Dealing with circular includes in the context of circular references - c++

As a follow up to an older question of mine, I wish to implement a client-server mockup simulation, where the client initiates a sequence of actions that involve calling methods on the server, which, in turn, can call methods on the client (let's ignore the issue that the stack may blow up).
More specifically, since I want to split the implementation from the definition, I will have server.h and server.cpp for the Server class and client.h and client.cpp for the Client class. Since Server holds a reference to Client and calls methods from it, it needs to #include "client.h". Also, Client holds a reference to Server and calls methods from it, it needs to #include "server.h". At this point, even if I use header guards in both server.h and client.h, it still messes up (yeah, it's expected) so I decided to forward-declare the Server class in client.h and the Client class in server.h. Unfortunately, this is not enough to solve the issue, because I'm also calling methods from the two classes, so I managed to make it compile & run (properly, as far as I can tell), by including server.h in client.cpp and client.h in server.cpp.
Does the above "hack" sound reasonable? Should I expect some unforeseen consequences? Is there any "smarter" way to do this without having to implement a proxy class?
Here's a rudimentary sample of how the implementation will look like:
file client.h:
#ifndef CLIENT_H
#define CLIENT_H
#include <iostream>
#include <memory>
class Server;
class Client
{
private:
std::shared_ptr<const Server> server;
public:
Client () {}
void setServer (const std::shared_ptr<const Server> &server);
void doStuff () const;
void doOtherStuff () const;
};
#endif
file client.cpp:
#include "client.h"
#include "server.h"
void Client::setServer (const std::shared_ptr<const Server> &server)
{
this->server = server;
}
void Client::doStuff () const
{
this->server->doStuff();
}
void Client::doOtherStuff () const
{
std::cout << "All done!" << std::endl;
}
file server.h:
#ifndef SERVER_H
#define SERVER_H
#include <iostream>
#include <memory>
class Client;
class Server
{
private:
std::weak_ptr<const Client> client;
public:
Server () {}
void setClient (const std::weak_ptr<const Client> &client);
void doStuff () const;
};
#endif
file sever.cpp:
#include "server.h"
#include "client.h"
void Server::setClient (const std::weak_ptr<const Client> &client)
{
this->client = client;
}
void Server::doStuff () const
{
this->client.lock()->doOtherStuff();
}
file main.cpp:
#include <iostream>
#include <memory>
#include "client.h"
#include "server.h"
int main ()
{
std::shared_ptr<Client> client(new Client);
std::shared_ptr<Server> server(new Server);
client->setServer(server);
server->setClient(client);
client->doStuff();
return 0;
}

That looks good to me. Forward declaring server in the client.h and forward declaring client in the server.h is the right thing to do.
It is perfectly fine to then include both header files in the .c or .cpp file - all you need to avoid is including the header files in a circle.

The "Hack" is none, it's perfectly common practice to separate declaration and implementation of the two classes as you did. And it's perfectly normal that the *.cpp include both Headers.
Sidenote: First consider different signatures for your setServer and setClient methods: In both methods, you copy the argument. Both copies are nontrivial, since the use_counts and/or weak_count have to be updated. If the argument indeed is an existing argument, that is ok, but if it is a temporary, the copy will increase the count and destruction of the temporary will decrease it again, each time an internal pointer has to be dereferenced. In contrast, moving a shared_ptr or weak_ptr does not affect the use counts but resets the temporary. Destruction of that reset temporary again does not affect the use count (it effectively is a null pointer).
Secondly, always prefer make_shared over simple new, because it saves you one allocation. So use this implementation instead:
void Client::setServer (std::shared_ptr<const Server> server)
{
this->server = std::move(server);
}
int main ()
{
auto client = std::make_shared<Client>(); //prefer make_shared
auto server = std::make_shared<Server>();
/* 1 */
client->setServer(server); //by copy, if you need to continue to use server
/* 2 */
server->setClient(std::move(client)); //by moving
}
Call 1 will be as expensive at it was, you make one copy pf the shared_ptr, only this time you make it while passing the argument, not inside the method. Call 2 will be cheaper, because the shared_ptr is moved around and never copied.
My following statement is false (see comments) and applies only to unique_ptr, not to shared_ptr:
But: Since you use a std::shared_ptr<const Server> in Client, you
will have to define Client's destructor inside client.cpp. The
reason is that if you don't, the compiler will generate it for you,
calling the shared_ptr's and thus Server's destructor which has
not been declared inside client.h. At reasonably high warning levels
you compiler should complain about calling delete on a an undefined
class' pointer.

Does the above "hack" sound reasonable? Should I expect some
unforeseen consequences? Is there any "smarter" way to do this without
having to implement a proxy class?
Forward declaration and use include directive to is the normal and right way to break circular include.

Related

Creating object(s) within a Class

I have this problem:
Problem:
I am trying to create a library (ard33WiFi) that manages and handles
a couple of other libraries (WiFiServer library for example)
I need to create the server object that I then use in functions in my Library (ard33WiFi):
WiFiServer myServer(iPort);
The problem is that when I call myServer in the members of the class I get:
'myServer' was not declared in this scope
Where/how do I declare myServer so that is becomes available to the entire class (ard33WiFi)? I have taken out any decleration because whatever I was trying was wrong. I have pasted a skeleton code below.
// HEADER FILE (.h)
// ----------------------------------------------------------------------------------------------
#ifndef Ard33WiFi_h
#define Ard33WiFi_h
#include <WiFiNINA.h>
#include <WiFiUdp.h>
class ard33WiFi{
public:
ard33WiFi(int iPort)
void someFunction();
void serverBegin();
private:
int _iPort;
};
#endif
// ----------------------------------------------------------------------------------------------
// C++ FILE (.cpp)
// -----------------------------------------------------------------------------------------------
#include <Ard33Wifi.h>
ard33WiFi::ard33WiFi(int iPort){
_iPort = iPort;
}
void ard33WiFi::someFunction(){
// code here required to prepare the server for initializing
// but ultimately not relevant to the question
}
void ard33WiFi::serverBegin(){
myServer.begin();
Serial.println("Server Online");
}
I run into the same problem with the UDP library as I need to call the UDP object in various functions to do UDP things.
Any help would be greatly appreciated.
I suppose you are using this:
https://www.arduino.cc/en/Reference/WiFiServer
I can see that you are not declaring the myServer in your class; that I guess is the error in your code. If I am not wrong, it should be something like this:
#ifndef Ard33WiFi_h
#define Ard33WiFi_h
#include <WiFiNINA.h>
#include <WiFiUdp.h>
#include <WiFi.h> // Not sure if you have to append this include
class ard33WiFi{
public:
ard33WiFi(int iPort)
void someFunction();
void serverBegin();
private:
int _iPort;
WiFiServer myServer;
};
#endif
The implementation, you would need to initialise the instance:
#include <Ard33Wifi.h>
ard33WiFi::ard33WiFi(int iPort):myServer(iPort), _iPort(iPort) {
}
void ard33WiFi::someFunction(){
// code here required to prepare the server for initializing
// but ultimately not relevant to the question
}
void ard33WiFi::serverBegin(){
myServer.begin();
Serial.println("Server Online");
}

Classes included within main() method

If I have some code like
main(int argc, char *argv[])
{
...
#include "Class1.H"
#include "Class2.H"
...
}
Generally the main() method is the starting point of every application and the content within main() is to be executed. Am I right in the assumption that the content of all classes included into main() will be executed when main() is started?
greetings
Streight
No, no, NO.
First of all, you don't #include a file within a function. You #include a file at the beginning of a file, before other declarations. OK, you can use #include anywhere, but you really just shouldn't.
Second, #include doesn't execute anything. It's basically just a copy-paste operation. The contents of the #included file are (effectively) inserted exactly where you put the #include.
Third, if you're going to learn to program in C++, please consider picking up one of our recommended texts.
You commented:
I am working with the multiphaseEulerFoam Solver in OpenFoam and
inside the main() of multiphaseEulerFoam.C are classes included. I
assume that the classes have the right structure to be called in
main()
That may be the case, and I don't doubt that the classes have the right structure to be called from main. The problem is main will be malformed after the #includes because you'll have local class definitions and who knows what else within main.
Consider this. If you have a header:
foo.h
#ifndef FOO_H
#define FOO_H
class Foo
{
public:
Foo (const std::string& val)
:
mVal (val)
{
}
private:
std::string mVal;
};
#endif
And you try to include this in main:
main.cpp
int main()
{
#include "foo.h"
}
After preprocessing the #include directive, the resulting file that the compiler will try to compile will look like this:
preprocessed main.cpp
int main()
{
#ifndef FOO_H
#define FOO_H
class Foo
{
public:
Foo (const std::string& val)
:
mVal (val)
{
}
private:
std::string mVal;
};
#endif
}
This is all kinds of wrong. One, you can't declare local classes like this. Two, Foo won't be "executed", as you seem to assume.
main.cpp should look like this instead:
#include "foo.h"
int main()
{
}
#define and #include are just textual operations that take place during the 'preprocessing' phase of compilation, which is technically an optional phase. So you can mix and match them in all sorts of ways and as long as your preprocessor syntax is correct it will work.
However if you do redefine macros with #undef your code will be hard to follow because the same text could have different meanings in different places in the code.
For custom types typedef is much preferred where possible because you can still benefit from the type checking mechanism of the compiler and it is less error-prone because it is much less likely than #define macros to have unexpected side-effects on surrounding code.
Jim Blacklers Answer # #include inside the main () function
Try to avoid code like this. #include directive inserts contents of the file in its place.
You can simulate the result of your code by copy-pasting file content from Class1.H and Class2.H inside the main function.
Includes do not belong into any function or class method body, this is not a good idea to do.
No code will be executed unless you instantiate one of your classes in your header files.
Code is executed when:
Class is instantiated, then it's constructor method is called and the code inside the method is executed.
If there are variables of a class type inside your instantiated class, they will too run their constructors.
When you call a class method.
Try this example:
#include <iostream>
using namespace std;
int main()
{
class A
{ public:
A() { cout << "A constructor called" << endl; }
};
// A has no instances
class B
{ public:
B() { cout << "B constructor called" << endl; }
void test() { cout << "B test called" << endl; }
} bbb;
// bbb will be new class instance of B
bbb.test(); // example call of test method of bbb instance
B ccc; // another class instance of B
ccc.test(); // another call, this time of ccc instance
}
When you run it, you'll observe that:
there will be no instance of class A created. Nothing will be run from class A.
if you intantiate bbb and ccc, their constructors will be run. To run any other code you must first make a method, for example test and then call it.
This is an openFoam syntax he is correct in saying that open Foam treats #include like calling a function. In OpenFoam using #include Foo.H would run through the code not the class declaration that is done in a different hierarchy level. I would recommend all openFoam related question not be asked in a C++ forum because there is so much stuff built onto C++ in openFoam a lot the rules need to be broken to produce a working code.
You're only including declarations of classes. To execute their code, you need to create class instances (objects).
Also, you shouldn't write #include inside a function or a class method. More often than not it won't compile.

What does *New() mean when declaring member function?

I'm working on a VTK program and have found a class (specifically this one: Image Region) which i need to incorporate into my code. To do so I have made a separate ImageRegion.h and ImageRegion.cpp files so they can be easily included in the project. My Problem here is the
static vtkBorderCallback *New()
function which i do not know how to implement in the .cpp file or, to be quite honest, what purpose it serves at all. What does it do? Is it even necessary to have it?
When compiling I get the error:
/home/Desktop/test/src/ImageRegion.cpp:7:10: error: ‘vtkBorderCallback::vtkBorderCallback’ names the constructor, not the type
My .h file:
//ImageRegion.h
#pragma once
#include <vtkSmartPointer.h>
#include <vtkActor.h>
#include <vtkAssemblyNode.h>
#include <vtkAssemblyPath.h>
#include <vtkBorderRepresentation.h>
#include <vtkCommand.h>
#include <vtkCoordinate.h>
#include <vtkImageMapper3D.h>
#include <vtkImageActor.h>
#include <vtkInteractorStyleImage.h>
#include <vtkPolyData.h>
#include <vtkPropPicker.h>
#include <vtkProperty2D.h>
#include <vtkBorderWidget.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkRenderer.h>
class vtkBorderCallback : public vtkCommand
{
public:
vtkBorderCallback();
static vtkBorderCallback *New();
virtual void Execute(vtkObject *caller, unsigned long, void*);
void SetRenderer(vtkSmartPointer<vtkRenderer> ren);
void SetImageActor(vtkSmartPointer<vtkImageActor> im);
double coords[6];
private:
vtkSmartPointer<vtkRenderer> Renderer;
vtkSmartPointer<vtkImageActor> ImageActor;
};
And my .cpp file:
//ImageRegion.cpp
#include "ImageRegion.h"
vtkBorderCallback::vtkBorderCallback(){}
static vtkBorderCallback::vtkBorderCallback* New()
{
return new vtkBorderCallback;
}
void vtkBorderCallback::Execute(vtkObject *caller, unsigned long, void*)
{
//Do stuff, from original VTK example code
}
void vtkBorderCallback::SetRenderer(vtkSmartPointer<vtkRenderer> ren) {this->Renderer = ren;}
void vtkBorderCallback::SetImageActor(vtkSmartPointer<vtkImageActor> im) {this->ImageActor = im;}
Any help is much appreciated.
This
static vtkBorderCallback *New();
is a static member function called New, taking no arguments, and returning a pointer to vtkBorderCallback.
In the implementation, you should omit the static. You also need to place the function in the scope of its class:
vtkBorderCallBack* vtkBorderCallback::New()
{// ^^^^^^^^^^^^^^^^^^^
return new vtkBorderCallback; // danger! Caller needs to delete this eventually
}
In VTK nearly all of the classes derive from vtkObjectBase. They should use New() and Delete() to create and delete the objects (the constructor and destructor are protected). These methods include referencing counting to make sure that they get properly shared among other vtkObjects that may use them. There is a VTK macro (vtkStandardNewMacro) that takes care of the implementation of New() and the base class implements Delete(). So for VTK, the best way to implement the static New() method is to use that macro. For your class called vtkBorderCallBack it would look like:
vtkStandardNewMacro(vtkBorderCallBack);
This should go in your .cpp file.
To solve the error, put vtkBorderCallBack:: before New():
vtkBorderCallBack* vtkBorderCallBack::New()
~~~~~~~~~~~~~~~~~~~
{
...
}
He should not omit static since New() is meant as constructor. In this scenario I would rather expect the real constructor to be private. The implementation
static vtkBorderCallback::vtkBorderCallBack* New()
{
return new vtkBorderCallback;
}
is syntactically wrong. It has so be
vtkBorderCallBack* vtkBorderCallback::New()
{
return new vtkBorderCallback;
}
Finally the whole approach is strange. New() is not really required here, and possibly leads to a memory leak. To establish a class-specific memory management overload operators new and delete on a per-class basis. Alternatively, to prevent leaks, do not return a raw pointer; return std::auto_ptr (deprecated) or std::unique_ptr:
std::unique_ptr<vtkBorderCallBack> vtkBorderCallback::New()
{
return std::unique_ptr<vtkBorderCallBack>(new vtkBorderCallback); // uses move c'tor
}
However, std::unique_ptrs are movable but not copyable. But that's the point when leaks have to be prevented. When the pointer returned by New() is spreaded all over the code better use a std::shared_ptr.
If you have only a C++03 compiler I recommend Herb Sutter's Using auto_ptr Effectively.

Hiding library internal class to clients C++: use friend?

I'm trying to develop a networking part in my basic game engine in C++, but I'm faced with a rather strange problem (it is for me).
I got a Singleton Networker class that handles the set-up of a socket (UDP) and will try to register the username the client provided to the server, returning a bool that represents if the username is already taken by a different player or not. The class itself does not handle the packets itself, I aim to use a Packethandler class for that.
The problem is that I don't want a client to be able to create a PacketHandler directly because the entire point of the Networker class is to provide some kind of interface for handling that. The first idea I had is 'make the constructor of Packethandler private and create a Singleton' but then ofcourse the client can still ask the instance. Then I quickly though 'well protected then' but Networker is not extending PacketHandler. Then I thought 'well let's make Networker a friend of PacketHandler'. All the info I found up till now seems to discourage Friend usage however, so I wonder:
Is my plan for the simple networking part flawed thus facing me with an issue like this or is my problem the reason friend functionality exists in the first place? What is the take of you guys on this, do you have a much better idea?
Edit: code of my idea.
Packethandler.h
#ifndef FANCY_PACKET_HANDLER
#define FANCY_PACKET_HANDLER
#include <SFML/Network.hpp>
namespace fancy {
namespace network {
class PacketHandler
{
friend class Networker;
private:
sf::IpAddress _hostAddress;
sf::UdpSocket _socket;
PacketHandler(std::string hostAdress);
public:
~PacketHandler();
void sendPacket(const char* packet_data, const unsigned int packet_size);
char* receivePacket();
};
}
}
#endif
Networker.h
#ifndef FANCY_NETWORKER
#define FANCY_NETWORKER
#include <SFML/Network.hpp>
#include <Engine/PacketHandler.h>
namespace fancy {
namespace network {
class Networker
{
public:
static Networker* instance();
bool openUdpSocket(unsigned short port);
bool registerUsername(char* username);
char* receiveOnSocket();
protected:
Networker();
private:
static Networker* _instance;
static PacketHandler* _packetHandler;
const char* _username;
sf::IpAddress _hostAddress;
sf::UdpSocket _socket;
};
}
}
#endif
If you wish to provide an interface and hide all the implementation details, you'd better define a pure virtual class (e.g. Networker) which is visible to a user (say in .h-file), and write an implementation in a descendant class (e.g. NetworkerImpl) which is NOT visible to a user. You can then declare in the header file a function (or even a static method of the Networker class) like Networker *CreateNetworker(); (so the declaration is visible to a user), and then implement the function in .cc file along with your NetworkerImpl (so that the implementation is not visible to a user) like return new NetworkerImpl();.

C++ private access to other classes' members

I'm writing a multi-threaded server using boost::asio (for sockets), boost::thread (for threading), libconfig++ (for configuration files reading) and protocol buffers (for the protocol implementation).
The server follows more or less this route: main() -> creates an Application object -> runs application object. Application loads configuration file, then creates the server object (which is passed the configuration class as a const). Server object configures itself and binds the port, starts accepting, blah. Whenever a new client is detected, the server creates a new Client object, and then creates a thread running the client's connection handler.
All of this is to explain that the configuration file is loaded from my Application class, and then passed all the way down to my Client class. This shouldn't pose any kind of trouble if the libconfig object was passed directly all the way to the Client, yet as we all know, multi-threading implies that memory corrupts when accessed simultaneously by two or more threads.
The way to solve this was discussed in other post and ended up with the implementation of a wrapper which automagically solves the mutex problem.
The magical class
app_config.h
#ifndef _APP_CONFIG_H_
#define _APP_CONFIG_H_ 1
#include <boost/shared_ptr.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/thread/locks.hpp>
#include <boost/noncopyable.hpp>
#include <libconfig.h++>
#include <string>
namespace BBCP {
namespace App {
class ConfigLock;
class Config {
public:
friend class BBCP::App::ConfigLock;
Config(std::string const &file) :
cfg(new libconfig::Config()),
mutex(new boost::mutex())
{
cfg->readFile(file.c_str());
}
private:
boost::shared_ptr<libconfig::Config> cfg;
boost::shared_ptr<boost::mutex> mutex;
};
class Server;
class Client;
class ConfigLock : boost::noncopyable {
public:
ConfigLock(BBCP::App::Config const &wrapper) :
cfg(wrapper.cfg),
mutex(wrapper.mutex),
lock(new LockType(*mutex))
{ }
libconfig::Config &get() throw() { return *cfg; };
private:
boost::shared_ptr<libconfig::Config> cfg;
boost::shared_ptr<boost::mutex> mutex;
typedef boost::lock_guard<boost::mutex> LockType;
boost::shared_ptr<LockType> lock;
};
}
}
#endif
For lazy people, this class consists of... well, two classes (irony?): BBCP::App::Config and BBCP::App::ConfigLock. BBCP::App::Config simply loads a file, while BBCP::App::ConfigLock takes a BBCP::App::Config as an argument, and then locks BBCP::App::Config's mutex. Once the lock has been created, one calls BBCP::App::ConfigLock::get, which returns a reference to the libconfig::Config object!.
THE problem
Well:
server.cpp:
void BBCP::App::Server::startAccept() {
newClient.reset(new BBCP::App::Client(io_service, config_wrapper));
acceptor.async_accept(newClient->getSocket(), boost::bind(&BBCP::App::Server::acceptHandler, this, boost::asio::placeholders::error));
}
This function creates a new client object, loaded with the boost::asio::io_service object and BBCP::App::Config object.
server.cpp
void BBCP::App::Server::acceptHandler(boost::system::error_code const &e) {
if (!acceptor.is_open()) {
// ARR ERROR!
return;
}
if (!e) {
client_pool.create_thread(*newClient);
}
else {
// HANDLE ME ERROR
throw;
}
startAccept();
}
This function creates a new thread or (not implemented yet) errors in case of... well, errors, then starts the accept loop again.
Client code mostly doesn't matter until this part:
client.cpp:
void BBCP::App::Client::parseBody() {
BBCP::Protocol::Header header;
BBCP::Protocol::Hello hello;
boost::scoped_ptr<BBCP::App::ConfigLock> lock;
libconfig::Config *cfg;
(...)
switch ((enum BBCP::Protocol::PacketType)header.type()) {
case BBCP::Protocol::HELLO:
(...)
// config_wrapper is a private variable in the client class!
lock.reset(new BBCP::App::ConfigLock(config_wrapper));
// ARRRRRRR HERE BE DRAGOONS!!
*cfg = lock->get();
(...)
lock.reset();
break;
(...)
}
(...)
}
Well, truth be told, I didn't expect this kind of error:
/usr/include/libconfig.h++: In member function ‘void BBCP::App::Client::parseBody()’:
/usr/include/libconfig.h++:338:13: error: ‘libconfig::Config& libconfig::Config::operator=(const libconfig::Config&)’ is private
client.cpp:64:30: error: within this context
client.cpp:71:21: error: request for member ‘exists’ in ‘cfg’, which is of non-class type ‘libconfig::Config*’
client.cpp:77:51: error: request for member ‘lookup’ in ‘cfg’, which is of non-class type ‘libconfig::Config*’
But here it is, and I need some way to solve it :(. I've tried making BBCP::App::Client a friend class of BBCP::App::ConfigLock, but then it went like:
In file included from ../include/app_config.h:4:0,
from ../include/app_main.h:6,
from main.cpp:18:
../include/app_client.h:15:53: error: ‘BBCP::App::Config’ has not been declared
In file included from ../include/app_config.h:4:0,
from ../include/app_main.h:6,
from main.cpp:18:
../include/app_client.h:32:5: error: ‘Config’ in namespace ‘BBCP::App’ does not name a type
In file included from ../include/app_config.h:4:0,
from ../include/app_main.h:6,
from main.cpp:18:
../include/app_client.h: In constructor ‘BBCP::App::Client::Client(boost::asio::io_service&, const int&)’:
../include/app_client.h:15:120: error: class ‘BBCP::App::Client’ does not have any field named ‘config_wrapper’
And then I went like O_O, so I just gave up and came here, once again looking for some über C++ guru hackz0r's help and scolding for doing such a misdeed as trying to access another class's private members is.
The first thing is figuring out if you are going in the right direction, and the next step is getting there.
Why is the assignment operator of the Config type private? By default the compiler generated assignment operator is public, so if it has been declared as private, chances are that there is a reason for the object not to be copied, or else you should make it public and the problem would no longer be a problem.
As of your particular problem after adding the friend declaration, it seems to indicate that you have missed including the header where the Config type is declared/defined. And then there are some more errors in the code (a member that has not been defined - result of the previous error?), or in the original code trying to access the object referred by a pointer without dereferencing it...
You probably wanted to store a pointer to the config object in cfg instead of creating a copy (and dereferencing an uninitialized pointer):
cfg = &local->get();