Making a member function static makes the program fail to compile. Can't figure out why

Question

I am making a program for arduino. I am using avr-g+ 4.9.2 with STL from here.

I have a class Cocktail. I want all objects of type Cocktail to be able to access a vector of pointers. These pointers point to objects of type Alcohol. Seeing as this vector of pointers is the same for each instance of Cocktail, I wanted to make it static. But then my program fails to compile if I make them static. Here is the code:

Ineb.hpp

class Alcohol
{
    private:
        float flow_rate_;
        Pump * which_pump_;

    public:
        std::string this_alcohol_;
        Alcohol(std::string this_alcohol, float flow_rate);
        Alcohol(std::string this_alcohol, float flow_rate, Pump which_pump);
        float HowLong(float percentage_of_drink, uint8_t drink_size); //How long in seconds the pump should be on
        void ChangeByteToRegister(uint8_t& byte_to_register);
};

class Cocktail
{
    private:
        bool order_matter_;
        uint8_t byte_to_register_;
        static std::vector<Alcohol*> alcohol_directory_; 
    public:
        static void test(Alcohol *ba) {alcohol_directory_.push_back(ba);} //STATIC KEYWORD HERE
        Cocktail(bool ordr_matter);
        std::vector<std::string> GetIngredients(const uint8_t& num_ingredients, PGM_P& string_table);
        uint8_t GetByteToRegister();
        void MakeDrink(const uint8_t& num_ingredients, PGM_P& string_table);
};

main.cpp

#include "src/Ineb.hpp"
#include "src/Pins.hpp"
#include "ingredients.h"
#include <pnew.cpp>

extern "C" void __cxa_pure_virtual() {
  for(;;);
}

int main(void) {
  init();
  setup();

  Ineb::Pump A(1,8);

  Ineb::Alcohol Vodka("vodka", 2.5, A);

  Ineb::Cocktail::test(&Vodka);

  for(;;)
    loop();

  return 0; // not reached
}

undefined reference to `Ineb::Cocktail::alcohol_directory_'

I'm mainly confused why this compiles when I take away static. What is static doing under the hood??

Answer 1

static members of classes must be defined outside the class definition.

Having the line

std::vector<Alcohol*> Cocktail::alcohol_directory_; 

in Cocktail.cpp will do it.