Unresolved External Symbol how to fix it? abstract class - c++

Question

Here's my abstract class Storestate.h:
#ifndef STORESTATE_H_
#define STORESTATE_H_
class Store;
class StoreState
{
public:
virtual void Close(Store&);
virtual void Open(Store&);
virtual void Sell(Store&);
};
#endif
The Derived class header file ConcreteStateOpened.h:
#ifndef CONCRETESTATEOPENED_H_
#define CONCRETESTATEOPENED_H_
#include "StoreState.h"
#include "Store.h"
class ConcreteStateOpened : public StoreState
{
public:
ConcreteStateOpened() {}
void Open(Store&) override;
void Close(Store&) override;
void Sell(Store&) override;
};
#endif
The Dervied class cpp file ConcreteStateOpened.cpp:
#include "ConcreteStateOpened.h"
#include <iostream>
using namespace std;
void ConcreteStateOpened::Open(Store &store)
{
cout << store.Name << " is already opened!" << endl;
}
void ConcreteStateOpened::Close(Store &store)
{
store.State = ConcreteStateOpened();
}
void ConcreteStateOpened::Sell(Store &store)
{
std::cout << "Sell Opened";
}
I don't know how to fix this. I tried removing override keywords, aswell as virtual ones. Even removing the definition etc. I just need help from pros :,)
Here are the unresolved external symbol errors:
1>ConcreteStateOpened.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall StoreState::Close(class Store &)" (?Close#StoreState##UAEXAAVStore###Z)
1>Data.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall StoreState::Close(class Store &)" (?Close#StoreState##UAEXAAVStore###Z)
1>StateStore.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall StoreState::Close(class Store &)" (?Close#StoreState##UAEXAAVStore###Z)
1>ConcreteStateOpened.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall StoreState::Open(class Store &)" (?Open#StoreState##UAEXAAVStore###Z)
1>Data.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall StoreState::Open(class Store &)" (?Open#StoreState##UAEXAAVStore###Z)
1>StateStore.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall StoreState::Open(class Store &)" (?Open#StoreState##UAEXAAVStore###Z)
1>ConcreteStateOpened.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall StoreState::Sell(class Store &)" (?Sell#StoreState##UAEXAAVStore###Z)
1>Data.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall StoreState::Sell(class Store &)" (?Sell#StoreState##UAEXAAVStore###Z)
1>StateStore.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall StoreState::Sell(class Store &)" (?Sell#StoreState##UAEXAAVStore###Z)
1>D:\Local-Git-Repos\DesignPatterns\StateC++\StateStore\Debug\StateStore.exe : fatal error LNK1120: 3 unresolved externals

Answer 1

You didn't make the methods in the abstract base pure virtual.
Also you should add : public virtual destructor, protected default constructor and remove copy/move/assignment constructors
#ifndef STORESTATE_H_
#define STORESTATE_H_
class Store;
class StoreState
{
public:
virtual ~StoreState() = default;
StoreState(const StoreState&) = delete;
StoreState(StoreState&&) = delete;
StoreState& operator=(const StoreState&) = delete;
virtual void Close(Store&) = 0;
virtual void Open(Store&) = 0;
virtual void Sell(Store&) = 0;
protected:
StoreState() = default; // prevent accidental creation
};
#endif

Answer 2

Your StoreState isn't actually abstract. You have declared Close, Open, Sell as actual virtual functions with definitions. To declare a virtual function with no definition in that class, use the "pure virtual" syntax = 0.
Also, it's best practice for a polymorphic class type to have a virtual destructor. And per the Rule Of Five, when you declare a destructor, be sure to think about the copy/move constructor and copy/move assignment. For an interface class, it's often best to just make it uncopyable and unassignable.
class StoreState
{
public:
StoreState() = default;
StoreState(const StoreState&) = delete;
StoreState& operator=(const StoreState&) = delete;
virtual ~StoreState() = default;
virtual void Close(Store&) = 0;
virtual void Open(Store&) = 0;
virtual void Sell(Store&) = 0;
};

Answer 3

For starters there is no abstract class in the presented code by you because the class StoreState does not contain pure virtual functions.
From the C++ 20 Standard (11.7.3 Abstract classes)
2 A virtual function is specified as a pure virtual function by using
a pure-specifier (11.4) in the function declaration in the class
definition. [Note: Such a function might be inherited: see below. —
end note] A class is an abstract class if it has at least one pure
virtual function. [Note: An abstract class can be used only as a
base class of some other class; no objects of an abstract class can be
created except as subobjects of a class derived from it (6.2, 11.4). —
end note] A pure virtual function need be defined only if called with,
or as if with (11.4.6), the qualified-id syntax
You need either to provide definitions of the virtual functions declared in the class StoreState or to make them pure virtual functions like for example
class StoreState
{
public:
virtual void Close(Store&) = 0;
virtual void Open(Store&) = 0;
virtual void Sell(Store&) = 0;
};
Even if a function is declared as a pure virtual function nevertheless you may provide its definition (outside the class definition where it is declared as a pure virtual function) if it is required.
Pay attention to that when you have polymorphic classes then you should declare the destructor also as virtual. For example
class StoreState
{
public:
virtual ~StoreState() = default;
virtual void Close(Store&) = 0;
virtual void Open(Store&) = 0;
virtual void Sell(Store&) = 0;
};

Answer 4

You may need to add virtual to the concrete class methods
#ifndef CONCRETESTATEOPENED_H_
#define CONCRETESTATEOPENED_H_
#include "StoreState.h"
#include "Store.h"
class ConcreteStateOpened : public StoreState
{
public:
ConcreteStateOpened() {}
virtual void Open(Store&) override;
virtual void Close(Store&) override;
virtual void Sell(Store&) override;
};
#endif