c++ 可变参数模板构造函数初始化成员列表和mixins

Question

我正在尝试从我的主机类 (BaseSensor) 初始化可变数量的 mixins (ThresholdSensor<>, TrendSensor, EdgeSensor)，它们也恰好需要作为构造函数参数，主机类的成员(BaseSensor::bsd)。我在这两项任务中都失败了:)

我正在使用 c++11 ，gcc 4.7.1（因此 std::map 没有“emplace”），代码如下：

#include <iostream> // std::cout std::endl
#include <string>   // std::string
#include <map>   // std::map

using namespace std;

struct BaseSensorData
{
    BaseSensorData(const string & _sensorName)
        : sensorName(_sensorName)
    {}

    const string sensorName;
};

class TrendSensor //mixin for BaseSensor
{
    public:
        TrendSensor( BaseSensorData & bsd , size_t _windowSize , const double & _trainRangefactor)
        {
            cout << "TrendSensor: " << _windowSize << " " << bsd.sensorName << " " << _trainRangefactor << endl;
        }
};

typedef struct{ double trough; double peak; } Edges;
class EdgeSensor  //mixin for BaseSensor
{
    public:
        EdgeSensor( BaseSensorData & bsd , size_t _windowSize , const double _rangeFactor)
        {
            cout << "EdgeSensor: " << _windowSize << " " << bsd.sensorName << " " << _rangeFactor << endl;
        }
};

class CO2Threshold //mixin for ThresholdSensor
{
    std::map<std::string , double>thresholds;

    public:
        CO2Threshold( const double & _toxicThres , const double & _zeroThres , const double & )
        {
            thresholds["toxic"] = _toxicThres;
            thresholds["zero"] = _zeroThres;
            cout << "CO2Threshold: " << _toxicThres << " " << thresholds["zero"] << endl;
        }
};
class O2Threshold //mixin for ThresholdSensor
{
    std::map<std::string , double>thresholds;

    public:
        O2Threshold( const double & _toxicThres , const double & _lowThres , const double & _elecChemThres )
        {
            thresholds["toxic"] = _toxicThres;
            thresholds["low"] = _lowThres;
            thresholds["elecchem"] = _elecChemThres;
            cout << "O2Threshold: " << _toxicThres << " " << thresholds["low"] << " " << thresholds["elecchem"] << endl;
        }
};

template<typename ThresholdMixin> //CO2Threshold , O2Threshold , or others ...
class ThresholdSensor : public ThresholdMixin  //mixin for BaseSensor
{
    public:
        ThresholdSensor ( BaseSensorData & bsd
                        , const size_t & _windowSize
                        , const double & _toxicThres)
            : ThresholdMixin
                ( _toxicThres //3x the same arg
                , _toxicThres //just for the sake
                , _toxicThres //of this exercise
                )
        {
            cout << "ThresholdSensor: " << _windowSize << " " << bsd.sensorName << " " << endl;
        }

};

template<typename ... SensorType>
class BaseSensor : public SensorType ... //to my BaseSensor class
{
    BaseSensorData bsd;

    public:
        BaseSensor(const string& _sensorName , size_t _windowSize , const double d)
            :
            bsd(_sensorName) //this causes "Wreorder" warning. 
            ,
            SensorType( bsd , _windowSize , d )...
        {
            cout << "BaseSensor: " << _windowSize << " " << bsd.sensorName << " " << endl;
        }

};

int main() {
    BaseSensor<ThresholdSensor<CO2Threshold> , TrendSensor, EdgeSensor> bs{string("test"),60U , 14.5f};
    //BaseSensor<EdgeSensor> bs2;
}

专注于 BaseSensor ，我使用 gdb 运行可执行文件，我意识到没有办法让“bsd”成员首先初始化。 “SensorType” mixins 将首先被初始化。那么如何先初始化bsd呢？

其次，通过一步一步的执行，我意识到执行只进入“ThresholdSensor”，而不是“TrendSensor”或“EdgeSensor”。这是为什么？我虽然所有的 mixins 都将使用省略号语法进行评估。 initializer_list 会以某种方式帮助 init 排序吗？如果我包含以下信息，initializer_list 是否仍然是建议的解决方案

提前感谢您的帮助

Answer 1

我不太喜欢下面的解决方案，但是...

首先：您可以修改SensorType 类以接收（作为构造函数的第一个参数）指向BaseSensorData 的指针，而不是引用；例如，TendSensor 变为

class TrendSensor //mixin for BaseSensor
 {
   public:
      TrendSensor (BaseSensorData * pBsd, size_t _windowSize,
                   const double & _trainRangefactor)
       { std::cout << "TrendSensor: " << _windowSize << " "
            << pBsd->sensorName << " " << _trainRangefactor << std::endl; }
 };

第二：可以修改BaseSensor派生自BaseSensorData（重要：在SensorType...包之前）并删除bsd成员； BaseSensor成为

template <typename ... SensorType>
class BaseSensor : public BaseSensorData, public SensorType ...
 {
   public:

      BaseSensor (const std::string & _sensorName, size_t _windowSize,
                  const double d)
         : BaseSensorData(_sensorName), SensorType(this, _windowSize, d )...
            { std::cout << "BaseSensor: " << _windowSize << " "
              << sensorName << " " << std::endl; } 
 };

完整的例子如下

#include <iostream> // std::cout std::endl
#include <string>   // std::string
#include <map>      // std::map

struct BaseSensorData
 {
   BaseSensorData (const std::string & _sensorName)
      : sensorName(_sensorName)
    {}

   const std::string sensorName;
 };

class TrendSensor //mixin for BaseSensor
 {
   public:
      TrendSensor (BaseSensorData * pBsd, size_t _windowSize,
                   const double & _trainRangefactor)
       { std::cout << "TrendSensor: " << _windowSize << " "
            << pBsd->sensorName << " " << _trainRangefactor << std::endl; }
 };

typedef struct { double trough; double peak; } Edges;

class EdgeSensor  //mixin for BaseSensor
 {
   public:
      EdgeSensor (BaseSensorData * pBsd, size_t _windowSize,
                  const double _rangeFactor)
       { std::cout << "EdgeSensor: " << _windowSize << " "
            << pBsd->sensorName << " " << _rangeFactor << std::endl; }
 };

class CO2Threshold //mixin for ThresholdSensor
 {
   private:
      std::map<std::string, double> thresholds;

   public:
      CO2Threshold (const double & _toxicThres, const double & _zeroThres,
                    const double & )
       {
         thresholds["toxic"] = _toxicThres;
         thresholds["zero"] = _zeroThres;
         std::cout << "CO2Threshold: " << _toxicThres << " "
            << thresholds["zero"] << std::endl;
       }
};

class O2Threshold //mixin for ThresholdSensor
 {
   private:

      std::map<std::string , double> thresholds;

    public:

      O2Threshold (const double & _toxicThres, const double & _lowThres,
                   const double & _elecChemThres)
       {
         thresholds["toxic"] = _toxicThres;
         thresholds["low"] = _lowThres;
         thresholds["elecchem"] = _elecChemThres;
         std::cout << "O2Threshold: " << _toxicThres << " "
            << thresholds["low"] << " " << thresholds["elecchem"]
            << std::endl;
       }
 };

template <typename ThresholdMixin> //CO2Threshold , O2Threshold , or others ...
class ThresholdSensor : public ThresholdMixin  //mixin for BaseSensor
 {
   public:
      ThresholdSensor (BaseSensorData * pBsd, const size_t & _windowSize,
                       const double & _toxicThres)
         : ThresholdMixin
            ( _toxicThres, //3x the same arg
              _toxicThres, //just for the sake
              _toxicThres)  //of this exercise
       { std::cout << "ThresholdSensor: " << _windowSize << " "
         << pBsd->sensorName << " " << std::endl; }
 };

template <typename ... SensorType>
class BaseSensor : public BaseSensorData, public SensorType ...
 {
   public:

      BaseSensor (const std::string & _sensorName, size_t _windowSize,
                  const double d)
         : BaseSensorData(_sensorName), SensorType(this, _windowSize, d )...
            { std::cout << "BaseSensor: " << _windowSize << " "
              << sensorName << " " << std::endl; } 
 };

int main()
 {
   BaseSensor<ThresholdSensor<CO2Threshold>,
      TrendSensor, EdgeSensor> bs { std::string("test"), 60U, 14.5f };

   //BaseSensor<EdgeSensor> bs2;
 }