单例模式

[caven]

class Singleton

{

public:

static Singleton * Instance()

{

if( 0== _instance)

{

_instance = new Singleton;

}

return _instance;

};

void method1()
{
};

protected:

Singleton(void)

{

}

virtual ~Singleton(void)

{

}

static Singleton* _instance;

};



int main(int argc, char* argv[])
{


Singleton::Instance()->method1();

return 0;
}


提示：
P1.obj : error LNK2001: unresolved external symbol "protected: static class Singleton * Singleton::_instance" (?_instance@Singleton@@1PAV1@A)
Debug/P1.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

为什么？

[晨星]

静态成员在类中声明，同时需在类外定义并初始化。

class Singleton {
public:
static Singleton * Instance() {
if( 0 == _instance) {
_instance = new Singleton;
}
return _instance;
};
void method1(){};

protected:
Singleton(void){}
virtual ~Singleton(void){}

static Singleton* _instance;
};

// 定义并初始化。
Singleton* Singleton::_instance = 0;

int main(int argc, char* argv[]) {
Singleton::Instance()->method1();
return 0;
}

[cunsh]

static成员变量要在类外边写这个呀.
Singleton* Singleton::_instance;