ザ・コンパイルタイム・ポリモーフィズム

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[]namespace[] []poost_extension[][] {[]
    []struct[] []hello[][] { };[]
[]}[]

[]namespace[] []poost[][] {[]
    []namespace[] []detail[][] {[]
        []template[][]< [][]class[] []T[][] >[]

        []int[] []poost_hello[][]([][]T[] []const[][]& [][]x[][])[]

[]        {[]

            []return[] []poost_[][]([][]poost_extension[][]::[][]hello[][](), [][]x[][]);[]

[]        }[]
[]    }[]
    []template[][]< [][]class[] []T[][] >[]

    []int[] []hello[][]([][]T[] []const[][]& [][]x[][])[]

[]    {[]

        []using[] []namespace[] []detail[][];[]

        []return[] []poost_hello[][]([][]x[][]);[]

[]    }[]
[]}[]


[]// user code[]

[]//[]
[]struct[] []bar_base[]

[]{[]

    []int[] []m_i[][];[]

[]};[]


[]namespace[] []my[][] {[]
    []struct[] []foo[]

[]    {[]

    []private[][]:[]

        []int[] []i[][];[]
        []friend[] []int[] []poost_hello[][]([][]foo[] []const[][]& [][]self[][])[]

[]        {[]

            []return[] []self[][].[][]i[][];[]

[]        }[]

[]    };[]
    []struct[] []foo_[]

[]    {[]

        []int[] []i[][];[]

[]    };[]
        []inline[]

        []int[] []poost_hello[][]([][]foo_[] []const[][]& [][]f_[][])[]

[]        {[]

            []return[] []f_[][].[][]i[][];[]

[]        }[]
    []struct[] []bar[][] : [][]bar_base[]

[]    { };[]

[]}[]


[]namespace[] []your[][] {[]
    []template[][]< [][]class[] []Int[][] >[]

    []struct[] []foo[]

[]    {[]

    []private[][]:[]

        []Int[] []i[][];[]
        []friend[] []int[] []poost_hello[][]([][]foo[][]<[][]Int[][]> [][]const[][]& [][]self[][])[]

[]        {[]

            []Int[] []i[][]; [][]// (1)[]

            []return[] []self[][].[][]i[][];[]

[]        }[]

[]    };[]
    []struct[] []bar[][] : [][]bar_base[]

[]    { };[]

[]}[]


[]namespace[] []poost_extension[][] {[]
    []int[] []poost_[][]([][]hello[][], [][]bar_base[] []const[][]& [][]b[][])[]

[]    {[]

        []return[] []b[][].[][]m_i[][];[]

[]    }[]
[]}[]


[]int[] []main[][]()[]

[]{[]

    []poost[][]::[][]hello[][]([][]my[][]::[][]foo[][]());[]

    []poost[][]::[][]hello[][]([][]my[][]::[][]foo_[][]());[]

    []poost[][]::[][]hello[][]([][]your[][]::[][]foo[][]<[][]int[][]>());[]

    []poost[][]::[][]hello[][]([][]my[][]::[][]bar[][]());[]

    []poost[][]::[][]hello[][]([][]your[][]::[][]bar[][]());[]

[]}[]