C++ private static member variables

Question

This C++ code is producing linker errors at compile time:

// A.h
class A {
    public:
        static void f();
    private:
        static std::vector<int> v;
};

// A.cpp
void A::f() {
    // this line is causing trouble
    int i = v.size();
}

Moving the vector declaration into the cpp file works. However I want to understand the linker error "Undefined symbols" cause in the above code. What is causing the linker error in the above code?

Answer 1

Static members have to be defined in a compilation unit:

// A.cpp

vector<int> A::v;

Answer 2

// A.h
class A {
    public:
        static void f();
    private:
        static std::vector<int> v;
};

// A.cpp
//modify add this line
static std::vector<int> A::v;
void A::f() {
    // this line is causing trouble
    int i = v.size();
}