C++/QT: Using constant pointer to private member as read-only data-sharing

Question

I am writing a program in QT, which currently has a GameEngine class and a MainWindow class.

GameEngine.h:

#include "Player.h"

class GameEngine : public QObject
{
    Q_OBJECT

public:
    void test();

signals:
    void newPlayerDataReady(const std::vector<Player>* playerList);

private:
    std::vector<Player> mPlayers; // List of player objects
}

GameEngine.cpp

#include "GameEngine.h"

// Omitting constructor and destructor for simplicity of the example

void GameEngine::test() {
    emit newPlayerDataReady(&mPlayers);
}

The GameEngine is storing a lot of data in the array of Player instances (Player is a rather complex class itself with a lot of different members), which has to be stored there for centralizing the logic of the program (long-story short).

The MainWindow is a GUI class, which is supposed to visualize the data, when the signal newPlayerDataReady is emitted and represent a lot of the data on a graph, a list etc. This is done by connecting the signal to a dedicated slot in the MainWindow class. The connection is created in Main.cpp, which owns the instance of both classes.

So, my question is: If I want to share all this data from one class to another without letting the receiver alter the data (read-only access), is there a better common practice to do this, rather than using constant pointers to the data?

It is not that I don't like this solution. I just want to be absolutely sure that I doing it right from the beginning instead of having to rework a lot later in the process.

I know that one alternative could be done exchange the data as copies of the private member, but I don't think it would be a smooth solution, as the amount of data will increase to a point, where it might affect the speed of the program.

score 1 · Accepted Answer · answered Oct 11 '18 at 05:46

So, my question is: If I want to share all this data from one class to another without letting the receiver alter the data (read-only access), is there a better common practice to do this, rather than using constant pointers to the data?

Sharing via const-pointers is fine, although sharing a const-reference (i.e. const std::vector<Player> &) might be very slightly preferable, if only because any code receiving a const-reference can be 100% assured that any reference it receives isn't NULL, since NULL references aren't allowed by the language (whereas NULL pointers are allowed).

One other thing to consider is object-lifetime issues -- i.e. if you pass a const std::vector<Player>* to your MainWindow class, and the MainWindow class keeps a copy of that pointer-value (i.e. copies the passed-in pointer to a local member-variable), and then the vector is destructed, the MainWindow will be left holding a dangling pointer that will invoke undefined behavior (i.e. probably crash, or worse, silently corrupt your program's memory space in some very-hard-to-debug fashion) if/when it ever tries to dereference the pointer. If you want to avoid any possibility of that particular fiasco, you can pass the data via a smart-pointer instead (i.e. either a shared_ptr, or if you prefer to use the Qt-specific APIs, a QSharedPointer or QPointer object)

C++/QT: Using constant pointer to private member as read-only data-sharing

1 Answers1