to check whether reshape made a separate instance or not to keep the variables’ behavior consistent during mutations.
julia> x = zeros(2); y = reshape(x, 2);

julia> x === y
true

julia> push!(y, 7.0); x === y # x seems corrupted if you didn't check
true