Question

Possible Duplicate:
What is an undefined reference/unresolved external symbol error and how do I fix it?

I’m relatively new to C++ (as you can probably tell by the question) and I’ve hit a problem. I have two files: Drives.h and Drives.cpp

Drives.h

#pragma once

enum MountMode
{
    User,
    System,
    Both,
    Auto
};

class Drive
{
public:
    Drive(void);
    ~Drive(void);

    BOOL Mount(MountMode mode);
    VOID Unmount(void);
    BOOL IsConnected(void);

    static char* DeviceName;
    static char* DrivePath;
};

class Drives
{
public:
    Drives(void);
    ~Drives(void);
};

and my Drives.cpp:

#include "stdafx.h"
#include "Drives.h"

Drives::Drives(void)
{
    Drive USB0; //Error happening here
}

Drives::~Drives(void)
{
}

The error is saying that the Drives class constructor, destructor and IsConnected() are all unresolved externals. I’m not sure what I’m missing since I set this class up like the one on cplusplus.com

Thanks in advance

Answer 1

As the error message says, you have not implemented the constructor and destructor of Drive:

Drive::Drive(void) {
    ...
}

Drive::~Drive(void) {
    ...
}

Creating a local variable of class type (as you do in Drive USB0;) will invoke that class’ constructor, and the destructor will be invoked at the end of the variable’s scope; hence the error.

You should implement the other functions of Drive too – declaring a function in a class declaration is essentially a promise that the function will be implemented somewhere.