利用map動態建立C 類物件

mfc的cruntimeclass利用鍊錶實現了c++類的動態建立。但是如果專案中對動態建立的要求比較低，我們完全可以利用map實現簡單的動態建立。以下三個檔案做了乙個簡單的實現。

/*
* author: yejingx
* date: 2011-12-29
* file: base.h
*/#ifndef base_h_
#define base_h_
#include "dyncreate.h"
class base
};public:
base(void);
virtual ~base(void);
virtual void run();
};#endif

/*
* author: yejingx
* date: 2011-12-29
* file: base.cpp
*/#include #include "base.h"
std::mapbase::class_set; 
base* base::create(const std::string& class_name) // 動態建立函式實現
return null;
}base::base(void)
public:
derived(void);
virtual ~derived(void);
void run();
};#endif

/*
* author: yejingx
* date: 2011-12-29
* file: derived.cpp
*/#include #include "derived.h"
static derived::_auto_register _register_derived("derived",derived::create); // 宣告乙個靜態變數，自動呼叫它的建構函式對類進行自動註冊
derived::derived(void)};
#define implement_dynbase(base) \
std::mapbase::class_set; \
base* base::create(const std::string& class_name) \
return null; }
#define declare_dyncreate(derived,base) \
public: \
static base* create()
#define implement_dyncreate(derived) \
static derived::_auto_register _register_##derived(#derived,derived::create); 
#endif

有了這些巨集，base.h, base.cpp, derived.h和derived.cpp就可以變成以下這樣子。

/*
* author: yejingx
* date: 2011-12-29
* file: base.h
*/#ifndef base_h_
#define base_h_
#include "dyncreate.h"
class base
;#endif

/*
* author: yejingx
* date: 2011-12-29
* file: base.cpp
*/#include #include "base.h"
implement_dynbase(base) // 實現動態建立基類
implement_dyncreate(base) // 實現基類的動態建立
base::base(void);
#endif

/*
* author: yejingx
* date: 2011-12-29
* file: derived.cpp
*/#include #include "derived.h"
implement_dyncreate(derived) // 實現動態建立子類
derived::derived(void) \struct _auto_register };
#define implement_dynbase(base) \
base* base::create(const std::string& class_name) \
return null; }
#define declare_dyncreate(derived,base) \
public: \
static base* create()
#define implement_dyncreate(derived) \
static derived::_auto_register _register_##derived(#derived,derived::create); 
#endif

編寫測試檔案

/* * author: yejingx * date: 2011-12-29 * file: dyn_test.cpp */#include "derived.h" int main()

輸出

base constructor called
derived constructor called
derived class run
derived destructor called
base destructor called
base constructor called
base class run
base destructor called

到此，我們已經保證在第一次使用class_set前對其進行建立並初始化，但是仍然存在乙個問題。設想，假如我們編寫的程式在_auto_register物件初始化前呼叫了base::create(class_name)，這時候的base::class_set是空的，base::create(class_name)就返回乙個空指標！有一次我把base, derived等類打包成乙個lib，然後寫了另外乙個檔案呼叫這個lib中的base::create，就出現了上述問題。之後我用了很土的辦法臨時解決了這個問題，在此不提也罷。

如何確保第一次呼叫base::create之前_auto_register物件都已經建立？期待高手指點。。。

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