我写了个小程序,但是得到未定义的参考错误。我没有弄错我在做什么
#include <iostream>
using namespace std;
class A()
{
private:
static A *m_object;
A()
{
m_object = NULL;
}
public:
static A* getSingleton();
int returnVar()
{
return 45;
}
};
A* A::getSingleton()
{
if(A::m_object == NULL)
{
A::m_object = new A();
}
return m_object;
}
int main()
{
cout<<"Hello World";
A *a = A::getSingleton();
cout<<"Address of a"<<&a<<endl;
cout<<"return a"<<a->returnVar()<<endl;
A *b = A::getSingleton();
cout<<"Address of a"<<&b<<endl;
return 0;
}
错误
/home/main.o: In function `A::getSingleton()':
main.cpp:(.text+0x3): undefined reference to `A::m_object'
main.cpp:(.text+0x21): undefined reference to `A::m_object'
/home/main.o: In function `main':
main.cpp:(.text.startup+0x16): undefined reference to `A::m_object'
main.cpp:(.text.startup+0x7e): undefined reference to `A::m_object'
main.cpp:(.text.startup+0xcb): undefined reference to `A::m_object'
/home/main..o:main.cpp:(.text.startup+0xde): more undefined references to `A::m_object' follow
collect2: error: ld returned 1 exit status
答案 0 :(得分:0)
当您在类内声明静态变量时,只会声明它,而未定义。因此,您需要在cpp中拥有该类的实例。
#include <iostream>
using namespace std;
class A
{
private:
static A *m_object;
A()
{
m_object = NULL;
}
public:
static A* getSingleton();
int returnVar()
{
return 45;
}
};
A* A::getSingleton()
{
if(A::m_object == NULL)
{
A::m_object = new A();
}
return m_object;
}
A* A::m_object; // You forgot this line
int main()
{
cout<<"Hello World";
A *a = A::getSingleton();
cout<<"Address of a"<<&a<<endl;
cout<<"return a"<<a->returnVar()<<endl;
A *b = A::getSingleton();
cout<<"Address of a"<<&b<<endl;
return 0;
}