Undefined Reference when Compiling to Executable - c++

Question

When I try to compile all of my object files into an executable, I get the error
lab5.o:lab5.cpp: undefined reference to 'Card::suitMap'
I assume it's an issue with correctly linking dependencies, but I don't really know how to verify whether I'm on the right track.
Here's the source file containing the main function:
#include "Card.h"
#include "Deck.h"
#include <iostream>
using namespace std;
int main()
{
Deck* deck1 = new Deck();
Deck* deck2 = new Deck();
for(int i = 0; i < deck1->deck.size(); i++)
{
Card card = deck1->draw();
cout << card.getSuit() << " " << card.getValue() << endl;
}
deck2->shuffle();
for(int i = 0; i < deck2->deck.size(); i++)
{
Card card = deck2->draw();
cout << card.getSuit() << " " << card.getValue() << endl;
}
}
Here's my Card.h which contains suitMap
#ifndef CARD_H
#define CARD_H
#include <string>
#include <map>
class Card {
private:
int value;
int suit;
public:
std::map <int, std::string> suitMap;
Card(int value, int suit);
int getValue() { return this->value; }
std::string getSuit() { return this->suitMap.find(this->suit)->second; }
};
#endif
I'm not sure if my makefile could contain the problem, but here it is as well
CC=g++
CFLAGS=-c -Wall -g -std=c++11
LDFLAGS=
SOURCES=lab5.cpp Card.cpp Deck.cpp
OBJECTS=$(SOURCES:.cpp=.o)
EXECUTABLE=lab5
all: $(SOURCES) $(EXECUTABLE)
$(EXECUTABLE): $(OBJECTS)
$(CC) $(LDFLAGS) $(OBJECTS) -o $#
.cpp.o:
$(CC) $(CFLAGS) $< -o $#
clean:
rm -f *.o $(EXECUTABLE)

Answer 1

You must have had suitMap previously denoted static, and somehow not obtained a clean build after changing that. Be sure to perform a full clean build after modifying headers.

Answer 2

Your problem is due to a flaw in the makefile. A correctly designed makefile automatically manages all dependencies and thereby eliminates the need for running make clean to fix a build.
Below is an example makefile that would have prevented your problem. Rather than doing incremental builds, it always recompiles every source file. As you create larger projects, this approach may eventually become too slow, in which case you will want to invest in a more sophisticated build management solution. For your existing project, with this makefile, builds will likely take around one second on a modern PC.
EXECUTABLE = lab5
SOURCES += lab5.cpp Card.cpp Deck.cpp
CXXFLAGS += -Wall -Werror
CXXFLAGS += -std=c++11
CXXFLAGS += -g
.PHONY: build
build:
$(CXX) $(CXXFLAGS) $(LDFLAGS) $(SOURCES) $(LDLIBS) -o $(EXECUTABLE)
.PHONY: clean
clean:
rm -f $(EXECUTABLE)