模板依赖构造函数参数长度 [英] template dependent constructor argument lengths

问题描述

我尝试创建一个通用但仍然有效的多维点类。

I try to make a generic, but still efficient multi dimension Point class.

维度 枚举

enum Dimension : std::size_t { _2D = 2, _3D = 3 };

和点 class

template <typename T, Dimension D>
class Point : public std::array<T, D>
{
    public:
        T&        at(size_t idx)       { return std::array<T,D>::at(idx); };
        const T&  at(size_t idx) const { return std::array<T,D>::at(idx); };
        Dimension dim()          const { return D; }
        ...
};

我想创建nices构造函数，所以我添加了（在我的类定义之外）

I would like to create nices constructors so I added (outside my class definition)

template <typename T>
Point<T,_2D>::Point(T x, T y)      { at(0) = x; at(1) = y;            }
template <typename T>
Point<T,_3D>::Point(T x, T y, T z) { at(0) = x; at(1) = y; at(2) = z; }

但是我不能使用thoses。编译器告诉我只注册默认（空）和复制构造函数。

But still I cannot use thoses. The compiler tells me only default (empty) and copy constructors are registered.

定义构造函数的参数列表长度取决于我的 Dimension 模板？

How do I define constructors with arguments list length being dependant on my Dimension template ?

推荐答案

有几种方法可以实现这一点。在您的情况下，最容易使用 std :: enable_if ：

There are a few ways to accomplish this. In your case, it might be easiest to use std::enable_if:

template <typename T, Dimension D>
class Point : public std::array<T, D>
{
    public:
        template <
            Dimension D2=D,
            typename = typename std::enable_if<D2==_2D>::type
        >
        Point(T x,T y);

        template <
            Dimension D2=D,
            typename = typename std::enable_if<D2==_3D>::type
        >
        Point(T x,T y,T z);

        T&        at(size_t idx)       { return std::array<T,D>::at(idx); };
        const T&  at(size_t idx) const { return std::array<T,D>::at(idx); };
        Dimension dim()          const { return D; }
        ...
};

另一种选择是将其分成多个类并使用专门化：

Another option would be to break this into multiple classes and use specialization:

template <typename T, Dimension D>
class PointBase : public std::array<T, D>
{
    public:
        T&        at(size_t idx)       { return std::array<T,D>::at(idx); };
        const T&  at(size_t idx) const { return std::array<T,D>::at(idx); };
        Dimension dim()          const { return D; }
        ...
};

template <typename T, Dimension D> class Point;

template <typename T>
class Point<T,_2D> : public PointBase<T,_2D> {
    public:
        Point(T x,T y);
};

template <typename T>
class Point<T,_3D> : public PointBase<T,_3D> {
    public:
        Point(T x,T y,T z);
};

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