Currently I am working on a project where I want to control a model train for a nice showcase.
I have multiple locomotives which all have a unique address (just think of it as a UUID). Some locomotives have a headlight, some of them have a flashing light, some have both and some of them have none.
My base class is this:
class GenericLocomotive : public Nameable, public Describable {
private:
uint16_t address;
public:
GenericLocomotive(const char* name, const char* description, uint16_t address);
void setFunction(uint8_t command, bool val);
Now I want to have a different class which provides the functionality to enable and disable the headlight:
class HasHeadLight {
public:
void activateHeadlight();
void deactivateHeadlight();
}
My goal is to have a specific class for every locomotive (with different functionality) which looks something like this:
class <SpecificLocomotive> : public GenericLocomotive, public HasHeadlight, public HasFlashlight,... {
...
}
The problem is, that I must have access to the private field 'address' of my GenericLocomotive class and I also have to call the function setFunction(...) from my HasHeadlight class.
I am quite new to C++ and just found out about the concept of friend classes and methods, but I can not quite get it to work, because even with the declaration of the method setFunction(...) as a friend, I can not just call something like
this->setFunction(HEADLIGHT_COMMAND, true);
from my HasHeadlight-class, because the function is not declared in 'this'.
How can I access the method from my other class? Is this friend thing even needed or is there a completely different way to structure my C++ program?
You have misunderstood how class inheritance works:
Inheritance establishes an is-a relationship between a parent and a child. The is-a relationship is typically stated as as a specialization relationship, i.e., child is-a parent.
There are many ways you can tackle what you want to achieve here, but this is not it. You're on the right track as far as treating the different train components as separate objects, and one way to achieve that would be to instead make each component a member of the specialized locomotive:
class HeadLight {
public:
void activateHeadlight();
void deactivateHeadlight();
}
class SpecialLocomotive : public GenericLocomotive {
HeadLight mHeadlight;
Flashlight mFlashlight;
public:
SpecialLocomotive(const char* name, const char* description, uint16_t address)
: GenericLocomotive(name, description, address) {
setFunction(HEADLIGHT_COMMAND, true);
}
void toggleLight(bool on) {
if (on) {
mHeadlight.activateHeadlight();
} else {
mHeadlight.void deactivateHeadlight();
}
}
/* so on and so forth /*
}
There's not enough details to go further with it. If you need to call setFunction from within Headlight, I would consider that a poor design choice, but there are other ways.
Related
Say I have the following classes:
class Car
{
...
public:
void drive(Direction direction);
void open_door();
void refuel();
double get_speed();
...
};
class SelfDrivingCar: public Car
{
CarDrivingProgram *driving_program;
...
};
class CarDrivingProgram
{
public:
virtual void drive_car(Car *car) = 0;
};
class ExampleCarDrivingProgram: public CarDrivingProgram
{
void drive_car(Car *car) override;
...
};
SelfDrivingCar is a Car in every way except the user is forbidden from manually driving it. Instead, CarDrivingProgram drives it via drive_car(...). Using public inheritance seems correct because most operations on a Car could also happen to a SelfDrivingCar. For example, the user might create an array of Cars and then refuel all the Cars in the array, some of which happen to be SelfDrivingCars.
However, using public inheritance exposes the drive(...) method, which the user should not call on a SelfDrivingCar.
One solution would be to make the drive(...) method private in SelfDrivingCar, but that's messy, requires all CarDrivingPrograms to be friends in order to access drive(...), and can be circumvented if the user casts a SelfDrivingCar to a Car and then calls drive(...) from the Car.
Another solution would be to keep a boolean flag that indicates whether a Car is currently allowed to be driven or not and printing an error if drive(...) is called when the flag is set to "false". For a SelfDrivingCar, this flag would be "false" for most of the time, and CarDrivingProgram would temporarily set this to "true" for the duration of drive_car(...). However, this catches mistakes at runtime rather than at compile time, and the user can still toggle the flag and drive a SelfDrivingCar manually if they really want to.
Using protected/private inheritance would be another solution, but that prevents the user from doing something like adding SelfDrivingCars to an array of Cars that are to be later refuelled (described earlier).
How do I cleanly forbid the user from calling drive() on a SelfDrivingCar, ideally catching things at compile thing?
For clarification: The SelfDrivingCar class still needs a drive(...) function or something like it because CarDrivingProgram needs a way to tell SelfDrivingCar the direction to drive in. CarDrivingPrograms can drive any Car, not just SelfDrivingCars. I want CarDrivingProgram to be able to drive SelfDrivingCars but for the user to be unable to. For example, the user could have an array of pairs of CarDrivingPrograms and Cars, and call carDrivingProgram[i]->drive_car(car[i]) for each pair. I could make all CarDrivingPrograms friends but that's messy.
You could do something like this:
class Car {
...
public:
void open_door();
void refuel();
double get_speed();
...
};
class NormalCar: public Car{
public:
void drive(Direction direction);
};
class SelfDrivingCar: public Car
{
CarDrivingProgram *driving_program;
...
};
This is a very clean way. When there are no reasons you can't introduce another class, this should be fine. You can still store them in an array and refuel them.
When SelfDrivingCar is a Car that can drive, you can make it a Car and call the function (like TheUndeadFish mentioned in his comment). But you can't make a SelfDrivingCar a NormalCar.
How do I allow global functions to have access to private members?
The constraints are that you are not allowed to directly friend the global function in the class declaration. The reason is because I do not want the users to have to see all of these global functions in the header file. The functions themselves are defined in implementation files, and I'd like to keep them hidden there as best as possible.
Now you're probably wondering why I have so many of these global functions. To keep it simple, I'm registering various WNDPROC functions with windows as callbacks, and they must be global. Furthermore, they must be able to update information that is otherwise private to various classes.
I have come up with 2 solutions, but both are a bit sticky.
Solution 1. Make all of the members that need back doors protected rather than private. In the implementation file, declare a class changer that inherits from the original class but provides public getters to protected members. When you need protected members, you can simply cast to the changer class:
//Device.h
class Device{
protected:
std::map<int,int> somethingPrivate;
};
//Device.cpp
DeviceChanger : public Device{
private:
DeviceChanger(){} //these are not allowed to actually be constructed
public:
inline std::map<int,int>& getMap(){ return somethingPrivate; }
};
void foo(Device* pDevice){ ((DeviceChanger*)pDevice)->getMap(); }
Of course, users that inherit this class now have access to the protected variables, but it allows me to at least hide most of the important private variables because they can stay private.
This works because DeviceChanger instances have the exact same memory structure as Device, so there aren't any segfaults. Of course, this is creeping into undefined C++ domain since that assumption is compiler dependent, but all compilers that I care about (MSVC and GCC) will not change the memory footprint of each instance unless a new member variable has been added.
Solution 2. In the header file, declare a friend changer class. In the implementation file, define that friend class and use it to grab private members via static functions.
//Device.h
class DeviceChanger;
class Device{
friend DeviceChanger;
private:
std::map<int,int> somethingPrivate;
};
//Device.cpp
class DeviceChanger{
public:
static inline std::map<int,int>& getMap(Device* pDevice){ return pDevice->somethingPrivate; }
};
void foo(Device* pDevice){ DeviceChanger::getMap(pDevice); }
While this does add a friend to all my classes (which is annoying), it is only one friend which can then forward the information to any global functions that need it. Of course, the users could simply define their own DeviceChanger class and freely change any of the private variables themselves now.
Is there a more accepted way to achieve what I want? I realize I'm trying to sneak around C++ class protections, but I really do not want to friend every global function in every class that needs its private members accessed; it is ugly in the header files and not easy enough to add/remove more functions.
EDIT: Using a mixture of Lake and Joel's answers, I came up with an idea that does exactly what I wanted, however it makes the implementations very dirty. Basically, you define a class with various public/private interfaces, but it's actual data is stored as a pointer to a struct. The struct is defined in the cpp file, and therefore all of it's members are public to anything in that cpp file. Even if users define their own version, only the version in the implementation files will be used.
//Device.h
struct _DeviceData;
class Device {
private:
_DeviceData* dd;
public:
//there are ways around needing this function, however including
//this makes the example far more simple.
//Users can't do anything with this because they don't know what a _DeviceData is.
_DeviceData& _getdd(){ return *dd; }
void api();
};
//Device.cpp
struct _DeviceData* { bool member; };
void foo(Device* pDevice){ pDevice->_getdd().member = true; }
This basically means that each instance of Device is completely empty except for a pointer to some data block, but it lays an interface over accessing the data that the user can use. Of course, the interface is completely implemented in the cpp files.
Additionally, this makes the data so private that not even the user can see the member names and types, but you can still use them in the implementation file freely. Finally, you can inherit from Device and get all of the functionality because the constructor in the implementation file will create a _DeviceData and assign it to the pointer, which gives you all of the api() power. You do have to be more careful about move/copy operations, as well as memory leaks though.
Lake gave me the base of the idea, so I give him credit. Thank you sir!
I usually solve this problem by extracting the application programmer interface in the form of abstract classes, which is the set of types and operations that the application programmer (i.e. the user of your library) will be able to use.
Then, in my implementation, I declare public all methods and types that will be used within my package by other classes.
For example:
API: IDevice.h
Internal: Device.h Device.cpp
I define the API classes in a way similar to:
class IDevice {
public:
// What the api user can do with the device
virtual void useMe() = 0;
};
Then, in my library (not exposed to user interface):
class Device : public IDevice {
public:
void useMe(); // Implementation
void hiddenToUser(); // Method to use from other classes, but hidden to the user
}
Then, for every header(interface) that is part of the API, i will use the IDevice type instead of the Device type, and when internally i will have to use the Device class, i will just cast the pointer down to Device.
Let's say you need a Screen class that uses the class Device, but is completely hidden to the user (and won't therefore have any API abstract class to implement):
#include "Device.h"
class Screen {
void doSomethingWithADevice( Device* device );
}
// Screen.cpp
void Screen::doSomethingWithADevice( Device* device ){
device->hiddenToUser();
}
This way, you don't have to make something private just because you don't want the user to see/use it. You obtain a further layer of abstraction (1 above public) which I call API. You will have:
API // Method/Type visible to the application programmer
public // Method/Type visible to your whole library package, but NOT to the api user
protected // Method/Type visible only to subclasses of the class where it is defined
private // Method/Type local to the defining class
Therefore, you can declare public methods you need to register as callback method, without the user seeing them.
Finally, I deliver the content of API to the user together with the binary, so that the user will have access exactly to what i explicitly defined in the API and nothing else.
You may be asking a specific coding question, but I'd like to take a step back and examine the reason why you'd want to do this, and the solutions to that.
Breaking abstraction
Are you making a decision based on private state?
class Kettle {
private:
int temperatureC;
public:
void SwitchOff();
};
void SwitchOffKettleIfBoiling(Kettle& k) {
if (k.temperatureC > 100) { // need to examine Kettle private state
k.SwitchOff();
}
}
This is relatively bad because the abstraction of Kettle now leaks outside into the SwitchOffKettleIfBoiling function, in the form of coupling to the private temperatureC. This is a bit better:
class Kettle {
private:
int temperatureC;
public:
void SwitchOffIfBoiling() {
if (temperatureC > 100) {
SwitchOff();
}
}
};
void SwitchOffKettleIfBoiling(Kettle& k) {
k.SwitchOffIfBoiling();
}
This practice is called Tell, don't Ask.
Multiple responsibilities
Sometimes you have data that is clearly related but used in different roles. Look at this example:
class Car {
private:
int statusFactor;
public:
void Drive();
};
void DriveSomewhere(Car& c) {
c.Drive();
// ...
}
void ShowOffSomething(const Car &c) {
// How can we access statusFactor, without also exposing it to DriveSomewhere?
}
One way to deal with this is to use interfaces which represent those responsibilities.
class IVehicle {
public:
virtual void Drive() = 0;
};
class IStatusSymbol {
public:
virtual int GetStatusFactor() const = 0;
};
class Car : public IVehicle, public IStatusSymbol {
// ...
};
void DriveSomewhere(IVehicle& v) {
v.Drive();
// ...
}
void ShowOffSomething(const IStatusSymbol &s) {
int status = s.GetStatusFactor();
// ...
}
This pattern is called the Facade pattern. It's useful for maintaining good abstraction without limiting your implementation.
Here's a (very) rough example of pimpl.
//Device.h
class DeviceImpl;
class Device {
public:
Device();
private:
std::unique_ptr<DeviceImpl> pimpl;
};
//Device.cpp
class DeviceImpl {
public:
friend LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
private:
std::map<int,int> somethingPrivate;
};
Device::Device()
: pimpl(new DeviceImpl)
{
}
LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
DeviceImpl* pimpl = reinterpret_cast<DeviceImpl*>(GetWindowLongPtr(hWnd, GWLP_USERDATA));
use(pimpl->somethingPrivate);
// omitting the SetWindowLongPtr that you have to do before calling GetWindowLongPtr,
// but the concept is the same - you'd probably do it in WM_CREATE
}
Now you're probably wondering why I have so many of these global
functions. To keep it simple, I'm registering various WNDPROC
functions with windows as callbacks, and they must be global.
Furthermore, they must be able to update information that is otherwise
private to various classes.
You can use static member functions to do this instead of global functions. Then you can get at the private members just fine. The code would look a bit like this.
class MyClass {
private:
std::string some_data;
static void onEvent( void * user_data );
};
void MyClass::onEvent( void * user_data ) {
MyClass* obj = (MyClass*)(user_data);
std::cout<<some_data<<std::endl;
};
...
register_callback( &MyClass::onEvent, &myClassInstance);
The only issue is then the exposing of the onEvent function name. The solution to that is to extract an interface so that none of your private data or functions are exposed (as IMO leaking the private implementation is about as bad as leaking the names of private functions.)
// Header File.
class IMyClass {
//...
// public stuff goes here
//...
};
// Implementation file.
class MyClass : public IMyClass {
private:
std::string some_data;
static void onEvent( void * user_data );
};
void MyClass::onEvent( void * user_data ) {
MyClass* obj = (MyClass*)(user_data);
std::cout<<some_data<<std::endl;
};
...
register_callback( &MyClass::onEvent, &myClassInstance);
EDIT: Based on some of the responses to other answers it looks like a viable solution would look more like this.
// IUSBDeviceBackend.h (private)
class IUSBDeviceBackend {
public:
virtual void update(USBUpdateData data)=0;
virtual bool resondsTo(USBUpdateCode code)=0
virtual ~IUSBDeviveBackend() {}
};
// IUSBDeviceUI.h (public)
class IUSBDeviceUI {
public:
virtual void showit()=0;
};
// MyDevice.h & MyDevice.cpp (both private)
class MyDevice : public IUSBDeviceBackend, public IUSBDeviceUI {
void update(USBUpdateData data) { dataMap[data.key]=data.value; }
bool resondsTo(USBUpdateCode code) { return code==7; }
void showit(){ ... }
};
// main.cpp
main() {
std::vector<IUSBDeviceBackedn*> registry;
MyDevice dev;
registry.push_back(this);
set_user_data(®istry);
// ...
}
void mycallback(void* user_daya) {
std::vector<IUSBDeviceBackedn>* devices = reinterpret_cast<std::vector<IUSBDeviceBackedn>*>(user_data);
for(unsigned int i=0; i<devices->size(); ++i) {
if( (*devices)[i]->resondsTo( data.code ) ) { (*devices)[i]->update(data); }
}
}
Why not use factory methods to return an interface to your internal class, but still give the globals access to those internal classes? Example:
// IDriver.h public interface:
class IDriver {
public:
virtual int getFoo() = 0;
// ... other public interface methods.
// The implementation of this method will contain code to return a Driver:
static IDriver* getDriver();
};
// Driver.h internal interface (available to WNDPROC functions):
class Driver : public IDriver {
public:
int getFoo(); // Must provide this in the real Driver.
void setFoo(int aFoo); // Provide internal methods that are not in the public interface,
// but still available to your WNDPROC functions
}
// In Driver.cc
IDriver* IDriver::getDriver() { return new Driver(); }
Using this approach, IDriver.h would be a well-known public header, but you would only use Driver.h internally in your own code. This approach is well known and used my many existing C+ libraries (such as Java's JNI) to allow access to native low-level bits of your classes, without exposing it to users.
I use game state manager (intro, main menu, gameplay etc) from here. However there is one problem. A very minimalistic example:
class cApp //manages the states and gives them access to window
{
public:
cApp (RenderWindow & ref) : window(ref) {}
void changeState(cState *); //these function realy doesn't matter
void update();
void draw();
RenderWindow & window; //the same as in the article, this class not only manages state but gives them access to window etc
private:
std::vector <cState *> states;
}
The state:
class cState
{
public:
cState(cApp * ptr) : app(ptr) {}
virtual void update() = 0;
virtual void draw() = 0;
protected:
cApp * app;
}
So far everything is good. The problem is this is the part of basic framework. So the cApp is only very basic and gives access only to window. However there may be the case where the user wants to use networking in his game. Network engine isn't a part of single state so it must be at the more global (that means, cApp) level.
So the user does:
class cNetworkedApp : public cApp
{
public:
cNetworkedApp(RenderWindow & ref1, NetworkEngine & ref2)
: networking(ref2), cApp(ref1)
NetworkEngine & networking; //initialized in cNetworkedApp constructor
}
class CharacterCreationState : public cState
{
CharacterCreationState(cApp * ptr) : cState(ptr) {}
//implement pure virtual functions
void draw()
{}
void update()
{
//THE PROBLEM
//the state needs to access the network engine so casting is required
cNetworkedApp * ptr = static_cast<cNetworkedApp*>(app))
ptr->networking.sendSomething();
}
}
The only obvious solution is to include everything what may be possible in cApp, however as I said this is a framework. Of course some engines like physics engine or sound engine are things which you put into a state so that's not problem, but things like networking system must be the one object available for all states. And not every app uses it.
Do I need to redesign this code or is it okay?
Your cApp may keep with it a named list of a polymorphic type Engine, ie map<string,Engine*>, then, your user may ask the cApp if it has a given engine.
NetworkEngine would be a subclass of the pure abstract Engine.
Update
When dealing with a pointer that I you are sure that it is of the given specialized type, you should use static_cast, when you wanna query if the pointer can be casted to a type you should use dynamic_cast.
I, myself, have a safer approach for the first case, I use an assertion to guarantee that the type can be casted and use the static_cast in normal code:
Engine* fetchedEngine = cApp.fetch("network");
assert( dynamic_cast<NetworkEngine*>(fetchedEngine) != NULL );
NetworkEngine* network = static_cast<NetWorkEngine*>(fetchedEngine);
Only a object of type NetworkEngine should be putted on the "network" name, but maybe someone mistakenly put something else, the assert will make us safer without needing to worry about the overhead.
The qml viewer (for 4.8 and 5.0) is implemented like that:
In the .h(eader) we have:
class QtQuick2ApplicationViewer : public QQuickView
{
Q_OBJECT
...
private:
class QtQuick2ApplicationViewerPrivate *d;
};
Then in the .CPP file:
class QtQuick2ApplicationViewerPrivate
{
QString mainQmlFile;
friend class QtQuick2ApplicationViewer;
static QString adjustPath(const QString &path);
};
QtQuick2ApplicationViewer::QtQuick2ApplicationViewer(QWindow *parent)
: QQuickView(parent)
, d(new QtQuick2ApplicationViewerPrivate())
{
connect(engine(), SIGNAL(quit()), SLOT(close()));
setResizeMode(QQuickView::SizeRootObjectToView);
#ifdef Q_OS_ANDROID
engine()->setBaseUrl(QUrl::fromLocalFile("/"));
#endif
}
Why is using friend necessary here? I don't see any reason why would anybody use a friend class. Is there any real use for friend classes (except for exotics that anybody could live without)?
.h
#include
class QtQuick2ApplicationViewer : public QQuickView
{
Q_OBJECT
public:
explicit QtQuick2ApplicationViewer(QWindow *parent = 0);
virtual ~QtQuick2ApplicationViewer();
void setMainQmlFile(const QString &file);
void addImportPath(const QString &path);
void showExpanded();
private:
class QtQuick2ApplicationViewerPrivate *d;
};
.cpp
#include "qtquick2applicationviewer.h"
#include <QtCore/QCoreApplication>
#include <QtCore/QDir>
#include <QtQml/QQmlEngine>
class QtQuick2ApplicationViewerPrivate
{
QString mainQmlFile;
friend class QtQuick2ApplicationViewer;
static QString adjustPath(const QString &path);
};
QString QtQuick2ApplicationViewerPrivate::adjustPath(const QString &path)
{
#ifdef Q_OS_UNIX
#ifdef Q_OS_MAC
if (!QDir::isAbsolutePath(path))
return QString::fromLatin1("%1/../Resources/%2")
.arg(QCoreApplication::applicationDirPath(), path);
#elif !defined(Q_OS_ANDROID)
const QString pathInInstallDir =
QString::fromLatin1("%1/../%2").arg(QCoreApplication::applicationDirPath(), path);
if (QFileInfo(pathInInstallDir).exists())
return pathInInstallDir;
#endif
#endif
return path;
}
QtQuick2ApplicationViewer::QtQuick2ApplicationViewer(QWindow *parent)
: QQuickView(parent)
, d(new QtQuick2ApplicationViewerPrivate())
{
connect(engine(), SIGNAL(quit()), SLOT(close()));
setResizeMode(QQuickView::SizeRootObjectToView);
#ifdef Q_OS_ANDROID
engine()->setBaseUrl(QUrl::fromLocalFile("/"));
#endif
}
QtQuick2ApplicationViewer::~QtQuick2ApplicationViewer()
{
delete d;
}
void QtQuick2ApplicationViewer::setMainQmlFile(const QString &file)
{
d->mainQmlFile = QtQuick2ApplicationViewerPrivate::adjustPath(file);
setSource(QUrl::fromLocalFile(d->mainQmlFile));
}
void QtQuick2ApplicationViewer::addImportPath(const QString &path)
{
engine()->addImportPath(QtQuick2ApplicationViewerPrivate::adjustPath(path));
}
void QtQuick2ApplicationViewer::showExpanded()
{
#if defined(Q_WS_SIMULATOR)
showFullScreen();
#else
show();
#endif
}
Friends examine friends' privates. You sure can do without access restrictions at all, but once you use it, being friendly helps in intimate situations.
class Me;
class You {
friend class Me;
private:
Home _home;
Car _car;
public:
void bar(Me my);
};
class Me {
Stuff _stuff;
public:
foo(You you) {
//If you consider me a friend
you._home.enter(); //I can enter your `private _home`
you._car.drive(); //I can drive your `private _car`.
}
};
void You::bar(Me my) {
my.stuff //this is an error because I don't consider you a friend so you can't touch my `private _stuff`.
}
Knowing you can always count on me, for sure. That's what friends are for. http://www.youtube.com/watch?v=xGbnua2kSa8
But I guess you're asking about friend classes in C++.
The whole point of "scope" is to define exactly who can see what in another class. You don't "need friends" any more than you need "protected" or "private", in the sense that you could make everything in all your classes public, and your program would successfullly compile and run. But the idea is to establish -- and document -- exactly what is the public interface of a class, and thus cannot be changed without considering the impact on other classes, and what is an internal implementation, which can be freely re-worked or re-organized without fear of impacting other classes.
So the point of a "friend" is to say: Hey, I have this class X, and this other class Y. And in general other classes don't need to know how X goes about doing it's job. But Y interacts with some low-level thing in X, so it needs to see it. Thus I make Y a friend of X. Like, I have an Investor class that has a function that (presumably among other things) has a function to calculate the total amount of a customer's investments. In general, other classes shouldn't care how I do that calculation: they just want the total. But now I have a TaxReporting class that needs to know how much of that balance is in taxable securities and how much is in non-taxable securities. Maybe I don't want to make these functions public because the information is confidential and I want to limit access for real-world privacy reasons. More often, I don't want to make it public because the calculation is tricky or subject to frequent change, and I want to keep tight control on what classes access it to limit the problems caused when things change. So I make TaxReporting a friend so it can access some functions that make the distinction, without opening these to the world.
In practice, when I was doing C++ I rarely used friends. But "rarely" is not "never". If you find yourself saying, "Oh, I have to make this public just so this one other class can see it", then maybe instead of making it public you should make a friend.
"friend" is super useful and something you want to use all the time.
Typical use cases are:
You have a class that uses subclasses where the subclass is allowed to use private functions of the class that owns the subclasses:
class ManagedObject
{
public:
void doStuff() { mMgr->updateManager(); }
private:
Manager* mMgr;
};
class Manager
{
friend ManagedObject;
public:
ManagedObject* createManagedObject();
private:
void updateManager() { }
};
So in this case you have a class that creates and deals with "managedObject". Whenever this object is manipulated it needs to update the object that created it. You want users of your class to know that they don't ever need to call "updateManager" and in fact wat to generate a compile time error if they do.
Another common case is when you have a function which acts like a class member but cannot for some reason be a class member. An example is operator<<. If you write your own io stream class, or if you want to create a serialization system that users operator<<:
class serializedObject
{
public:
friend Serializer& operator<< ( Serializer& s, const serializedObject& obj );
protected:
u32 mSecretMember;
};
Serializer& operator<<( Serializer& s, serializedObject& obj )
{
serializer << obj.mSecretMember;
return s;
}
In this case the serialization function cannot be a member of serializedObject, but needs to look at the internals of serializedObject to serialize it. You will see similar patterns of you create other operators ( like addition ) where the RHS of the operator is not the same class as the LHS
In Qt, there is something called a 'guarantee of binary compatibility', which means that your app can run against Qt4.8, 4.8.1, and 4.8.2 and so forth without recompiling.
In order to achieve this the vtable for objects cannot change. So, Qt classes are written using the "PIMPL" (pointer to implementation) idiom.
The "Private" class is the PRIVATE implementation of the public class - it is an implementation detail of QtQuick2ApplicationViewer. No one in the whole world knows about the private class except the public class. These two classes are deeply intertwined by design. In fact, they are really different aspects of a single object that has been partitioned c++ wise in order to achieve the binary compatibility guarantee.
It is reasonable in this context that the private class can access the public class.
2) In this context quit is not QApplication::quit(), that is slot of cause, but some internal signal of engine().
I feel like the answer to this question is really simple, but I really am having trouble finding it. So here goes:
Suppose you have the following classes:
class Base;
class Child : public Base;
class Displayer
{
public:
Displayer(Base* element);
Displayer(Child* element);
}
Additionally, I have a Base* object which might point to either an instance of the class Base or an instance of the class Child.
Now I want to create a Displayer based on the element pointed to by object, however, I want to pick the right version of the constructor. As I currently have it, this would accomplish just that (I am being a bit fuzzy with my C++ here, but I think this the clearest way)
object->createDisplayer();
virtual void Base::createDisplayer()
{
new Displayer(this);
}
virtual void Child::createDisplayer()
{
new Displayer(this);
}
This works, however, there is a problem with this:
Base and Child are part of the application system, while Displayer is part of the GUI system. I want to build the GUI system independently of the Application system, so that it is easy to replace the GUI. This means that Base and Child should not know about Displayer. However, I do not know how I can achieve this without letting the Application classes know about the GUI.
Am I missing something very obvious or am I trying something that is not possible?
Edit: I missed a part of the problem in my original question. This is all happening quite deep in the GUI code, providing functionality that is unique to this one GUI. This means that I want the Base and Child classes not to know about the call at all - not just hide from them to what the call is
It seems a classic scenario for double dispatch. The only way to avoid the double dispatch is switching over types (if( typeid(*object) == typeid(base) ) ...) which you should avoid.
What you can do is to make the callback mechanism generic, so that the application doesn't have to know of the GUI:
class app_callback {
public:
// sprinkle const where appropriate...
virtual void call(base&) = 0;
virtual void call(derived&) = 0;
};
class Base {
public:
virtual void call_me_back(app_callback& cb) {cb.call(*this);}
};
class Child : public Base {
public:
virtual void call_me_back(app_callback& cb) {cb.call(*this);}
};
You could then use this machinery like this:
class display_callback : public app_callback {
public:
// sprinkle const where appropriate...
virtual void call(base& obj) { displayer = new Displayer(obj); }
virtual void call(derived& obj) { displayer = new Displayer(obj); }
Displayer* displayer;
};
Displayer* create_displayer(Base& obj)
{
display_callback dcb;
obj.call_me_back(dcb);
return dcb.displayer;
}
You will have to have one app_callback::call() function for each class in the hierarchy and you will have to add one to each callback every time you add a class to the hierarchy.
Since in your case calling with just a base& is possible, too, the compiler won't throw an error when you forget to overload one of these functions in a callback class. It will simply call the one taking a base&. That's bad.
If you want, you could move the identical code of call_me_back() for each class into a privately inherited class template using the CRTP. But if you just have half a dozen classes it doesn't really add all that much clarity and it requires readers to understand the CRTP.
Have the application set a factory interface on the system code. Here's a hacked up way to do this. Obviously, apply this changes to your own preferences and coding standards. In some places, I'm inlining the functions in the class declaration - only for brevity.
// PLATFORM CODE
// platformcode.h - BEGIN
class IDisplayer;
class IDisplayFactory
{
virtual IDisplayer* CreateDisplayer(Base* pBase) = 0;
virtual IDisplayer* CreateDisplayer(Child* pBase) = 0;
};
namespace SystemDisplayerFactory
{
static IDisplayFactory* s_pFactory;
SetFactory(IDisplayFactory* pFactory)
{
s_pFactory = pFactory;
}
IDisplayFactory* GetFactory()
{
return s_pFactory;
}
};
// platformcode.h - end
// Base.cpp and Child.cpp implement the "CreateDisplayer" methods as follows
void Base::CreateDisplayer()
{
IDisplayer* pDisplayer = SystemDisplayerFactory::GetFactory()->CreateDisplayer(this);
}
void Child::CreateDisplayer()
{
IDisplayer* pDisplayer = SystemDisplayerFactory::GetFactory()->CreateDisplayer(this);
}
// In your application code, do this:
#include "platformcode.h"
class CDiplayerFactory : public IDisplayerFactory
{
IDisplayer* CreateDisplayer(Base* pBase)
{
return new Displayer(pBase);
}
IDisplayer* CreateDisplayer(Child* pChild)
{
return new Displayer(pChild);
}
}
Then somewhere early in app initialization (main or WinMain), say the following:
CDisplayerFactory* pFactory = new CDisplayerFactory();
SystemDisplayFactory::SetFactory(pFactory);
This will keep your platform code from having to know the messy details of what a "displayer" is, and you can implement mock versions of IDisplayer later to test Base and Child independently of the rendering system.
Also, IDisplayer (methods not shown) becomes an interface declaration exposed by the platform code. Your implementation of "Displayer" is a class (in your app code) that inherits from IDisplayer.