单例类中未解析的外部符号 [英] Unresolved External Symbol in Singleton Class
问题描述
我已经编码了很长时间,但我不明白这个错误。我编写一个自定义系统,为对象的特定实例提供唯一的整数ID(我称之为标签)。
I've been coding for a long time now but I do not understand this error. I am writing a custom system for providing unique integer ids to specific instances of objects (I call them tags). And I am implementing one of the classes as a Singleton.
Tagging系统的两个类被定义为:
The two classes for the Tagging system are defined as such:
#include "singleton.h"
class Tag: public bear::Singleton<Tag>
{
public:
static dUINT32 RequestTag(Tagged* requester);
static void RevokeTags(void);
private:
Tag(void);
~Tag(void);
Tagged** m_tagTable; // list of all objects with active tags
dUINT32 m_tagTable_capacity, // the maximum capacity of the tag table
m_tagIndexer; // last given tag
};
class Tagged
{
friend class Tag;
public:
inline dUINT32 GetTag(void) {return m_tag;}
private:
inline void InvalidateTag(void) {m_tag=INVALID_TAG;}
dUINT32 m_tag;
protected:
Tagged();
virtual ~Tagged();
};
Singleton类的定义如下:
The Singleton class is defined as such:
template <typename T>
class Singleton
{
public:
Singleton(void);
virtual ~Singleton(void);
inline static T& GetInstance(void) {return (*m_SingletonInstance);}
private:
// copy constructor not implemented on purpose
// this prevents copying operations any attempt to copy will yield
// a compile time error
Singleton(const Singleton<T>& copyfrom);
protected:
static T* m_SingletonInstance;
};
template <typename T>
Singleton<T>::Singleton (void)
{
ASSERT(!m_SingletonInstance);
m_SingletonInstance=static_cast<T*>(this);
}
template <typename T>
Singleton<T>::~Singleton (void)
{
if (m_SingletonInstance)
m_SingletonInstance= 0;
}
我尝试编译和链接文件时收到以下错误:
I am getting the following error upon trying to compile and link together the files:
1> test.obj:error LNK2001:未解析的外部符号protected:static class util :: Tag * bear :: Singleton :: m_SingletonInstance(?m_SingletonInstance @? $ singleton @ VTag @ util @@@ bear @@ 1PAVTag @ util @@ A)
1> C:... \tools\Debug\util.exe:致命错误LNK1120:1 unresolved externals
1>test.obj : error LNK2001: unresolved external symbol "protected: static class util::Tag * bear::Singleton::m_SingletonInstance" (?m_SingletonInstance@?$Singleton@VTag@util@@@bear@@1PAVTag@util@@A) 1>C:...\tools\Debug\util.exe : fatal error LNK1120: 1 unresolved externals
有人知道我为什么会收到这个错误吗?
Does anyone have any idea why I am getting this error?
推荐答案
您应该在命名空间范围为您的静态数据成员提供一个定义(目前您只有一个声明):
You should provide a definition for your static data member at namespace scope (currently, you only have a declaration):
template <typename T>
class Singleton
{
// ...
protected:
static T* m_SingletonInstance; // <== DECLARATION
};
template<typename T>
T* Singleton<T>::m_SingletonInstance = nullptr; // <== DEFINITION
如果使用C ++ 03, code> nullptr 与 NULL
或 0
。
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