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Augment convert's documentation with a note on aliasing #17519

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Jul 26, 2016
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Augment convert's documentation with a note on aliasing #17519

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32 changes: 32 additions & 0 deletions base/docs/helpdb/Base.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -8129,6 +8129,38 @@ julia> convert(Rational{Int32}, x)
julia> convert(Rational{Int64}, x)
6004799503160661//18014398509481984
```

If `T` is a collection type and `x` a collection, the result of `convert(T, x)` may alias
`x`.
```jldoctest
julia> x = Int[1,2,3];

julia> y = convert(Vector{Int}, x);

julia> y === x
true
```
Similarly, if `T` is a composite type and `x` a related instance, the result of
`convert(T, x)` may alias part or all of `x`.
```jldoctest
julia> x = speye(5);

julia> typeof(x)
SparseMatrixCSC{Float64,Int64}

julia> y = convert(SparseMatrixCSC{Float64,Int64}, x);

julia> z = convert(SparseMatrixCSC{Float32,Int64}, y);

julia> y === x
true

julia> z === x
false

julia> z.colptr === x.colptr
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this partial aliasing seems really undesirable to me.

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Relevant discussion in #12441, #8759, and #9575. This section should be augmented with a pointer to either the copy!(similar(... idiom or a future copy(type, inst) interface, whichever gains favor in #12441.

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sure, all aliasing is a bit scary when mutation is widespread, but partial aliasing of only some fields is even moreso

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Agreed, it's disquieting and warrants care.

true
```
"""
convert

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33 changes: 33 additions & 0 deletions doc/stdlib/base.rst
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Expand Up @@ -387,6 +387,39 @@ All Objects
julia> convert(Rational{Int64}, x)
6004799503160661//18014398509481984

If ``T`` is a collection type and ``x`` a collection, the result of ``convert(T, x)`` may alias ``x``\ .

.. doctest::

julia> x = Int[1,2,3];

julia> y = convert(Vector{Int}, x);

julia> y === x
true

Similarly, if ``T`` is a composite type and ``x`` a related instance, the result of ``convert(T, x)`` may alias part or all of ``x``\ .

.. doctest::

julia> x = speye(5);

julia> typeof(x)
SparseMatrixCSC{Float64,Int64}

julia> y = convert(SparseMatrixCSC{Float64,Int64}, x);

julia> z = convert(SparseMatrixCSC{Float32,Int64}, y);

julia> y === x
true

julia> z === x
false

julia> z.colptr === x.colptr
true

.. function:: promote(xs...)

.. Docstring generated from Julia source
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