c – boost.proto在构建表达式树之前检测无效终端

我正在玩Boost.Proto,主要是为了好玩,看看将来我是否可以在我自己的项目中使用它.那说,
可能是这个库的大多数初学者,我玩过“懒惰矢量”示例的修改版本,但使用变换而不是上下文来执行评估.向量定义如下(好吧,我知道,’vector’对于在全局命名空间范围内定义的东西不是一个好名字……)

template <std::size_t D,class T>
class vector { 
    T data_[D];
    enum { dimension = D };
    // Constructors,destructors...
};

// expression wrapper
template <class> class vector_expr;

它是关于维度和数据类型的模板,boost :: array的种类(我没有使用它,因为我想重载operator =来接受表达式树,就像通常在这类事情中所做的那样).我使用proto手册中的代码定义了标量

// scalar = everything convertible to double
struct scalar_terminal :
    proto::terminal<proto::convertible_to <double> >
{};

// vector = everything for which the is_vector returns true_
template <class T> struct is_vector : mpl::false_ {};
template <std::size_t D,class T> struct is_vector <vector <D,T> > : mpl::true_ {};

struct vector_terminal :
    proto::and_ <
       proto::terminal<_>,proto::if_<is_vector<proto::_value>()>
  >
{};

// domain   
struct vector_domain
    : proto::domain <proto::generator <vector_expr> >
{};

// expression wrapper
template <class Expr>
struct vector_expr : proto::extends <Expr,vector_expr <Expr>,vector_domain>
{
    typedef proto::extends <Expr,vector_domain> base_type;

    // Construct from expression (enough to compile)
    vector_expr (Expr const &e) : base_type (e) {}
};

// Bring in operators
BOOST_PROTO_DEFINE_OPERATORS(is_vector,vector_domain)

现在,我想要做的第一件事是：检查表达式中的所有向量终端是否都相同
维D.我最终得到了以下工作代码

// a meta-function that returns the vector dimension
template <class T>
struct vector_dim
{
    typedef mpl::int_ <T::dimension> type;
};

// a meta-function that combines dimensions from subtrees. int<-1> means
// that sub-trees store vectors of differing static dimension. No good.
template <class D1,class D2>
struct dim_combine
{
   typedef mpl::int_ < -1 > type;
};

// ok,dimensions are the same,propagate up the value
template <class D>
struct dim_combine <D,D>
{
   typedef D type;
};

// 0 is used to mark scalars. It is ok to mix vectors and scalars
// but propagate up the vector dimension only. This is for vector
// on the left and scalar on the right.
template <class D>
struct dim_combine <D,mpl::int_ <0> >
{
   typedef D type;
};

// this is for scalar on the left,vector to the right of some 
// binary operator.
template <class D>
struct dim_combine <mpl::int_ <0>,D>
{
   typedef D type;
};

// need this too to avoid ambiguity between the two specializations
// above when D is int_ <0>. Even if this combination should never
// happen
template <>
struct dim_combine <mpl::int_ <0>,mpl::int_<0> >
{
   typedef mpl::int_<0> type;
};


// A transform that check that all arrays have the same dimension
struct vec_dim_check
    : proto::or_ <
        proto::when <
            vector_terminal,vector_dim<proto::_value>()
        >,proto::when <
            scalar_terminal,boost::mpl::int_<0>()
        >,proto::when <
            proto::nary_expr<_,proto::vararg<_> >,proto::fold<_,boost::mpl::int_<0>(),dim_combine<vec_dim_check,proto::_state>()>
        >
    >
{};

template <class E>
void check_dim (E const&)
{
    typedef typename boost::result_of<vec_dim_check(E)>::type type;
    BOOST_ASSERT(type::value == 3);
}

int main (int,char**)
{
    vector <3,double> a,b,c;
    check_dim (2*a+b/c);
    return 0;
}

问题是：由于数组的维度已经在表达式中编码,因此它应该
可以在编译时检测到无效组合.应该甚至可以避免
首先创建树.这是如何实现的？

在此先感谢,最好的问候

struct vector_grammar_untyped
  : proto::or_<
        scalar_terminal,vector_terminal,proto::nary_expr<proto::_,proto::vararg<vector_grammar_untyped> >
    >
{};

struct vector_grammar
  : proto::and_<
        vector_grammar_untyped,proto::if_<mpl::not_equal_to< mpl::int_<-1>,vec_dim_check >()>
    >
{};

struct vector_domain
    : proto::domain <proto::generator <vector_expr>,vector_grammar >
{};