如何在模板元编程中使用"默认"值

Question

我面临以下问题:

我有一些通用容器,可以对类型进行一些操作.为简单起见,操作在请求时是线程安全的.并且,要求表示容器中的类型具有typedef std::true_type needs_thread_safety;.

struct thread_safe_item {
    typedef std::true_type needs_thread_safety;
    /* */ 
};

struct thread_unsafe_item {
    typedef std::false_type needs_thread_safety;
    /* */
};
template<typename TItem> container {
    /* some algorithms, that are std::enable_if selected, according to needs_thread_safety */
};

但是,我希望needs_thread_safety选择加入,而不需要定义(=默认false_type).我试过以下:

    struct thread_unsafe_item {
        /* */ 
    };

template<typename TItem>
struct thread_safety_selector
{
    template<typename T>
    struct has_defined_thread_safety
    {
        typedef char yes[1];
        typedef char no[2];

        template <typename C> static yes& test(typename C::needs_thread_safety*);
        template <typename> static no& test(...);

        static const bool value = sizeof(test<T>(0)) == sizeof(yes);
    };

    typedef 
        typename std::conditional<
            has_defined_thread_safety<TItem>::value,
            typename TItem::needs_thread_safety,
            std::false_type
        >::type needs_thread_safety;
};
....
struct <typename TItem> container {
    /* changed all TItem::needs_thread_safety selectors to thread_safety_selector<TItem>::needs_thread_safety */
};

但显然没有懒惰评估,因为错误是error C2039: 'needs_thread_safety' : is not a member of 'thread_unsafe_item'.

如何为未指定的参数实现默认值？

它是出于教育目的,所以我不需要不同的方法来解决这个问题.

谢谢!

Answer 1

你无法使用std::conditional它,因为它总是会解析它的所有参数.您可以创建自己的谓词:

template <bool, class>
struct get_thread_safety;

template <class T>
struct get_thread_safety<true, T>
{
  typedef typename T::needs_thread_safety type;
};

template <class T>
struct get_thread_safety<false, T>
{
  typedef std::false_type type;
};

// Used like this:

typedef 
    typename get_thread_safety<
        has_defined_thread_safety<TItem>::value,
        TItem
    >::type needs_thread_safety;