从模板类派生类的指针容器，以共同祖先作为模板参数。

Question

我有一个超级类，模型是 Measurement 和两个派生类。PointCloudMeasurement 和 ImageMeasurement.

又是一个模板化的超级类。Handler<MeasT> 定义了处理测量的接口。

两个处理器类。PointCloudHandler 和 ImageHandler 专攻 Handler 类，并将相应的测量类型作为模板参数。

我想用一个指向超级类的指针容器来跟踪所有的处理程序，并根据手头处理程序的派生类型调用相应的方法。

下面是代码(联系):

// Example program
#include <iostream>
#include <string>
#include <vector>
#include <memory>

class Measurement {
public:
  uint64_t time;
  virtual ~Measurement() {}
};

class PointCloudMeasurement : public Measurement {
public:
  uint64_t n_points = 64*1024;

};

class ImageMeasurement : public Measurement {
public:
  uint64_t n_pixels = 640*480;

};

template<class MeasT = Measurement>
class Handler {
public:
  virtual void addMeasurement(MeasT& m) = 0;
};

class PointCloudHandler : public Handler<PointCloudMeasurement> {
public:
  void addMeasurement(PointCloudMeasurement& m) override {
    std::cout << m.n_points << std::endl;
  }
};

class ImageHandler : public Handler<ImageMeasurement> {
  void addMeasurement(ImageMeasurement& m) override {
    std::cout << m.n_pixels << std::endl;

  }

};


int main(void){
  std::vector<std::unique_ptr<Handler<Measurement>>> handlers;

  handlers.push_back(std::make_unique<PointCloudHandler>());
  handlers.push_back(std::make_unique<ImageHandler>());

  PointCloudMeasurement pcm;
  ImageMeasurement im;

  handlers[0]->addMeasurement(pcm);
  handlers[1]->addMeasurement(im);
}

这给了我:

In function 'int main()':
50:59: error: no matching function for call to 'std::vector<std::unique_ptr<Handler<Measurement> > >::push_back(std::_MakeUniq<PointCloudHandler>::__single_object)'
50:59: note: candidates are:
In file included from /usr/include/c++/4.9/vector:64:0,
                 from 4:
/usr/include/c++/4.9/bits/stl_vector.h:913:7: note: void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::unique_ptr<Handler<Measurement> >; _Alloc = std::allocator<std::unique_ptr<Handler<Measurement> > >; std::vector<_Tp, _Alloc>::value_type = std::unique_ptr<Handler<Measurement> >]
       push_back(const value_type& __x)
       ^
/usr/include/c++/4.9/bits/stl_vector.h:913:7: note:   no known conversion for argument 1 from 'std::_MakeUniq<PointCloudHandler>::__single_object {aka std::unique_ptr<PointCloudHandler, std::default_delete<PointCloudHandler> >}' to 'const value_type& {aka const std::unique_ptr<Handler<Measurement> >&}'
/usr/include/c++/4.9/bits/stl_vector.h:931:7: note: void std::vector<_Tp, _Alloc>::push_back(std::vector<_Tp, _Alloc>::value_type&&) [with _Tp = std::unique_ptr<Handler<Measurement> >; _Alloc = std::allocator<std::unique_ptr<Handler<Measurement> > >; std::vector<_Tp, _Alloc>::value_type = std::unique_ptr<Handler<Measurement> >]
       push_back(value_type&& __x)
       ^
/usr/include/c++/4.9/bits/stl_vector.h:931:7: note:   no known conversion for argument 1 from 'std::_MakeUniq<PointCloudHandler>::__single_object {aka std::unique_ptr<PointCloudHandler, std::default_delete<PointCloudHandler> >}' to 'std::vector<std::unique_ptr<Handler<Measurement> > >::value_type&& {aka std::unique_ptr<Handler<Measurement> >&&}'
51:54: error: no matching function for call to 'std::vector<std::unique_ptr<Handler<Measurement> > >::push_back(std::_MakeUniq<ImageHandler>::__single_object)'
51:54: note: candidates are:
In file included from /usr/include/c++/4.9/vector:64:0,
                 from 4:
/usr/include/c++/4.9/bits/stl_vector.h:913:7: note: void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::unique_ptr<Handler<Measurement> >; _Alloc = std::allocator<std::unique_ptr<Handler<Measurement> > >; std::vector<_Tp, _Alloc>::value_type = std::unique_ptr<Handler<Measurement> >]
       push_back(const value_type& __x)
       ^
/usr/include/c++/4.9/bits/stl_vector.h:913:7: note:   no known conversion for argument 1 from 'std::_MakeUniq<ImageHandler>::__single_object {aka std::unique_ptr<ImageHandler, std::default_delete<ImageHandler> >}' to 'const value_type& {aka const std::unique_ptr<Handler<Measurement> >&}'
/usr/include/c++/4.9/bits/stl_vector.h:931:7: note: void std::vector<_Tp, _Alloc>::push_back(std::vector<_Tp, _Alloc>::value_type&&) [with _Tp = std::unique_ptr<Handler<Measurement> >; _Alloc = std::allocator<std::unique_ptr<Handler<Measurement> > >; std::vector<_Tp, _Alloc>::value_type = std::unique_ptr<Handler<Measurement> >]
       push_back(value_type&& __x)
       ^
/usr/include/c++/4.9/bits/stl_vector.h:931:7: note:   no known conversion for argument 1 from 'std::_MakeUniq<ImageHandler>::__single_object {aka std::unique_ptr<ImageHandler, std::default_delete<ImageHandler> >}' to 'std::vector<std::unique_ptr<Handler<Measurement> > >::value_type&& {aka std::unique_ptr<Handler<Measurement> >&&}'

我不明白为什么指针转换不成功, 因为所有的派生类都继承自一个... Handler<MeasT> 哪儿 MeasT 是由 Measurement.

我见过类似的问题，但大多数都是在派生类中保留模板（我没有）或者参考CRTP。然而，CRTP直接使用派生类作为模板参数，而我这里使用的是不同的派生类。

Answer 1

想到C++编译器在我们做模板的时候会用类名来代替。所以你的哈希图声明是这样翻译的std::vector>> Handlers; --> std::vector> Handlers;意味着编译器会自动从类模板Handler中创建一个 "模板类"，如下图。

class Handler_Measurement {
  public:
   virtual void addMeasurement(Measurement& m) = 0;
 };

但是当编译器看到handlers.push_back(std:make_unique())时；它创建了以下模板类。

  class Handler_PointCloudMeasurement {
  public:
   virtual void addMeasurement(PointCloudMeasurement& m) = 0;
 };

  class PointCloudHandler : public Handler_PointCloudMeasurement {
    public:
      void addMeasurement(PointCloudMeasurement& m) override {
         std::cout << m.n_points << std::endl;
      }
    };

而对于handlers.push_back(std::make_unique())；编译器创建了类Handler_ImageMeasurement { public: virtual void addMeasurement(ImageMeasurement& m) = 0; }。

     class ImageHandler : public Handler_ImageMeasurement{
       void addMeasurement(ImageMeasurement& m) override {
         std::cout << m.n_pixels << std::endl;
       }
      };

从手工制作的代码示例中，你可以看到基类是不同的。所以像下面这样的解决方案可能会帮助你。

#include <iostream>
#include <string>
#include <vector>
#include <memory>

class Measurement {
 public:
uint64_t time;
virtual ~Measurement() {}
virtual void display() = 0;
 };

 class PointCloudMeasurement : public Measurement {
  public:
uint64_t n_points = 64 * 1024;

void display()
{
    std::cout << n_points << std::endl;
}

  };

  class ImageMeasurement : public Measurement {
   public:
uint64_t n_pixels = 640 * 480;

void display()
{
    std::cout << n_pixels << std::endl;
}
  };

  class Handler {
  public:
virtual void addMeasurement(Measurement& m) = 0;
  };

  class PointCloudHandler : public Handler {
  public:
void  addMeasurement(Measurement& m) override {
    m.display();
}
  };

  class ImageHandler : public Handler {
void  addMeasurement(Measurement& m) override {
    m.display();
}
  };


  int main(void) {
std::vector<std::unique_ptr<Handler>> handlers;

handlers.push_back(std::make_unique<PointCloudHandler>());
handlers.push_back(std::make_unique<ImageHandler>());

PointCloudMeasurement pcm;
ImageMeasurement im;

handlers[0]->addMeasurement(pcm);
handlers[1]->addMeasurement(im);
 }

另外，你的派生类在数据成员方面也是不同的。它们唯一的共同点是 "它们是一种测量类型"，但它们没有任何共同点。

希望对你有所帮助。