C++ error undefined reference to `vtable for XXX' - c++

Question

I have been starting to write a mock shell program but came into an issue right away.
I am getting a undefined reference to `vtable for Cmd' when compiling my project
Shell.h
#ifndef SHELL_H
#define SHELL_H
#include <iostream>
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <stdlib.h>
#include <cstring>
using namespace std;
class Shell
{
protected:
char* cmd;
public:
Shell(): cmd(NULL){};
Shell(char* userInput): cmd(userInput){};
virtual bool execute() = 0;
};
#endif
Cmd.h
#ifndef CMD_H
#define CMD_H
#include <unistd.h>
#include <iostream>
#include <stdio.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <stdlib.h>
#include <cstring>
#include "Shell.h"
using namespace std;
class Cmd : public Shell
{
public:
Cmd(): Shell(NULL) {};
Cmd(char* userInput): Shell(userInput) {};
bool execute();
};
#endif
Cmd.cpp
#include "Cmd.h"
bool Cmd::execute()
{
char * list[1000];
bool status = true;
return status;
}
I researched this and i found out it could be from my makefile or my virtual functions
my virtual function is declared
Makefile
COMPILE = g++
FLAGS = -Werror -Wall -ansi
all:
mkdir -p ./bin
$(COMPILE) $(FLAGS) ./src/main.cpp -o ./bin/rshell
main:
$(COMPILE) $(FLAGS) ./src/main.cpp
Cmd:
$(COMPILE) $(FLAGS) ./src/Cmd.cpp
Exit:
$(COMPILE) $(FLAGS) ./src/Exit.cpp
Connector:
$(COMPILE) $(FLAGS) ./src/Connector.cpp
And:
$(COMPILE) $(FLAGS) ./src/And.cpp
Or:
$(COMPILE) $(FLAGS) ./src/Or.cpp
Semi:
$(COMPILE) $(FLAGS) ./src/Semi.cpp
clean:
rm -rf ./bin

Answer 1

The error means you are missing a reference to the first virtual method in the class.
The problem is you hosed your makefile.
General form of the solution is as follows:
CXX=g++ -Werror -Wall -ansi -c
LD=g++
OBJS=/src/main.o src/Cmd.o src/Exit.o src/Connector.o src/And.o src/Or.o src/Semi.o
all: .PHONY
mkdir bin
$(LD) -o bin/rshell $(OBJS)
%.o: %.cpp Shell.h Cmd.h
$(CXX) $(FLAGS) -o $# $<
There's a couple of things going on here.
Use of OBJS and a rule to save lots of typing.
compiling each .cpp file with -c to avoid trying to link too soon.
Linking as the last step doesn't compile anything.
Use of .PHONY so compile doesn't choke if somebody made a file called all.