Undefined reference to member in custom class

[Squirrel96]

I attempted to compile the following code using g++ and I got the following error:

$ g++ test.cpp

/tmp/ccjE1ooy.o: In function `A::A()':
test.cpp:(.text._ZN1AC2Ev[_ZN1AC5Ev]+0x16): undefined reference to `A::allA'
/tmp/ccjE1ooy.o: In function `A::~A()':
test.cpp:(.text._ZN1AD2Ev[_ZN1AD5Ev]+0x1d): undefined reference to `A::allA'
test.cpp:(.text._ZN1AD2Ev[_ZN1AD5Ev]+0x40): undefined reference to `A::allA'
test.cpp:(.text._ZN1AD2Ev[_ZN1AD5Ev]+0x6f): undefined reference to `A::allA'
test.cpp:(.text._ZN1AD2Ev[_ZN1AD5Ev]+0xa8): undefined reference to `A::allA'
/tmp/ccjE1ooy.o:test.cpp:(.text._ZN1AD2Ev[_ZN1AD5Ev]+0xc3): more undefined references to `A::allA' follow
collect2: ld returned 1 exit status

Minimal Code example:

//test.cpp
#include <vector>
class A
{
	public:
		static std::vector<A*> allA;
		A()
		{
			allA.push_back(this);
		}
		
		~A()
		{
			for(int i=0; i<allA.size(); i++)
			{
				if(allA[i] == this)
				{
					delete allA[i];
					allA.erase(allA.begin() + (i-1));
					break;
				}
			}
		}
};

int main()
{
	A a1;
	return 0;
}

[vlad from moscow]

You shall define your vector. For example

#include <vector>

class A
{
    // your class definition
};

std::vector<A*> A::allA;

int main()
{
	A a1;
	return 0;
}

[Squirrel96]

Thanks vlad, I forgot that I had to define it outside of the class.