回顧前面的文章，實現了一個簡單工廠模式來創建不同類對象，但由於c++沒有類似new "Circle"之類的語法，導致CreateShape 函
數中需要不斷地ifelse地去判斷，如果有多個不同類對象需要創建，顯然這是很費神的，下面通過宏定義注冊的方法來實現動態創
建對象。
Shape.h:
#ifndef _SHAPE_H_
#define _SHAPE_H_
class Shape
{
public:
virtual void Draw() = 0;
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 0;
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_;
//頭文件被包含多次，也只定義一次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 0;
}
在DynBase.h 中#define了一個宏定義REGISTER_CLASS(class_name)，且在Shape.cpp 中調用宏定義，拿REGISTER_CLASS(Circle);
來說，程序編譯預處理階段會被替換成：
class CircleRegister {
public:
static void* NewInstance()
{
return newCircle;
}
private:
static Register reg_;
};
Register CircleRegister::reg_("Circle",CircleRegister::NewInstance);
也即定義了一個新類，且由於含有static 成員，則在main函數執行前先執行初始化，調用Register類構造函數，在構造函數中調用
DynObjectFactory::Register(name, func); 即調用DynObjectFactory 類的靜態成員函數，在Register函數中通過map容器完成了
字符串與函數指針配對的注冊，如mapCls_[name] = func;
進入main函數，調用DynObjectFactory::CreateObject("Circle") ，CreateObject函數中通過string找到對應的函數指針
（NewInstance），並且調用後返回創建的對象指針，需要注意的是 return it->second(); 中it->second 是函數指針，後面加括
號表示調用這個函數。對宏定義中的#,##用法不熟悉的可以參考這裡。
這樣當需要創建多個不同類對象的時候，就不再需要寫很多ifelse的判斷了。
