具有非 void 返回类型的虚函数

Question

考虑以下继承和组合方案。

// 示例程序

#include <iostream>
#include <valarray>

using namespace std;

class TapProcessing
{
    public:
          TapProcessing(){};
          virtual ~TapProcessing(){};

          virtual void getWeights(valarray<double> &weights) {};
          virtual void getPrecisionWeights (valarray<double> &precWeights) {};
          virtual const uint &getTapIndex(const uint index) const
          {
              valarray<uint> a;
              a.resize(10);
              return a[index];
          }; //const {return 0;};
          virtual const uint &getTapIndexLow(const uint index) const
          {
              valarray<uint> a;
              a.resize(10);
              return a[index];
          }; //const {return 0;};;
          virtual const uint &getTapIndexHigh(const uint index) const
          {
              valarray<uint> a;
              a.resize(10);
              return a[index];
          }; //const {return 0;};

};

class StepPrecisionTapProcessing : public TapProcessing
{
    public:
        StepPrecisionTapProcessing() { _tapIndex.resize(10, 3); }
        ~StepPrecisionTapProcessing() {};

        void getWeights(valarray<double> &weights) { return weights.resize(10);}
        virtual const uint &getTapIndex(const uint index) const {return _tapIndex[index]; }

    private:
          valarray<uint> _tapIndex;

};

class HighPrecisionTapProcessing : public TapProcessing
{
    public:
        HighPrecisionTapProcessing()
        {
            _tapIndexLow.resize(10, 4);
            _tapIndexHigh.resize(10, 5);
        }
        ~HighPrecisionTapProcessing() {};

        void getPrecisionWeights (valarray<double> &precWeights) { return precWeights.resize(10); };
        virtual const uint &getTapIndexLow(const uint index) const {return _tapIndexLow[index]; }
        virtual const uint &getTapIndeHigh(const uint index) const {return _tapIndexHigh[index]; }

    private:
          valarray<uint> _tapIndexLow;
          valarray<uint> _tapIndexHigh;

};

class Generator
{
    public:
           Generator(bool isPrecision)
           {
               if (isPrecision) {_tp = new HighPrecisionTapProcessing();
               }
               else { _tp = new StepPrecisionTapProcessing(); }
           }

           ~Generator() { delete _tp; }

           const uint &getTapIndex(const uint index) const {return _tp->getTapIndex(index); }
           const uint &getTapIndexLow(const uint index) const {return _tp->getTapIndexLow(index); }
           const uint &getTapIndexHigh(const uint index) const {return _tp->getTapIndexHigh(index); }

    private:
           TapProcessing *_tp;
};



int main()
{
  Generator *G = new Generator(true);
  uint index = 5;

  cout<<"High tap index is = "<<G->getTapIndexHigh(index)<<endl;

  delete G;
  return 0;
}

当我运行 main 时，我得到以下输出，

高点指数 = 0

这里如果派生类中的getTapIndeHigh声明覆盖了基类中的声明，我们会在输出中看到值5而不是0。为什么派生类实现没有覆盖非类方法的基类实现无效的虚拟方法？

Answer 1

问题出在这几行...

      virtual uint getTapIndex(uint index) {};
      virtual uint getTapIndexLow(uint index) {};
      virtual uint getTapIndexHigh(uint index) {};

他们描述了一个返回uint的函数，提供了一个实现，但实际上并不返回一个值。

如果有一个合理的基础实现，那么最好提供它。但是在这里，您似乎想要一个pure virtual 方法，使您的课程成为abstract。您希望所有派生类都实现一个值。例如

      virtual uint getTapIndex(uint index) =0;  // require derived classes to implement
      virtual uint getTapIndexLow(uint index) { return 0 } // can be overridden, but defaults to returning zero.
      virtual uint getTapIndexHigh(uint index) = 0; // require derived classes to implement.