Errors when initializing class with an array as a member variable in C++

I'm just starting to learn C++ and can't figure out what I'm doing wrong.

I'm trying to build a class "Holder" that holds an array of up to 100 of another class "Obj". I have the following two classes:

class Obj {
public:
    Obj(int k, char v): key(k), val(v) {};
private:
    int key;
    char val;


   friend class Holder;
};


class Holder {
private:
    enum {MAX_SIZE = 100};
    Obj p[MAX_SIZE];
    int pSize = 0;
public:

    Holder();
    ~Holder();
//...
};

When initializing the class Holder from main(), as follows...

int main() {
    Holder test;


    return 0;
}

I'm receiving these errors after running the program:

undefined reference to "Holder::Holder()" and
undefined reference to "Holder::~Holder()"

I can't tell if I'm correctly using an array as a class member variable in "Holder"? Or if I'm missing something in the constructor?