Undefined reference to static member of class - c++

Question

I am about to learn basic OOP operations in C++ and now I encountered a problem with static members of class. I try to build simple card game. I create classes Rules, Deck and Card.
My Deck class taking rules and some constants from Rules class and then do something. I'm only use Deck::createDeck(); in main function, nothing else. When I try to compile the code I get in result error:
/usr/bin/ld: CMakeFiles/CardGame.dir/Sources/Rules.cpp.o: in function Rules::getSuits[abi:cxx11]()':
/home/bartosz/CLionProjects/CardGame/Sources/Rules.cpp:12: undefined reference toRules::suits[abi:cxx11]'
/usr/bin/ld: CMakeFiles/CardGame.dir/Sources/Rules.cpp.o: in function Rules::getRanks[abi:cxx11]()':
/home/bartosz/CLionProjects/CardGame/Sources/Rules.cpp:16: undefined reference toRules::ranks[abi:cxx11]'
collect2: error: ld returned 1 exit status
But I belive that static members (suits and ranks) are corectlly initialized, so why compiler doesn't see this variables?
My code:
Rules.h
#ifndef CARDGAME_RULES_H
#define CARDGAME_RULES_H
#include <string>
class Rules {
public:
static std::string suits[4];
static std::string ranks[13];
public:
static std::string * getSuits();
static std::string * getRanks();
};
#endif //CARDGAME_RULES_H
Rules.cpp
#include "../Headers/Rules.h"
std::string suits[4] = {"Diamonds", "Hearts", "Spades", "Clubs"};
std::string ranks[13] = {"2","3","4","5","6","7","8","9","10","J","Q","K","A"};
std::string * Rules::getSuits() {
return suits;
}
std::string * Rules::getRanks() {
return ranks;
}
Deck.h
#ifndef CARDGAME_DECK_H
#define CARDGAME_DECK_H
#include "Card.h"
class Deck {
private:
Card * cards;
Deck();
public:
static void createDeck();
void shuffle();
void dealCards();
};
#endif //CARDGAME_DECK_H
Deck.cpp
#include "../Headers/Deck.h"
#include "../Headers/Rules.h"
Deck::Deck() {
}
void Deck::createDeck() {
std::string * ranks = Rules::getRanks();
std::string * suits = Rules::getSuits();
// some operations
}
void Deck::shuffle() {
}
void Deck::dealCards() {
}

Answer 1

In Rules.cpp, you don't define the static members Rules::suits and Rules::ranks, but rather introduce 2 new global variables.
In order for the static definition to work, you need to specify the fully qualified name, e.g. Rules::suits.

Answer 2

Use a constant expression constexpr
In the header Rules.h file:
constexpr std::array<std::string_view, 4> suits = {"Diamonds", "Hearts", "Spades", "Clubs"};