XXX is parsed as a non-type, but instantiation yields a type 错误解决办法

最近在写程序的时候遇到了XXX is parsed as a non-type, but instantiation yields a type 的错误。程序上下文如下

template<typename FullKey,// base containe         typename PolicyTraits         >class container_with_policy{private:FullKey m_cache;//initialize firstpublic:struct container_element{typename PolicyTraits::policy_hook_type  policy_hook_;typename FullKey::iterator m_iterator;typedef typename PolicyTraits::template container_hook<container_element>::type PolicyHook;typename FullKey::iterator& operator&(){return m_iterator;};container_element& operator=(typename FullKey::iterator& it){m_iterator=it;return *this;}container_element(){};container_element(const typename FullKey::iterator& it):m_iterator(it){}};typedef typename FullKey::iterator iterator;typedef typename FullKey::const_iterator const_iterator;typedef typename PolicyTraits::template policy<FullKey,container_element,container_element::PolicyHook >::type policy_container;typedef typename FullKey::value_type base_element_type;///////////////////////////////////////////////////////////////////////////////container_with_policy();~container_with_policy();};

在

<pre name="code" class="cpp">container_element::PolicyHook >::type policy_container;

这一行里，编译器提示

error: dependent-name ‘ns3::ndn::container_with_policy<FullKey, PolicyTraits>::container_element:: PolicyHook’ is parsed as a non-type, but instantiation yields a type    container_element::PolicyHook >::type policy_container;

大意是说PolicyHook在编译器看来不是一个类型，但是却要它作为一个类型来使用。

但是没理由啊，

typedef typename PolicyTraits::template container_hook<container_element>::type PolicyHook;

这里已经把PolicyHook作为一个typedef了。

仔细看看，原来凡是typedef的内容，在使用的时候，要在前面加上typename， 否则编译器搞不清楚这个到底是你写错了还是真的有这个类型。

修改方法：

把container_element::PolicyHook前面添加typename，改成typename container_element::PolicyHook即可，结果错误消失了。