I'm writing a library for parsing expressions into a tree structure and I have an abstract type QCExpressionNode as my base class. It looks like this:
#ifndef QCEXPRESSIONNODE_H
#define QCEXPRESSIONNODE_H
#include <QString>
class QCExpressionNode
{
public:
virtual ~QCExpressionNode() {}
virtual float evaluate(float* x) = 0;
virtual bool containsVariable() = 0;
virtual QString infixNotation() = 0;
};
Q_DECLARE_INTERFACE(QCExpressionNode, "org.nathanmoos.qcalc.libexprtree-qt.QCExpressionNode/0.1")
#endif // QCEXPRESSIONNODE_H
When I compile some tests (another project in QtCreator) that work on subclasses (QCConstantNode, QCVariableNode, QCBinaryOperatorNode, and so on), the linker gives me a 'undefined reference to vtable' error for QCExpressionNode. What am I doing wrong?
#include <QtPlugin>, then the file should compile just fine.
Q_DECLARE_INTERFACE is declared in QtPlugin.
By the way: it's quite unusual to compile header files by themselves and it's unusual to have include guards outside header files.
Related
I have a question in regards to building a project in C++ with abstract classes.
Suppose you have an abstract class A in a header file and then you have class B in a cpp file to create an object from class A. So far we have:
Class_A.h, Class_B.cpp
Within the Class_B.cpp file, I have included the header Class_A.h.
I want to now create objects of Class_B in a main file, for example: main.cpp. I added the header Class_A.h in main.cpp and when compiled, I get 'Class_B’ was not declared in this scope as an error. All files are in the same directory. I'm not too sure how to fix this or whether or not I am not using abstract classes correctly. All the examples I found online did something along the lines of: defining Class_B inside main.cpp without creating a separate cpp file for Class_B.
Example:
Abstract class: Car.h
Instance of abstract class: Toyota.cpp
main.cpp:
#include 'Car.h'
...
Toyota new_car = new Toyota(...);
Just need to organize your code. There's no hard and fast rules about how that needs to be done, but generally you define classes in header files (.h or .hpp) and implement the class in source files (.cpp). In this situation I would:
Class_A.h
class Class_A
{
public:
virtual void myFunc() = 0; // Class_A is abstract
...
}
Class_B.h
#include "Class_A.h"
class Class_B : public Class_A
{
public:
void myFunc();
...
}
Class_B.cpp
Class_B::myFunc()
{
...
}
main.cpp
#include "Class_B.h"
int main()
{
Class_B myClassB;
myClassB.myFunc();
...
return 0;
}
And of course you need to feed all the source files to your compiler correctly, at least Class_B.cpp and main.cpp in this case.
I'm comparatively new to C++ so I tested some things out in Xcode, and found a really weird thing.
This is my 'Testing.h' file
#ifndef Testing_h
#define Testing_h
class Testing{
private:
int a;
public:
Testing(int a=3);
void hey(int b);
};
#endif
This is my 'Testing.cpp' file
#include "Testing.h"
Testing::Testing(int a){
a = 4;
}
And finally, this is the 'main.cpp' file
#include <iostream>
#include "Testing.h"
using namespace std;
int main(){
Testing a;
//Apparently not completing the definitions of every abstract methods in the class is not a problem
}
I only declared 'void hey(int b)' in 'Testing.h' but have not defined it in 'Testing.cpp'. So I was wondering how it is possible for the compiler to successfully compile the 'main.cpp' without having enough information of 'void hey(int b)'. Thanks in advance!
Because you never require there to be a definition for hey().
You can require a definition by calling it, for example :
a.hey(42);
And you'll see that the linker isn't too happy because hey is an undefined reference.
Testing a;//Apparently not completing the definitions of every abstract methods in the class is not a problem
You defined constructor with default value a=3 but calling both constructor argument and class parameter the same name is bad practice.
Instead you can write this:
//Testing.h
#ifndef Testing_h
#define Testing_h
using namespace std;
class Testing{
private:
int number;
public:
Testing(int a=3): number(a = 4){}//it's the same as your implementation in cpp file
void hey(int b);
int getNumber() {return number;}
};
#endif
//main.cpp
#include <iostream>
#include "Testing.h"
int main()
{
Testing object;
cout<<object.getNumber();// returns 4
return 0;
}
And why hey compiles?
During building your project compiler translates your source code into object code by verifying the syntax. After that process linker checks the definitions marked by whole phrases. Source code is compiled from each file provided. Linker doesn't care for the implementation presence, it only looks it up if a method is used by the program. So even without implementation of hey your program compiles.
Last remark
It's discouraged to include .cpp files use headers instead. Sometimes you can get yourself into multiple definitions of the same functions causing compiler errors.
First: My English is not that good yours is. Excuse me.
I'm using Ubuntu (I don't know if this is important) and I had issues with Code::Blocks since I started to use it. But I fixed them by re-opening the program. But now, I get a really crazy error when compiling the code. I included a file just like usual:
#include "GameObjectUtility.h"
and I used the class "GameObjectUtility" to declare a member object, just like this:
class GameObject
{
std::vector<GameObjectUtility> uts;
// Error here:
// GameObjectUtility was not declared in this scope
}
So, is this my fault or is there something buggy with Code::Blocks?
And, additionally, is there a way of saying to the Linker: First execute this file and then the other?
Thank you for your answers!
EDIT: .h and .ccp file GameObjectUtility:
So this is GameObjectUtility.h:
#ifndef GAMEOBJECTUTILITY_H
#define GAMEOBJECTUTILITY_H
#include <string>
#include "Collision.h"
class GameObjectUtility
{
public:
GameObjectUtility();
virtual ~GameObjectUtility();
virtual void Update() = 0;
virtual void LateUpdate() = 0;
virtual void FixedUpdate() = 0;
static void SendMsg(std::string msg);
protected:
private:
virtual void GetMsg(std::string msg) = 0;
};
#endif // GAMEOBJECTUTILITY_H
And in GameObjectUtility.cpp are just two empty definitions of constructor and destructor
Since class GameObjectUtility is pure virtual, you cannot instantiate it.
You can only store std::vector<GameObjectUtility*> in class GameObject.
I checked out a post similar to this but the linkage was different the issue was never resolved. The problem with mine is that for some reason the linker is expecting there to be a definition for the base class, but the base class is just a interface. Below is the error in it's entirety
c:\users\numerical25\desktop\intro todirectx\godfiles\gxrendermanager\gxrendermanager\gxrendermanager\gxdx.h(2) : error C2504: 'GXRenderer' : base class undefined
Below is the code that shows how the headers link with one another
GXRenderManager.h
#ifndef GXRM
#define GXRM
#include <windows.h>
#include "GXRenderer.h"
#include "GXDX.h"
#include "GXGL.h"
enum GXDEVICE {
DIRECTX,
OPENGL
};
class GXRenderManager {
public:
static int Ignite(GXDEVICE);
private:
static GXRenderer *renderDevice;
};
#endif
at the top of GxRenderManager, there is GXRenderer , windows, GXDX, GXGL headers. I am assuming by including them all in this document. they all link to one another as if they were all in the same document. correct me if I am wrong cause that's how a view headers. Moving on...
GXRenderer.h
class GXRenderer {
public:
virtual void Render() = 0;
virtual void StartUp() = 0;
};
GXGL.h
class GXGL: public GXRenderer {
public:
void Render();
void StartUp();
};
GXDX.h
class GXDX: public GXRenderer {
public:
void Render();
void StartUp();
};
GXGL.cpp and GXDX.cpp respectively
#include "GXGL.h"
void GXGL::Render()
{
}
void GXGL::StartUp()
{
}
//...Next document
#include "GXDX.h"
void GXDX::Render()
{
}
void GXDX::StartUp()
{
}
Not sure whats going on. I think its how I am linking the documents, I am not sure.
The problem is You need to have #include "GXRenderer.h" at the top of both: GXGL.h and also GXDX.h.
The base type must be defined not just declared before defining a derived type.
By the way, the error is a compiling error not linking error.
Edit: About your class type redefinition:
at the top of every header file you should have #pragma once.
The #pragma once directive specifies that the file will be included at most once by the compiler in a build.
You included them all into GXRenderManager.h, meaning that GXRenderManager.h is OK.
But you forgot to include them all into GXGL.cpp and GXDX.cpp. In these .cpp files GXRenderer class is completely unknown.
There are at least two "schools" of #include strategies. One says that header file must include everything that is needed for its own compilation. That would mean that GXGL.h and GXDX.h must include GXRenderer.h. If you followed that strategy, your GXGL.cpp and GXDX.cpp would be OK as they are now.
Another "school" says that header files must not include each other at all, i.e. all inclusions must be done through .cpp files. At first sight one could guess that your GXGL.h and GXDX.h follow that strategy (since you are not including anything into them), but then your GXRenderManager.h looks completely different.
You need to decide which strategy you are trying to follow and follow it. I'd recommend the first one.
I got an error C2504: 'CView' : base class undefined
where CView is not directly my base class from which I am inheriting.
I am inherting mYClass from MScrollView, "for this matter any class which is not actual Base Class is what the point is to be noted down here"
but the error is the C2504. When I have included it in the header where this problem is arising, this problem is resolved.
#include "stdafx.h"
where stdafx.h has #include which contains all the basic class defined...hope this answer resolves everyone who are facing this issue.
My CGameStateLogo which inherit from CGameState:
CGameState.h
#pragma once
#include "GameMain.h"
#include "MyBitmap.h"
class CGameMain;
class CMyBitmap;
class CGameState
{
public:
CMyBitmap* pbmCurrent;
CGameMain* pGM;
int GameStateID;
virtual void MessageEnter () = 0;
virtual void MessageUpdate( int iKey ) = 0;
virtual void MessagePaint( HDC* pDC ) = 0;
void StateHandler ( int msg, HDC* pDC, int key );
public:
CGameState(void);
~CGameState(void);
};
After creating and finding, problem comes from here :
I've created 2 classes: CTest and CGameStateLogo
#pragma once
#include "GameState.h"
class CTest:CGameState
{
public:
CTest(void);
~CTest(void);
};
#pragma once
#include "GameState.h"
class CGameStateLogo:CGameState // Bug at this line
{
public:
CGameStateLogo(void);
~CGameStateLogo(void);
};
Do VS has problem in naming ?
Thanks for reading this :). Things go WEIRD, I'll update my question later. Sorry for wasting your time .
pGameStates.push_back( (CGameState*)gameLogo );
Since CGameStateLogo inherits publically from CGameState, the cast is unneccesary. Upcasts are implicit. Simply write
pGameStates.push_back( gameLogo );
instead.
This does probably not solve your compile troubles, though. As to that, you seem to be mixing up GameState.h and CGameState.h. If that's an actual error in your code, instead of just a copy/paste mistake while writing your question, it could cause this problem.
It could also be that there is some circular dependency problem in your headers. You write
#include "GameState.h"
which should define the GameState class, unless the file has already been included once, but the class definition has not yet been read. This could happen for example in this way:
Some .cpp file includes GameState.h.
GameState.h includes GameMain.h.
GameMain.h includes GameStateLogo.h.
GameStateLogo.h includes GameState.h, but this has already been included, so the include is ignored due to #pragma once.
Generally, such circular dependencies in header files are a thing to avoid.
CGameStateLogo.h is including GameState.h and not CGameState.h. Is it possible you have a file called GameState.h on the include path (and hence you wouldn't get an error about not being able to include GameState.h)?