C++ template class inherit

Question

how to define a template class inherit from template class ?

I want to wrap std::queue and std::priority_queue to a base class. In my case is LooperQueue. I use StdQueue in this way auto queue = new StdQueue<LooperMessage *>().

my class define compiler complain

error log:

  In file included from /Users/rqg/ASProjects/PboTest/muses/src/main/cpp/Painter.cpp:10:
  /Users/rqg/ASProjects/PboTest/muses/src/main/cpp/util/StdQueue.h:14:5: error: unknown type name 'size_type'; did you mean 'size_t'?
      size_type size() override;
      ^~~~~~~~~
      size_t
  /Users/rqg/Library/Android/sdk/ndk-bundle/toolchains/llvm/prebuilt/darwin-x86_64/lib64/clang/5.0.300080/include/stddef.h:62:23: note: 'size_t' declared here
  typedef __SIZE_TYPE__ size_t;
                        ^
  In file included from /Users/rqg/ASProjects/PboTest/muses/src/main/cpp/Painter.cpp:10:
  /Users/rqg/ASProjects/PboTest/muses/src/main/cpp/util/StdQueue.h:16:5: error: unknown type name 'reference'
      reference front() override;
      ^
  /Users/rqg/ASProjects/PboTest/muses/src/main/cpp/util/StdQueue.h:20:21: error: unknown type name 'value_type'; did you mean 'ARect::value_type'?
      void push(const value_type &x) override;
                      ^~~~~~~~~~
                      ARect::value_type
  /Users/rqg/Library/Android/sdk/ndk-bundle/sysroot/usr/include/android/rect.h:44:21: note: 'ARect::value_type' declared here
      typedef int32_t value_type;

code:

#ifndef PBOTEST_LOOPERQUEUE_H
#define PBOTEST_LOOPERQUEUE_H

#include <queue>
#include <cstdlib>

template<typename Tp, typename Sequence = std::deque<Tp> >
class LooperQueue {
  public:

    typedef typename Sequence::value_type                value_type;
    typedef typename Sequence::reference                 reference;
    typedef typename Sequence::const_reference           const_reference;
    typedef typename Sequence::size_type                 size_type;
    typedef          Sequence                            container_type;


    virtual size_type size()  = 0;

    virtual reference front() = 0;

    virtual void pop()= 0;

    virtual void push(const value_type &x) = 0;
};

#endif //PBOTEST_LOOPERQUEUE_H

#ifndef PBOTEST_STDQUEUE_H
#define PBOTEST_STDQUEUE_H

#include "LooperQueue.h"

template<typename Tp, typename Sequence = std::deque<Tp> >
class StdQueue : public LooperQueue<Tp, Sequence> {
  public:
    size_type size() override;

    reference front() override;

    void pop() override;

    void push(const value_type &x) override;
    
  private:
    std::queue<Tp, Sequence> mQueue;
};

#endif //PBOTEST_STDQUEUE_H

Answer 1

The issue here is that the base class LooperQueue is a dependent base class, which depends on the template parameter Tp and Sequence, then its complete type can't be determined without knowing the template arguments. Standard C++ says that nondependent names (like size_type, reference and value_type) won't be looked up in dependent base classes.

To correct the code, it suffices to make the names qualified by the base class name; then these names can be looked up only at the time of instantiation, and at that time the exact base specialization that must be explored will be known. e.g.

template<typename _Tp, typename _Sequence = std::deque<_Tp> >
class StdQueue : public LooperQueue<_Tp, _Sequence> {
public:
    typename LooperQueue<_Tp, _Sequence>::::size_type size() override;
    typename LooperQueue<_Tp, _Sequence>::reference front() override;
    void pop() override;
    void push(const typename LooperQueue<_Tp, _Sequence>::value_type &__x) override;
private:
    std::queue<_Tp, _Sequence> mQueue;
};