与模板类成为朋友：编译错误

Question

我正在尝试使用“实现指针”来掩盖我正在使用Concurrency :: unbounded_buffer（来自VC ++ 2010）的事实。问题是，我正在使用模板执行此操作，并陷入了编译错误。这是代码：

BlockingQueue.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();
    };
}
}

BlockingQueue.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'

我尝试将类添加到好友声明中的template参数中，但没有成功

Answer 1

除了现有答案：

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

我有相同的错误消息，但是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

与现有答案相同：忽略类关键字（编译器已经知道这是我假定的类型）：

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