C++ Constructor and Destructor - c++

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...

Answer 2

You did not define your virtual function and/or g++ wants you to make your destructor virtual (because you have virtual functions which assumes inheritance)

Answer 3

Try
virtual void execute()=0;
This will make your class abstract, which seems to be what you intend since execute isn't defined.
If you ever want to use Instruction in more than one .cpp file, you should move the implementation of the class methods into a classes.cpp file.

Answer 4

As people already told, the problem is having execute() that is not implemented. Implement it, or make it pure virtual as says Dan Hook.
Just an extra remark: in many (maybe most depending on what you're coding on) cases, you don't need to implement the destructor. You just need if you want some specific functionnality (flushing data to a file for example).
As long as you have no pointer (as it is the case in your code), you won't have any memory tracking issues. Just remove the destructors: it's safe and it's less code.
However, if only one member is a pointer, then everything gets messy and you have to deal with memory management issues, memory leaks and segfaults ;)