This pattern is used with polymorphism and template which are representative feature of C++ language. The main advantage of factory method is that when making new object or extending previous object, define new sublayer class and then just overriding member function which is defined as a factory method. Therefore, we don't need to analyze existing source code detaily.
Here is a sample class diagram using Factory method pattern.
Implementation code is below.
sample.h
#include <map>
using namespace std;
typedef struct _controlInfo
{
unsigned int ctrlId;
struct _positionInfo
{
unsigned int x;
unsigned int y;
}Position;
struct _sizeInfo
{
unsigned int width;
unsigned int height;
}Size;
}ControlInfo;
class CControlBase
{
public:
virtual bool Create(const ControlInfo* info) = 0;
};
class CButton : public CControlBase
{
public :
bool Create(const ControlInfo* info)
{
//Make Button
//...
//...
return true;
}
};
class ControlMaker
{
public :
void Make(const ControlInfo* info)
{
CControlBase *base = CreateControl();
base->Create(info);
ctrlMap[info->ctrlId] = base;
}
protected :
virtual CControlBase* CreateControl() = 0;
private :
map<unsigned int, CControlBase *>ctrlMap;
};
template <class ctrlType>
class ConcreteControlMaker : public ControlMaker
{
protected :
CControlBase* CreateControl() { return new ctrlType; }
};
sample.cpp
//
#include "stdafx.h"
#include "Sample.h"
int _tmain(int argc, _TCHAR* argv[])
{
//get contrl info by using xml or ini
ControlInfo* info = NULL;
GetControlInfo(info);
//if need to make a button
ConcreteControlMaker<CButton> btn;
btn.Make(info);
return 0;
}
Because this pattern delegates sublayer classes to create object, therefore, factory method pattern has advantages when need to divide the responsibility of object creation.
