C++ 动态创建对象

转自：http://www.cnblogs.com/jisi5789/p/3190353.html

回顾前面的文章，实现了一个简单工厂模式来创建不同类对象，但由于c++没有类似new "Circle"之类的语法，导致CreateShape 函

数中需要不断地ifelse地去判断，如果有多个不同类对象需要创建，显然这是很费神的，下面通过宏定义注册的方法来实现动态创

建对象。

Shape.h:

#ifndef _SHAPE_H_
#define _SHAPE_H_ 

class Shape
{
public:
     virtual  void Draw() =  ;
     virtual ~Shape() {}
}; 

class Circle :  public Shape
{
public:
     void Draw();
    ~Circle();
}; 

class Square :  public Shape
{
public:
     void Draw();
    ~Square();
}; 

class Rectangle :  public Shape
{
public:
     void Draw();
    ~Rectangle();
}; 

#endif  // _SHAPE_H_

Shape.cpp:

#include  "Shape.h"
#include  "DynBase.h"
#include <iostream>
using  namespace std; 

void Circle::Draw()
{
    cout <<  "Circle::Draw() ..." << endl;
}
Circle::~Circle()
{
    cout <<  "~Circle ..." << endl;
} 

void Square::Draw()
{
    cout <<  "Square::Draw() ..." << endl;
}
Square::~Square()
{
    cout <<  "~Square ..." << endl;
} 

void Rectangle::Draw()
{
    cout <<  "Rectangle::Draw() ..." << endl;
} 

Rectangle::~Rectangle()
{
    cout <<  "~Rectangle ..." << endl;
} 

REGISTER_CLASS(Circle);
REGISTER_CLASS(Square);
REGISTER_CLASS(Rectangle); 

DynBase.h:

#ifndef _DYN_BASE_H_
#define _DYN_BASE_H_ 

#include <map>
#include <string>
using  namespace std; 

typedef  void *(*CREATE_FUNC)(); 

class DynObjectFactory
{
public:
     static  void *CreateObject( const string &name)
    {
        map<string, CREATE_FUNC>::const_iterator it;
        it = mapCls_.find(name);
         if (it == mapCls_.end())
             return  ;
         else
             return it->second();  //func();

    } 

     static  void Register( const string &name, CREATE_FUNC func)
    {
        mapCls_[name] = func;
    }
private:
     static map<string, CREATE_FUNC> mapCls_;
}; 

// g++
// __attribute ((weak))
__declspec(selectany) map<string, CREATE_FUNC> DynObjectFactory::mapCls_;
//头文件被包含多次，也只定义一次mapCls_;

class Register
{
public:
    Register( const string &name, CREATE_FUNC func)
    {
        DynObjectFactory::Register(name, func);
    }
}; 

#define REGISTER_CLASS(class_name) \
class class_name##Register { \
public: \
     static  void* NewInstance() \
    { \
         return  new class_name; \
    } \
private: \
     static Register reg_; \
}; \
Register class_name##Register::reg_(#class_name, class_name##Register::NewInstance)
//CircleRegister

#endif  // _DYN_BASE_H_

DynTest.cpp:

#include  "Shape.h"
#include  "DynBase.h"
#include <iostream>
#include <vector>
#include <string>
using  namespace std; 

void DrawAllShapes( const vector<Shape *> &v)
{
    vector<Shape *>::const_iterator it;
     for (it = v.begin(); it != v.end(); ++it)
    {
        (*it)->Draw();
    }
} 

void DeleteAllShapes( const vector<Shape *> &v)
{
    vector<Shape *>::const_iterator it;
     for (it = v.begin(); it != v.end(); ++it)
    {
         delete(*it);
    }
} 

int main( void)
{
    vector<Shape *> v; 

    Shape *ps;
    ps =  static_cast<Shape *>(DynObjectFactory::CreateObject( "Circle"));
    v.push_back(ps);
    ps =  static_cast<Shape *>(DynObjectFactory::CreateObject( "Square"));
    v.push_back(ps);
    ps =  static_cast<Shape *>(DynObjectFactory::CreateObject( "Rectangle"));
    v.push_back(ps); 

    DrawAllShapes(v);
    DeleteAllShapes(v); 

     return  ;
}

在DynBase.h 中#define了一个宏定义REGISTER_CLASS(class_name)，且在Shape.cpp 中调用宏定义，拿REGISTER_CLASS(Circle);

来说，程序编译预处理阶段会被替换成：

class CircleRegister {
public: 

static void* NewInstance() 

{ 

return newCircle; 

} 

private: 

static Register reg_; 

};

Register CircleRegister::reg_("Circle",CircleRegister::NewInstance);

也即定义了一个新类，且由于含有static 成员，则在main函数执行前先执行初始化，调用Register类构造函数，在构造函数中调用

DynObjectFactory::Register(name, func); 即调用DynObjectFactory 类的静态成员函数，在Register函数中通过map容器完成了

字符串与函数指针配对的注册，如mapCls_[name] = func;

进入main函数，调用DynObjectFactory::CreateObject("Circle") ，CreateObject函数中通过string找到对应的函数指针

（NewInstance），并且调用后返回创建的对象指针，需要注意的是 return it->second(); 中it->second 是函数指针，后面加括

号表示调用这个函数。对宏定义中的#,##用法不熟悉的可以参考这里。

这样当需要创建多个不同类对象的时候，就不再需要写很多ifelse的判断了。

参考：

C++ primer 第四版
Effective C++ 3rd
C++编程规范