c++可变模板构造函数的成员和混合的初始化列表

c++ variadic template constructor initialization list of members and mixins

本文关键字：成员 初始化 列表 混合 构造函数 c++      更新时间：2023-10-16

我试图从我的主机类(BaseSensor)初始化一个可变数量的mixins (ThresholdSensor<>, TrendSensor, EdgeSensor)，这也恰好需要作为构造函数参数，主机类(BaseSensor::bsd)的成员。我两个任务都失败了。

我使用的是c++11, gcc 4.7.1(所以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;
}

关注BaseSensor，我用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"混合将首先被初始化。那么我如何首先初始化bsd呢?

其次，通过一步一步的执行，我意识到执行步骤只进入"ThresholdSensor"，而没有进入"TrendSensor"或"EdgeSensor"。为什么呢?我认为所有的混合将与省略号语法进行评估。initializer_list会在某种程度上帮助init排序吗?initializer_list仍然是建议的解决方案，如果我包括的信息，最终basessensor mixins将有不同的数量(和类型)的参数，每个mixin将需要不同的值为这些参数。

?

提前感谢您的帮助

我不太喜欢下面的解决方案，但是…

第一:你可以修改SensorType类，接收(作为构造函数的第一个参数)一个指针到BaseSensorData而不是引用;例如，TendSensor变成

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:你可以修改BaseSensor从BaseSensorData派生(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; } 
 };

完整示例如下

#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;
 }
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