Pointers to pointers undefined reference c++

Question

I am trying to get a list (yes I know it's an array) of things out of a callback function. I seem to be going round in circles, I'm assuming it's something silly that I haven't noticed, or I'm going about it all wrong.

class.cpp

client_list* client::get_list(sqlite3* db) {
    char* err;
    sqlite3_exec(db, "SELECT client_id FROM clients", list_callback, 0, &err);
    if ( err ) { 
        fprintf(stderr, "There was a problem with the query: %s\n", err);
        sqlite3_free(err);
        return new client_list{0,0};
    }   
    return &list;
}

int client::list_callback(void *NotUsed, int argc, char **argv, char **azColName) {
    list.clients = new client[argc];
    for (int i = 0; i<argc; i++) {
            //We are only requesting one column, it can therefor be assumed that
            //the value will be in the first position in the row.
            list.clients[i] = new client(static_cast<int>(*argv[i]));
    }   

    return 0;
}

class.h

class client;

typedef struct {
    client** clients;
    int length;
} client_list;
class client {
    public:
        char* name;
        client();
        client(char* name);
        client(int client_id);
        int id();
        static client_list* get_list(sqlite3* db);
        ~client();
    private:
        int client_id;
        static client_list list;
        static int list_callback(void *NotUsed, int argc, char **argv, char **azColName);

};

output

undefined reference to `client::list'