Best practices to implement a Payload-containing class in C++? - c++

I have a question about hierarchy, references and pointers... The question comes to my mind when I had tried to do the following stuff:
class packet {
public:
int address;
int command; /**< Command select the type of Payload that I must decode */
Payload p; /**< Generic payload, first question:
Payload p or Payload * p or Payload &p ?
I have a background in C, for this reason I prefer
Payload p but I know that this is not recommended for C++ */
private:
/** All getter and setter for attributes */
/** Second question: What is the best way to implement a getter
and setter for Payload?... I prefer something
similar to Java if this is possible */
}
Now imagine that I have a lot of types of Payload, all these payloads are children of the super class (generic) Payload.
I want to read the header and switch o the command. For example, if command is 1 I create a PayloadReset : Payload and fill in all of its attributes, then I want to set on my packet this payload (up-casting). In other part of the program I want to read my current packet and then read the command field and down-cast to the appropriate type depending on the command field.
When I tried to do this, I could do the up-casting without problems but the problem comes when I tried to do the downcasting to the specific Payload, in our example PayloadReset.

To answer the first question (which was buried inside the comments in your first code example:
Payload *p;
The first thing you need to learn as part of your transition from Java to C++ is what pointers are and how they work. What will be confusing to you, for some time, is the fact that all objects in Java are really pointers. You never needed to know that, when working with Java. But you must know that now, in order to understand C++. So, declaring a C++ class as
Payload p;
Is not the same thing as making a similar declaration in Java. There is no equivalent to this declaration in Java. In Java you really have a pointer here, and you have to instantiate it using the new keyword. That part Java originally aped from C++. This is the same process as C++, except that you have to explicitly declare it as a pointer.
Payload *p;
Then, somewhere else, using your example of a PayloadReset subclass:
class PayloadReset : public Payload { /* Class declaration */ };
PayloadReset *r = new PayloadReset( /* Constructor argument */ };
p=r;
And the second thing you need to learn as part of your transaction from Java to C++ is when, and how, to delete all instantiated objects. You don't have Java's garbage collector here. This becomes your job, now.

Tagging onto Sam's answer.
Before you go any further, learn the difference between stack and heap allocation. In the example you posted, you're allocating your Payload p; object on the stack - implying that the size of the object is known at this point and said size will be allocated on the stack. If you wanted to assign an derived object to p, it wouldn't work, because said object will likely be of different size. This is why you instead declare a pointer to the object (8 bytes on 64-bit architecture, 4 bytes on 32 bit), and then when you know which type of derived object you want to allocate, you do it using the new operator, as such:
Payload *p;
p = new PayloadReset(...);
The above method would require manually managing memory, i.e. calling delete on the new allocated pointer. As of C++11, the recommendation is to use smart pointers from the <memory> header. These are essentially reference counted pointers that automatically call delete for you.
std::shared_ptr<Payload> p;
p = std::make_shared<PayloadReset>(...);

Your question is somewhat related to Java syntax, but mostly about Object Oriented Programming.
First of all, you should take a moment to get familiar with Java naming conventions. There are commonly used recommendations that you can find all over the web. Here is one example of Java Naming Conventions. I brought this up because single variable names is generally not a good idea and having descriptive variables names pays dividends as the program grows in size and especially if there are more than one person on a team. So, instead of Payload p use Payload payload.
Secondly, in OO (Object Oriented), it is best to always keep your Class instance variables private, not public. Give access to these variables only if necessary and shield access to them by providing public methods. So, in your example of class Packet, your public/private is backwards. Your class should look more like:
public class Packet{
//private fields
private int address;
private int command;
private Payload payload;
//Maybe provide a nice constructor to take in expected
//properties on instantiation
public Packet(Payload pay){
}
//public methods - as needed
public void getPayload(){
return this.payload;
}
public void setAddress(int addy){
this.address = addy;
}
public int getCommand(){
return this.command;
}
}
Also, to answer more of your question about the naming of Payload. Like i said earlier..use descriptive names. Java does not have pointer references like C and generally handles memory management for you, so the & is not required or supported.
Your last question/topic is really again about OO and Class heirarchy.
It seems that Payload would be a generic base class and you may have multiple, specific 'Payload types', like ResetPayload. If that is the case, you would then define Payload and create the ResetPayload class that extends Payload. I'm not sure exactly what you are trying to do, but think of Classes/objects ad nouns and methods as verbs. Also think about the 'is-a' and 'has-a' concept. From what I see, maybe all Payloads 'has-acommand and an address. Also, maybe eachPayloadalso has multiplePackets, whatever. Just as an example, you would then define yourPayload` class like this:
public class Payload{
private int address;
private int command;
private List<Packet> packets = new ArrayList<>();
public Payload(int addy, int comm){
this.address = addy;
this.command = comm;
}
public void addPacket(Packet p){
packets.add(p);
}
public List<Packet> getPackets(){
return this.packets;
}
public int getCommand(){
return this.command;
}
public int getAddress(){
return this.address;
}
}
Then if you had a type of Payload that is more specific, like Reset, you would create the class, extends Payload and provide the additional properties/operations specific to this type, something this like:
public class ResetPayload extends Payload{
public ResetPayload(int addy, int comm){
super(addy, comm);
}
public void reset(){
//Do stuff here to reset the payload
}
}
Hopefully, that answers your questions and moves you along further. Good luck.

Here is my take on the general problem, it extends the tagged union idea. Advantages are 1.) no inheritance/dynamic_cast 2.) no shared ptr 3.) POD 4.) rtti is used to generate unique tags:
using cleanup_fun_t = void(*)(msg*);
class msg
{
public:
template<typename T, typename... Args>
static msg make(Args&&... args);
private:
std::type_index tag_;
mutable std::atomic<cleanup_fun_t> del_fn_; // hell is waiting for me,
uint64_t meta_;
uint64_t data_;
};
Please fill in all the nice member functions. This class is move only. You are creating messages with payload by the static member function make:
template<typename T, typename... Args>
msg msg::make(Args&&... args)
{
msg m;
m.tag_ = typeid(T);
m.del_fn_ = nullptr;
if (!(std::is_empty<T>::value))
{
auto ptr = std::make_unique<T>(std::forward<Args>(args)...);
m.data_ = (uint64_t)ptr.release();
m.del_fn_ = &details::cleanup_t<T>::fun; // deleter template not shown
}
return m;
}
// creation:
msg m = msg::make<Payload>(params passed to payload constructor);
// using
if (m.tag() == typeid(Payload))
{
Payload* ptr = (Payload*)m.data;
ptr-> ...
}
Just check the tag if it contains your expected data (type) and cast the data to a pointer type.
Disclaimer: It is not the complete class. Some access member function are missing here.

Related

A better design pattern than factory?

In the code I am now creating, I have an object that can belong to two discrete types, differentiated by serial number. Something like this:
class Chips {
public:
Chips(int shelf) {m_nShelf = shelf;}
Chips(string sSerial) {m_sSerial = sSerial;}
virtual string GetFlavour() = 0;
virtual int GetShelf() {return m_nShelf;}
protected:
string m_sSerial;
int m_nShelf;
}
class Lays : Chips {
string GetFlavour()
{
if (m_sSerial[0] == '0') return "Cool ranch";
else return "";
}
}
class Pringles : Chips {
string GetFlavour()
{
if (m_sSerial.find("cool") != -1) return "Cool ranch";
else return "";
}
}
Now, the obvious choice to implement this would be using a factory design pattern. Checking manually which serial belongs to which class type wouldn't be too difficult.
However, this requires having a class that knows all the other classes and refers to them by name, which is hardly truly generic, especially if I end up having to add a whole bunch of subclasses.
To complicate things further, I may have to keep around an object for a while before I know its actual serial number, which means I may have to write the base class full of dummy functions rather than keeping it abstract and somehow replace it with an instance of one of the child classes when I do get the serial. This is also less than ideal.
Is factory design pattern truly the best way to deal with this, or does anyone have a better idea?
You can create a factory which knows only the Base class, like this:
add pure virtual method to base class: virtual Chips* clone() const=0; and implement it for all derives, just like operator= but to return pointer to a new derived. (if you have destructor, it should be virtual too)
now you can define a factory class:
Class ChipsFactory{
std::map<std::string,Chips*> m_chipsTypes;
public:
~ChipsFactory(){
//delete all pointers... I'm assuming all are dynamically allocated.
for( std::map<std::string,Chips*>::iterator it = m_chipsTypes.begin();
it!=m_chipsTypes.end(); it++) {
delete it->second;
}
}
//use this method to init every type you have
void AddChipsType(const std::string& serial, Chips* c){
m_chipsTypes[serial] = c;
}
//use this to generate object
Chips* CreateObject(const std::string& serial){
std::map<std::string,Chips*>::iterator it = m_chipsTypes.find(serial);
if(it == m_chipsTypes.end()){
return NULL;
}else{
return it->clone();
}
}
};
Initialize the factory with all types, and you can get pointers for the initialized objects types from it.
From the comments, I think you're after something like this:
class ISerialNumber
{
public:
static ISerialNumber* Create( const string& number )
{
// instantiate and return a concrete class that
// derives from ISerialNumber, or NULL
}
virtual void DoSerialNumberTypeStuff() = 0;
};
class SerialNumberedObject
{
public:
bool Initialise( const string& serialNum )
{
m_pNumber = ISerialNumber::Create( serialNum );
return m_pNumber != NULL;
}
void DoThings()
{
m_pNumber->DoSerialNumberTypeStuff();
}
private:
ISerialNumber* m_pNumber;
};
(As this was a question on more advanced concepts, protecting from null/invalid pointer issues is left as an exercise for the reader.)
Why bother with inheritance here? As far as I can see the behaviour is the same for all Chips instances. That behaviour is that the flavour is defined by the serial number.
If the serial number only changes a couple of things then you can inject or lookup the behaviours (std::function) at runtime based on the serial number using a simple map (why complicate things!). This way common behaviours are shared among different chips via their serial number mappings.
If the serial number changes a LOT of things, then I think you have the design a bit backwards. In that case what you really have is the serial number defining a configuration of the Chips, and your design should reflect that. Like this:
class SerialNumber {
public:
// Maybe use a builder along with default values
SerialNumber( .... );
// All getters, no setters.
string getFlavour() const;
private:
string flavour;
// others (package colour, price, promotion, target country etc...)
}
class Chips {
public:
// Do not own the serial number... 'tis shared.
Chips(std::shared_ptr<SerialNumber> poSerial):m_poSerial{poSerial}{}
Chips(int shelf, SerialNumber oSerial):m_poSerial{oSerial}, m_nShelf{shelf}{}
string GetFlavour() {return m_poSerial->getFlavour()};
int GetShelf() {return m_nShelf;}
protected:
std::shared_ptr<SerialNumber> m_poSerial;
int m_nShelf;
}
// stores std::shared_ptr but you could also use one of the shared containers from boost.
Chips pringles{ chipMap.at("standard pringles - sour cream") };
This way once you have a set of SerialNumbers for your products then the product behaviour does not change. The only change is the "configuration" which is encapsulated in the SerialNumber. Means that the Chips class doesn't need to change.
Anyway, somewhere someone needs to know how to build the class. Of course you could you template based injection as well but your code would need to inject the correct type.
One last idea. If SerialNumber ctor took a string (XML or JSON for example) then you could have your program read the configurations at runtime, after they have been defined by a manager type person. This would decouple the business needs from your code, and that would be a robust way to future-proof.
Oh... and I would recommend NOT using Hungarian notation. If you change the type of an object or parameter you also have to change the name. Worse you could forget to change them and other will make incorrect assumptions. Unless you are using vim/notepad to program with then the IDE will give you that info in a clearer manner.
#user1158692 - The party instantiating Chips only needs to know about SerialNumber in one of my proposed designs, and that proposed design stipulates that the SerialNumber class acts to configure the Chips class. In that case the person using Chips SHOULD know about SerialNumber because of their intimate relationship. The intimiate relationship between the classes is exactly the reason why it should be injected via constructor. Of course it is very very simple to change this to use a setter instead if necessary, but this is something I would discourage, due to the represented relationship.
I really doubt that it is absolutely necessary to create the instances of chips without knowing the serial number. I would imagine that this is an application issue rather than one that is required by the design of the class. Also, the class is not very usable without SerialNumber and if you did allow construction of the class without SerialNumber you would either need to use a default version (requiring Chips to know how to construct one of these or using a global reference!) or you would end up polluting the class with a lot of checking.
As for you complaint regarding the shared_ptr... how on earth to you propose that the ownership semantics and responsibilities are clarified? Perhaps raw pointers would be your solution but that is dangerous and unclear. The shared_ptr clearly lets designers know that they do not own the pointer and are not responsible for it.

Defineing a Typedef? (UDP Packet)

I'm using RakNet to create a program which involves networking. I however don't know how to define a typedef for a Packet. The function I am trying to get it working for is:
void UDP_ClientDisconnected(Packet *pPacket);
Packet needs to be a typedef for this function obviously, however I don't know how to do this? Thanks to anyone who offers a solution.
Edit:
void Connections::UpdateRakNetwork()
{
for(Packet = Peer ->Receive(); Packet; Peer ->DeallocatePacket(Packet), Packet = Peer ->Recieve())
{
PacketID = GetPacketIdentifier(Pacekt);
switch(PacketID)
{
case ID_DISCONNECTION_NOTIFICATION:
UDP_ClientDisconnected(Packet);
break;
}
Peer ->DeallocatePacket(Packet);
}
}
Information is passed from this Packet sorting also in Connections.cpp, to the .h file in order to allow me to access these features from other elements of the game. Therefore allowing me to call UDP_ClientDisconnected(..); from another file.
As of yet there is no errors with this part of the file but the .h declaration, with the "Packet is not a Type name" error. As the guy below suggested it might be the fact that I named something else packet therefore I renamed it RakPacket and gain the same error.
It seems that it is the solution :
void Connections::UpdateRakNetwork()
{
RakNet::Packet *RakPacket = NULL;
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
for(RakPacket = Peer->Receive(); NULL != RakPacket; Peer->DeallocatePacket(RakPacket), RakPacket = Peer->Recieve())
// ^^^^^^^^^ ^^^^^^^^^ ^^^^^^^^^ ^^^^^^^^^
{
RakNet::PacketID pID = GetPacketIdentifier(RakPacket);
// ^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^
switch(pID)
{
case ID_DISCONNECTION_NOTIFICATION:
UDP_ClientDisconnected(RakPacket);
// ^^^^^^^^^
break;
}
Peer->DeallocatePacket(RakPacket);
// ^^^^^^^^^
}
}
I'm not helping you by giving you that. Again try to understand RakNet::Packet *RakPacket = NULL; and why we are not using the classname as a parameter of a function but the pointer to an object...
EDIT :
In response of the first comment :
In C++, an object is a region of storage with associated semantics. In the context of the object model of C++, the term object refers to an instance of a class. A class defines the characteristics of its instances in terms of members: data members (state) and member functions (methods or operations), and the visibility of these members to other classes. C++ is statically typed.
In this explanation you can replace the word class by struct. The only difference between both is the default access right to their members (private for class and public for struct) : What are the differences between struct and class in C++?.
You can this easily by writing:
typedef Packet * pPacket;
After that you can create a pointer to a Packet object via:
pPacket somePacket(NULL);
somePacket = new Packet(...);
//etc...
Typedef's are great, but hiding pointers makes it unreadable. So if you really want to do that, you better have a good reason for this.
EDIT:
"Connections::Packet" is not a type name.
Make sure that you include the correct classes and libraries. Is that your only compiler error?

What is the proper way to handle a large number of interface implementations?

For one of my current projects I have an interface defined for which I have a large number of implementations. You could think of it as a plugin interface with many plugins.
These "plugins" each handle a different message type in a network protocol.
So when I get a new message, I loop through a list of my plugins, see who can handle it, and call into them via the interface.
The issue I am struggling with is how to allocate, initialize, and "load" all the implementations into my array/vector/whatever.
Currently I am declaring all of the "plugins" in main(), then calling an "plugin_manager.add_plugin(&plugin);" for each one. This seems less than ideal.
So, the actual questions:
1. Is there a standardized approach to this sort of thing?
2. Is there any way to define an array (global?) pre-loaded with the plugins?
3. Am I going about this the wrong way entirely? Are there other (better?) architecture options for this sort of problem?
Thanks.
EDIT:
This compiles (please excuse the ugly code)... but it kind of seems like a hack.
On the other hand, it solves the issue of allocation, and cleans up main()... Is this a valid solution?
class intf
{
public:
virtual void t() = 0;
};
class test : public intf
{
public:
test(){}
static test* inst(){ if(!_inst) _inst = new test; return _inst; }
static test* _inst;
void t(){}
};
test* test::_inst = NULL;
intf* ints[] =
{
test::inst(),
NULL
};
Store some form of smart pointer in a container. Dynamically allocate the plugins and register them in the container so that they can be used later.
One possible approach for your solution would be, if you have some form of message id that the plugin can decode, to use a map from that id to the plugin that handles that. This approach allows you to have fast lookup of the plugin given the input message.
One way of writing less code would be to use templates for the instantiation function. Then you only need to write one and put it in the interface, instead of having one function per implementation class.
class intf
{
public:
virtual void t() = 0;
template<class T>
static T* inst()
{
static T instance;
return &instance;
}
};
class test : public intf { ... };
intf* ints[] =
{
intf::inst<test>(),
NULL
};
The above code also works around two bugs you have in your code: One is a memory leak, in your old inst() function you allocate but you never free; The other is that the constructor sets the static member to NULL.
Other tips is to read more about the "singleton" pattern, which is what you have. It can be useful in some situations, but is generally advised against.

Design pattern to refactor switch statement

I have something like the following in the header
class MsgBase
{
public:
unsigned int getMsgType() const { return type_; }
...
private:
enum Types { MSG_DERIVED_1, MSG_DERIVED_2, ... MSG_DERIVED_N };
unsigned int type_;
...
};
class MsgDerived1 : public MsgBase { ... };
class MsgDerived2 : public MsgBase { ... };
...
class MsgDerivedN : public MsgBase { ... };
and is used as
MsgBase msgHeader;
// peeks into the input stream to grab the
// base class that has the derived message type
// non-destructively
inputStream.deserializePeek( msgHeader );
unsigned int msgType = msgHeader.getMsgType();
MsgDerived1 msgDerived1;
MsgDerived2 msgDerived2;
...
MsgDerivedN msgDerivedN;
switch( msgType )
{
case MSG_DERIVED_1:
// fills out msgDerived1 from the inputStream
// destructively
inputStream.deserialize( msgDerived1 );
/* do MsgDerived1 processing */
break;
case MSG_DERIVED_2:
inputStream.deserialize( msgDerived2 );
/* do MsgDerived1 processing */
break;
...
case MSG_DERIVED_N:
inputStream.deserialize( msgDerivedN );
/* do MsgDerived1 processing */
break;
}
This seems like the type of situation which would be fairly common and well suited to refactoring. What would be the best way to apply design patterns (or basic C++ language feature redesign) to refactor this code?
I have read that the Command pattern is commonly used to refactor switch statements but that seems only applicable when choosing between algorithms to do a task. Is this a place where the factory or abstract factory pattern is applicable (I am not very familiar with either)? Double dispatch?
I've tried to leave out as much inconsequential context as possible but if I missed something important just let me know and I'll edit to include it. Also, I could not find anything similar but if this is a duplicate just redirect me to the appropriate SO question.
You could use a Factory Method pattern that creates the correct implementation of the base class (derived class) based on the value you peek from the stream.
The switch isn't all bad. It's one way to implement the factory pattern. It's easily testable, it makes it easy to understand the entire range of available objects, and it's good for coverage testing.
Another technique is to build a mapping between your enum types and factories to make the specific objects from the data stream. This turns the compile-time switch into a run-time lookup. The mapping can be built at run-time, making it possible to add new types without recompiling everything.
// You'll have multiple Factories, all using this signature.
typedef MsgBase *(*Factory)(StreamType &);
// For example:
MsgBase *CreateDerived1(StreamType &inputStream) {
MsgDerived1 *ptr = new MsgDerived1;
inputStream.deserialize(ptr);
return ptr;
}
std::map<Types, Factory> knownTypes;
knownTypes[MSG_DERIVED_1] = CreateDerived1;
// Then, given the type, you can instantiate the correct object:
MsgBase *object = (*knownTypes[type])(inputStream);
...
delete object;
Pull Types and type_ out of MsgBase, they don't belong there.
If you want to get totally fancy, register all of your derived types with the factory along with the token (e.g. 'type') that the factory will use to know what to make. Then, the factory looks up that token on deserialize in its table, and creates the right message.
class DerivedMessage : public Message
{
public:
static Message* Create(Stream&);
bool Serialize(Stream&);
private:
static bool isRegistered;
};
// sure, turn this into a macro, use a singleton, whatever you like
bool DerivedMessage::isRegistered =
g_messageFactory.Register(Hash("DerivedMessage"), DerivedMessage::Create);
etc. The Create static method allocates a new DerivedMessage and deserializes it, the Serialize method writes the token (in this case, Hash("DerivedMessage")) and then serializes itself. One of them should probably test isRegistered so that it doesn't get dead stripped by the linker.
(Notably, this method doesn't require an enum or other "static list of everything that can ever exist". At this time I can't think of another method that doesn't require circular references to some degree.)
It's generally a bad idea for a base class to have knowledge about derived classes, so a redesign is definitely in order. A factory pattern is probably what you want here as you already noted.

C++ design - Network packets and serialization

I have, for my game, a Packet class, which represents network packet and consists basically of an array of data, and some pure virtual functions
I would then like to have classes deriving from Packet, for example: StatePacket, PauseRequestPacket, etc. Each one of these sub-classes would implement the virtual functions, Handle(), which would be called by the networking engine when one of these packets is received so that it can do it's job, several get/set functions which would read and set fields in the array of data.
So I have two problems:
The (abstract) Packet class would need to be copyable and assignable, but without slicing, keeping all the fields of the derived class. It may even be possible that the derived class will have no extra fields, only function, which would work with the array on the base class. How can I achieve that?
When serializing, I would give each sub-class an unique numeric ID, and then write it to the stream before the sub-class' own serialization. But for unserialization, how would I map the read ID to the appropriate sub-class to instanciate it?
If anyone want's any clarifications, just ask.
-- Thank you
Edit: I'm not quite happy with it, but that's what I managed:
Packet.h: http://pastebin.com/f512e52f1
Packet.cpp: http://pastebin.com/f5d535d19
PacketFactory.h: http://pastebin.com/f29b7d637
PacketFactory.cpp: http://pastebin.com/f689edd9b
PacketAcknowledge.h: http://pastebin.com/f50f13d6f
PacketAcknowledge.cpp: http://pastebin.com/f62d34eef
If someone has the time to look at it and suggest any improvements, I'd be thankful.
Yes, I'm aware of the factory pattern, but how would I code it to construct each class? A giant switch statement? That would also duplicade the ID for each class (once in the factory and one in the serializator), which I'd like to avoid.
For copying you need to write a clone function, since a constructor cannot be virtual:
virtual Packet * clone() const = 0;
Which each Packet implementation implement like this:
virtual Packet * clone() const {
return new StatePacket(*this);
}
for example for StatePacket. Packet classes should be immutable. Once a packet is received, its data can either be copied out, or thrown away. So a assignment operator is not required. Make the assignment operator private and don't define it, which will effectively forbid assigning packages.
For de-serialization, you use the factory pattern: create a class which creates the right message type given the message id. For this, you can either use a switch statement over the known message IDs, or a map like this:
struct MessageFactory {
std::map<Packet::IdType, Packet (*)()> map;
MessageFactory() {
map[StatePacket::Id] = &StatePacket::createInstance;
// ... all other
}
Packet * createInstance(Packet::IdType id) {
return map[id]();
}
} globalMessageFactory;
Indeed, you should add check like whether the id is really known and such stuff. That's only the rough idea.
You need to look up the Factory Pattern.
The factory looks at the incomming data and created an object of the correct class for you.
To have a Factory class that does not know about all the types ahead of time you need to provide a singleton where each class registers itself. I always get the syntax for defining static members of a template class wrong, so do not just cut&paste this:
class Packet { ... };
typedef Packet* (*packet_creator)();
class Factory {
public:
bool add_type(int id, packet_creator) {
map_[id] = packet_creator; return true;
}
};
template<typename T>
class register_with_factory {
public:
static Packet * create() { return new T; }
static bool registered;
};
template<typename T>
bool register_with_factory<T>::registered = Factory::add_type(T::id(), create);
class MyPacket : private register_with_factory<MyPacket>, public Packet {
//... your stuff here...
static int id() { return /* some number that you decide */; }
};
Why do we, myself included, always make such simple problems so complicated?
Perhaps I'm off base here. But I have to wonder: Is this really the best design for your needs?
By and large, function-only inheritance can be better achieved through function/method pointers, or aggregation/delegation and the passing around of data objects, than through polymorphism.
Polymorphism is a very powerful and useful tool. But it's only one of many tools available to us.
It looks like each subclass of Packet will need its own Marshalling and Unmarshalling code. Perhaps inheriting Packet's Marshalling/Unmarshalling code? Perhaps extending it? All on top of handle() and whatever else is required.
That's a lot of code.
While substantially more kludgey, it might be shorter & faster to implement Packet's data as a struct/union attribute of the Packet class.
Marshalling and Unmarshalling would then be centralized.
Depending on your architecture, it could be as simple as write(&data). Assuming there are no big/little-endian issues between your client/server systems, and no padding issues. (E.g. sizeof(data) is the same on both systems.)
Write(&data)/read(&data) is a bug-prone technique. But it's often a very fast way to write the first draft. Later on, when time permits, you can replace it with individual per-attribute type-based Marshalling/Unmarshalling code.
Also: I've taken to storing data that's being sent/received as a struct. You can bitwise copy a struct with operator=(), which at times has been VERY helpful! Though perhaps not so much in this case.
Ultimately, you are going to have a switch statement somewhere on that subclass-id type. The factory technique (which is quite powerful and useful in its own right) does this switch for you, looking up the necessary clone() or copy() method/object.
OR you could do it yourself in Packet. You could just use something as simple as:
( getHandlerPointer( id ) ) ( this )
Another advantage to an approach this kludgey (function pointers), aside from the rapid development time, is that you don't need to constantly allocate and delete a new object for each packet. You can re-use a single packet object over and over again. Or a vector of packets if you wanted to queue them. (Mind you, I'd clear the Packet object before invoking read() again! Just to be safe...)
Depending on your game's network traffic density, allocation/deallocation could get expensive. Then again, premature optimization is the root of all evil. And you could always just roll your own new/delete operators. (Yet more coding overhead...)
What you lose (with function pointers) is the clean segregation of each packet type. Specifically the ability to add new packet types without altering pre-existing code/files.
Example code:
class Packet
{
public:
enum PACKET_TYPES
{
STATE_PACKET = 0,
PAUSE_REQUEST_PACKET,
MAXIMUM_PACKET_TYPES,
FIRST_PACKET_TYPE = STATE_PACKET
};
typedef bool ( * HandlerType ) ( const Packet & );
protected:
/* Note: Initialize handlers to NULL when declared! */
static HandlerType handlers [ MAXIMUM_PACKET_TYPES ];
static HandlerType getHandler( int thePacketType )
{ // My own assert macro...
UASSERT( thePacketType, >=, FIRST_PACKET_TYPE );
UASSERT( thePacketType, <, MAXIMUM_PACKET_TYPES );
UASSERT( handlers [ thePacketType ], !=, HandlerType(NULL) );
return handlers [ thePacketType ];
}
protected:
struct Data
{
// Common data to all packets.
int number;
int type;
union
{
struct
{
int foo;
} statePacket;
struct
{
int bar;
} pauseRequestPacket;
} u;
} data;
public:
//...
bool readFromSocket() { /*read(&data); */ } // Unmarshal
bool writeToSocket() { /*write(&data);*/ } // Marshal
bool handle() { return ( getHandler( data.type ) ) ( * this ); }
}; /* class Packet */
PS: You might dig around with google and grab down cdecl/c++decl. They are very useful programs. Especially when playing around with function pointers.
E.g.:
c++decl> declare foo as function(int) returning pointer to function returning void
void (*foo(int ))()
c++decl> explain void (* getHandler( int ))( const int & );
declare getHandler as function (int) returning pointer to function (reference to const int) returning void