Placeholder Expressionの罠

C++ Boost.MPL
  • 型がMetafunctionであるかないかでapplyの動作が変わる
  • PlaceholderExpression自体がill-formedになり得る
  • 型でないパラメータを取る場合機能しない
  • デフォルトでは、template parameterの数が５個より多いと動作しない
    • boost::tupleなんかは駄目である
  • GCC3.4.4では、バグのためにMetafunctionにDefaultConstructibleを要求する
    • Boost1.34では対策されているが、それでも起こる場合、PlaceholderExpressionの継承を避けるといいようだ
  • これらを回避策するには、自分でMetafunctionClassを作ればいい

[]#include[] []<boost/mpl/apply.hpp>[]

[]#include[] []<boost/mpl/placeholders.hpp>[]

[]#include[] []<boost/type_traits/add_reference.hpp>[]




[]template[][]< [][]class[] []T[][], [][]class[] []U[][] >[]

[]struct[] []klass[]

[]{ };[]
[]template[][]< [][]class[] []T[][], [][]class[] []U[][] >[]

[]struct[] []has_type_self[]

[]{[]

    []typedef[] []has_type_self[] []type[][];[]

[]};[]
[]template[][]< [][]class[] []T[][], [][]class[] []U[][] >[]

[]struct[] []has_type_int[]

[]{[]

    []typedef[] []int[] []type[][];[]

[]};[]
[]template[][]< [][]class[] []T[][], [][]int[] []U[][] = [][]1[][] >[]

[]struct[] []non_type_parameter[]

[]{ };[]
[]template[][]< [][]class[] []T[][], [][]class[] []U[][] = [][]typename[] []T[][]::[][]hello[][] >[]

[]struct[] []hello[]

[]{ };[]
[]template[][]< [][]class[] []T0[][], [][]class[] []T1[][], [][]class[] []T2[][]=[][]int[][], [][]class[] []T3[][]=[][]int[][], [][]class[] []T4[][]=[][]int[][], [][]class[] []T5[][]=[][]int[][] >[]

[]struct[] []too_many_parameters[]

[]{ };[]
[]template[][]< [][]class[] []T[][], [][]class[] []U[][] >[]

[]struct[] []non_default_constructible[]

[]{[]

    []typedef[] []non_default_constructible[] []type[][];[]
    []explicit[] []non_default_constructible[][]([][]int[][])[]

[]    { }[]

[]};[]
[]namespace[] []mpl[][] = [][]boost[][]::[][]mpl[][];[]
[]void[] []test[][]()[]

[]{[]

[]    {[]

[]        ::[][]klass[][]<[][]int[][]&, [][]double[][]> [][]x[][] =[]

            []mpl[][]::[][]apply[][]<[]

[]                ::[][]klass[][]<[]

                    []boost[][]::[][]add_reference[][]<[][]mpl[][]::[][]_1[][]>, [][]mpl[][]::[][]_2[]

[]                >,[]

                []int[][], [][]double[]

[]            >::[][]type[][]();[]

[]    }[]

[]    {[]

[]        ::[][]has_type_self[][]<[][]int[][]&, [][]double[][]> [][]x[][] =[]

            []mpl[][]::[][]apply[][]<[]

[]                ::[][]has_type_self[][]<[]

                    []boost[][]::[][]add_reference[][]<[][]mpl[][]::[][]_1[][]>, [][]mpl[][]::[][]_2[]

[]                >,[]

                []int[][], [][]double[]

[]            >::[][]type[][]();[]

[]    }[]

[]    {[]

        []int[] []x[][] =[]

            []mpl[][]::[][]apply[][]<[]

[]                ::[][]has_type_int[][]<[]

                    []boost[][]::[][]add_reference[][]<[][]mpl[][]::[][]_1[][]>, [][]mpl[][]::[][]_2[]

[]                >,[]

                []int[][], [][]double[]

[]            >::[][]type[][]();[]

[]    }[]

[]    {[][]/*[]

[]        ::non_type_parameter<int&> error =[]

[]            mpl::apply<[]

[]                ::non_type_parameter<[]

[]                    boost::add_reference<mpl::_1>[]

[]                >,[]

[]                int[]

[]            >::type();[]

[]    */[][]}[]

[]    {[][]/*[]

[]        typedef ::hello<mpl::_1> error;[]

[]    */[][]}[]

[]    {[][]/*[]

[]        ::too_many_parameters<int&, double> error =[]

[]            mpl::apply<[]

[]                ::too_many_parameters<[]

[]                    boost::add_reference<mpl::_1>, mpl::_2[]

[]                >,[]

[]                int, double[]

[]            >::type();[]

[]    */[][]}[]

[]    {[]

[]        ::[][]non_default_constructible[][]<[][]int[][]&, [][]double[][]> [][]error_under_Boost_1_33[][] =[]

            []mpl[][]::[][]apply[][]<[]

[]                ::[][]non_default_constructible[][]<[]

                    []boost[][]::[][]add_reference[][]<[][]mpl[][]::[][]_1[][]>, [][]mpl[][]::[][]_2[]

[]                >,[]

                []int[][], [][]double[]

[]            >::[][]type[][]([][]1[][]);[]

[]    }[]

[]}[]