Question

假设我使用自己的自定义show方法来创建自己的自定义矢量类型：

struct MyVector{T} <: AbstractVector{T}
    v::Vector{T}
end

function Base.show(io::IO, v::MyVector{T}) where {T}
    println(io, "My custom vector with eltype $T with elements")
    for i in eachindex(v)
        println(io, "  ", v.v[i])
    end
end

如果我尝试在REPL上制造这些对象之一，则会遇到与我从未打算调用的函数有关的意外错误：

julia> MyVector([1, 2, 3])
Error showing value of type MyVector{Int64}:
ERROR: MethodError: no method matching size(::MyVector{Int64})
Closest candidates are:
  size(::AbstractArray{T,N}, ::Any) where {T, N} at abstractarray.jl:38
  size(::BitArray{1}) at bitarray.jl:77
  size(::BitArray{1}, ::Integer) at bitarray.jl:81
  ...
Stacktrace:
 [1] axes at ./abstractarray.jl:75 [inlined]
 [2] summary(::IOContext{REPL.Terminals.TTYTerminal}, ::MyVector{Int64}) at ./show.jl:1877
 [3] show(::IOContext{REPL.Terminals.TTYTerminal}, ::MIME{Symbol("text/plain")}, ::MyVector{Int64}) at ./arrayshow.jl:316
 [4] display(::REPL.REPLDisplay, ::MIME{Symbol("text/plain")}, ::Any) at /Users/mason/julia/usr/share/julia/stdlib/v1.3/REPL/src/REPL.jl:132
 [5] display(::REPL.REPLDisplay, ::Any) at /Users/mason/julia/usr/share/julia/stdlib/v1.3/REPL/src/REPL.jl:136
 [6] display(::Any) at ./multimedia.jl:323
 ...

好吧，无论如何，我都会实施Base.size，这样我就不用理会了：

julia> Base.size(v::MyVector) = size(v.v)

julia> MyVector([1, 2, 3])
3-element MyVector{Int64}:
Error showing value of type MyVector{Int64}:
ERROR: getindex not defined for MyVector{Int64}
Stacktrace:
 [1] error(::String, ::Type) at ./error.jl:42
 [2] error_if_canonical_getindex(::IndexCartesian, ::MyVector{Int64}, ::Int64) at ./abstractarray.jl:991
 [3] _getindex at ./abstractarray.jl:980 [inlined]
 [4] getindex at ./abstractarray.jl:981 [inlined]
 [5] isassigned(::MyVector{Int64}, ::Int64, ::Int64) at ./abstractarray.jl:405
 [6] alignment(::IOContext{REPL.Terminals.TTYTerminal}, ::MyVector{Int64}, ::UnitRange{Int64}, ::UnitRange{Int64}, ::Int64, ::Int64, ::Int64) at ./arrayshow.jl:67
 [7] print_matrix(::IOContext{REPL.Terminals.TTYTerminal}, ::MyVector{Int64}, ::String, ::String, ::String, ::String, ::String, ::String, ::Int64, ::Int64) at ./arrayshow.jl:186
 [8] print_matrix at ./arrayshow.jl:159 [inlined]
 [9] print_array at ./arrayshow.jl:308 [inlined]
 [10] show(::IOContext{REPL.Terminals.TTYTerminal}, ::MIME{Symbol("text/plain")}, ::MyVector{Int64}) at ./arrayshow.jl:345
 [11] display(::REPL.REPLDisplay, ::MIME{Symbol("text/plain")}, ::Any) at /Users/mason/julia/usr/share/julia/stdlib/v1.3/REPL/src/REPL.jl:132
 [12] display(::REPL.REPLDisplay, ::Any) at /Users/mason/julia/usr/share/julia/stdlib/v1.3/REPL/src/REPL.jl:136
 [13] display(::Any) at ./multimedia.jl:323
 ...

嗯，现在要getindex

julia> Base.getindex(v::MyVector, args...) = getindex(v.v, args...)

julia> MyVector([1, 2, 3])
3-element MyVector{Int64}:
 1
 2
 3

什么？那不是我告诉它的打印格式！这是怎么回事？

Answer 1

问题在于，在julia中，Base为Base.show(io::IO ::MIME"text/plain", X::AbstractArray)定义了一种方法Base.show(io::IO, v::MyVector)，实际上比display更具体。 This section of the julia manual describes the sort of custom printing that AbstractArray uses。因此，如果我们要使用自定义的show方法，则需要这样做

julia> function Base.show(io::IO, ::MIME"text/plain", v::MyVector{T}) where {T}
           println(io, "My custom vector with eltype $T and elements")
           for i in eachindex(v)
               println(io, "  ", v.v[i])
           end
       end

julia> MyVector([1, 2, 3])
My custom vector with eltype Int64 and elements
  1
  2
  3

另请参阅：https://discourse.julialang.org/t/extending-base-show-for-array-of-types/31289

如何重载Base.show自定义数组类型？

1 个答案: