从零开始学C++之动态创建对象
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() = 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_;
//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 0;
}
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 new Circle;
}
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
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