Implementing functions outside of sub class causing unknown error

Question

I am trying to write a basic gui library in c++ and i am having problems with seemingly basic inheritence. I have a base class Component here delcared in Component.h

class Component
{
public:
    virtual void add(Component &c);
    virtual void remove(Component &c);
    virtual void setBounds(int x, int y, int width, int height);
    virtual void setLocation(int x, int y);
    virtual void setSize(int width, int height);
    virtual void setVisible(bool b);
};

I also have a subclass frame declared in the same header shown here

class Frame : public Component
{
private:
    char* ftitle;
    HWND* hwnd;

public:
    Frame();
    Frame(char* title);
    void add(Component &c);
    void remove(Component &c);
    void setBounds(int x, int y, int width, int height);
    void setLocation(int x, int y);
    void setSize(int width, int height);
    void setVisible(bool b);
    void setTitle(char* title);
};

And I implement this classes functions in another file named Frame.cpp shown here

#include "Component.h"

Frame::Frame()
{
    Frame("");
}

Frame::Frame(char* title)
{
    ftitle = title;
    *hwnd = CreateWindow("static", title, WS_OVERLAPPEDWINDOW, 0, 0, 100, 100, NULL, NULL, GetModuleHandle(NULL), NULL);
}

void Frame::setVisible(bool visible)
{
    if(visible)
    {
        ShowWindow(*hwnd, SW_SHOW);
    }
    else
    {
        ShowWindow(*hwnd, SW_HIDE);
    }
}

void Frame::add(Component &c){}
void Frame::remove(Component &c){}
void Frame::setBounds(int x, int y, int width, int height){}
void Frame::setLocation(int x, int y){}
void Frame::setSize(int width, int height){}
void Frame::setTitle(char* title){}

However when I try to compile and build the project i get several errors shown like this

1>------ Build started: Project: GUI, Configuration: Debug Win32 ------
1>  Frame.cpp
1>  Generating Code...
1>  Compiling...
1>  Main.cpp
1>  Generating Code...
1>Frame.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Component::add(class Component &)" (?add@Component@@UAEXAAV1@@Z)
1>Frame.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Component::remove(class Component &)" (?remove@Component@@UAEXAAV1@@Z)
1>Frame.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Component::setBounds(int,int,int,int)" (?setBounds@Component@@UAEXHHHH@Z)
1>Frame.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Component::setLocation(int,int)" (?setLocation@Component@@UAEXHH@Z)
1>Frame.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Component::setSize(int,int)" (?setSize@Component@@UAEXHH@Z)
1>Frame.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Component::setVisible(bool)" (?setVisible@Component@@UAEX_N@Z)
1>C:\Users\Owner\Documents\Visual Studio 2012\Projects\GUI\Debug\GUI.exe : fatal error LNK1120: 6 unresolved externals
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

juanchopanza · Accepted Answer · 2014-02-13 15:53:05Z

3

The linker is complaining about a lack of implementations for your Component class methods, because they are not pure virtual. You can fix this by making them pure virtual:

virtual void add(Component &c) = 0;

and so on.

Or alternatively, provide implementations.

Note that you should also give Component a virtual destructor.

answered Feb 13, 2014 at 15:53

juanchopanza

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