c++ - 与模板类 : Compile error 成为 friend

Question

我正在尝试使用 Pointer to Implementation Idiom 来隐藏我正在使用 Concurrency::unbounded_buffer(来自 VC++ 2010)的事实。问题是，我正在使用模板进行操作，但遇到了编译错误。这是代码:

阻塞队列.h

#pragma once

namespace DRA{
namespace CommonCpp{
    //Forward declaration
    template <class Element> class BlockingQueueImpl;

    template <class Element>
    class BlockingQueue{
        BlockingQueueImpl<Element>* m_pImpl;
    //Forbid copy and assignment
        BlockingQueue( const BlockingQueue& );
        BlockingQueue& operator=( const BlockingQueue& );
    public:
        BlockingQueue();
        ~BlockingQueue();
        void Push( Element newElement );
        Element Pop();
    };
}
}

阻塞队列.cpp

#include "BlockingQueue.h"
#include "BlockingQueueImpl.h"

using namespace DRA::CommonCpp;

BlockingQueue<class Element>::BlockingQueue():
    m_pImpl( new BlockingQueueImpl<Element>() ){
}

BlockingQueue<class Element>::~BlockingQueue(){
    //Let the implementation's destructor handle pending Pops
    delete m_pImpl;
}

template<class Element>
void BlockingQueue<Element>::Push( Element newElement ){
    m_pImpl->Push( newElement );
}

template<class Element>
Element BlockingQueue<Element>::Pop(){
    return m_Impl->Pop();
}

BlockingQueueImpl.h

#pragma once

#include <agents.h> //This is VS2010 Exclusive (Concurrency Runtime)

namespace DRA{ namespace CommonCpp{ template <class Element> class BlockingQueue; } }

namespace DRA{
namespace CommonCpp{
template <class Element>
    class BlockingQueueImpl{
        Concurrency::unbounded_buffer<Element>  m_Queue;
    //Only friends can use this class
        friend class BlockingQueue<Element>;
        BlockingQueueImpl();
        ~BlockingQueueImpl();
    //Forbid copy and assignment
        BlockingQueueImpl( const BlockingQueueImpl& );
        BlockingQueueImpl& operator=( const BlockingQueueImpl& );
    //Members
        void Push( Element newElement );
        Element Pop();
};
}
}

BlockingQueueImpl.cpp

#include "BlockingQueueImpl.h"

using namespace DRA::CommonCpp;

BlockingQueueImpl<class Element>::BlockingQueueImpl(){
}

BlockingQueueImpl<class Element>::~BlockingQueueImpl(){
}

template<class Element>
void BlockingQueueImpl<class Element>::Push( Element newElement ){
    send( m_Queue, newElement );
}

template<class Element>
Element BlockingQueueImpl<class Element>::Pop(){
    return receive( m_Queue );
}

错误是:

BlockingQueueImpl.cpp(12): error C2649: 'typename' : is not a 'class'

我尝试在友元声明中的模板参数中添加类，但没有成功

Answer 1

对现有答案的补充:

error C2649: 'typename' : is not a 'class'

我有相同的错误消息，但我的代码看起来略有不同:

template< class T, class TNodeType = SimpleTreeNode<T> ...>
class Tree {
    friend class TNodeType; // *** Turns out: WRONG friend declaration

同样适用于 existing answer : 省略 class 关键字(编译器已经知道这是我假设的类型):

template< class T, class TNodeType = SimpleTreeNode<T> ...>
class Tree {
    friend TNodeType; // *** OK: note missing `class`keyword