C++ Constructor and Destructor

Question

I'm getting some errors when compiling my program. They relate to the constructor and destructor of my class Instruction.

Errors are:

/tmp/ccSWO7VW.o: In function `Instruction::Instruction(std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int)':
ale.c:(.text+0x241): undefined reference to `vtable for Instruction'
/tmp/ccSWO7VW.o: In function `Instruction::Instruction(std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int)':
ale.c:(.text+0x2ab): undefined reference to `vtable for Instruction'
/tmp/ccSWO7VW.o: In function `Instruction::~Instruction()':
ale.c:(.text+0x315): undefined reference to `vtable for Instruction'
/tmp/ccSWO7VW.o: In function `Instruction::~Instruction()':
ale.c:(.text+0x38d): undefined reference to `vtable for Instruction'
collect2: ld returned 1 exit status

Here is my code:

//classses.h

#include <iostream>
#include <string>
using namespace std;

class Instruction{

  protected:
    string name;
    int value;

  public:
    Instruction(string _name, int _value);
    ~Instruction();
    void setName(string _name);
    void setValue(int _value);
    string getName();
    int getValue();
    virtual void execute();
};

//constructor
Instruction::Instruction(string _name, int _value){
    name = _name;
    value = _value;
}
//destructor
Instruction::~Instruction(){
    name = "";
    value = 0;
}
void Instruction::setName(string _name){
     name = _name;
}

void Instruction::setValue(int _value){
    value = _value;
}

string Instruction::getName(){
       return name;
}

int Instruction::getValue(){
    return value;
}

/////////////////////////////////////////////////////////////////////

//ale.cpp

    #include "headers.h"
    #include "functions.h"
    #include "classes.h"
    #include <list>


    using namespace std;

    int main(){

    return 0;
    }

Answer 1

I would guess the problem is due to you declaring a virtual method 'execute' in the Instruction class, and never defining it anywhere. Compilers have to produce a vtable object for a class with virtual methods and really only want one copy of it, so they usually just do it in the compilation unit (source file) that defines the first virtual function...