I'm working on an assignment for my C++ programming class. The assignment is basically to take code given to us and fill it in to make a working, low level, text editor. Problem is, I've never seen this syntax before. If I knew what it were called, I could find information about it, I'm sure. It might even be in my textbook, I just haven't been able to find it...
Anyway, if anyone could point me in the right direction for learning about this type of class, I'd appreciate it greatly.
class DisplayManager: vector<string>
(A display is a vector of strings called "rows". Each line of the document may occupy one or more rows.)
This is called inheritance. Please have a look here.
Your DisplayManager class inherits from vector<string>. That means, it is essentially the same as vector<string>, plus the changes you make in the class body. However, since you did not add the public keyword, users of the class will not be able to access the members it inherited from vector.
Related
Im a bit confused. When I create a derived class, I get the constructor and destructor of the parent class. This, in my mind, would mean that I create 2 classes instead of one.
Now if i want to write the object to a file with fstream for example, using:
file.write((char*)&DerivedClass, sizeof(&DerivedClass));
Would that mean that i only write whatever is between the constructor and destructor of the derived class?
Yes, i tried typing it out myself, but i get mixed results. So thats why I am confused.
I hope someone could explain the mechanics behind it, and not so much a solution to writing it correctly.
First off, I'm new to c++, currently learning it online.
I greatly appreciate any help...
I have a vector of classes set up for vehicles, however it should also contain derived classes for vehicles of different types (car, truck, van).
My class is set up to have functions to gather user input and assign that to different variables, I do not use the constructor for this. Is that wrong?
In my main this all works fine and dandy but when I go to add the class to the vector I run into issues, first off, My instructor informed us to use smart pointers for this, so I initialize my vector like so...
vector<unique_ptr<Vehicle>> vehicles;
This is how I'm adding classes to that vector...
// For base class...
vehicles.push_back(make_unique<Vehicle>());
// For derived class...
vehicles.emplace_back(make_unique<Car>());
This is where I am stuck, adding the class this way will cause it to be called and therefor the constructor called as well. What I would like to do is just add a class that I have already created and defined beforehand, for example..
// Functions to gather user input, validate and display...
Vehicle vehicle;
vehicle.getMake();
vehicle.validateName(vehicle.make);
vehicle.getModel();
vehicle.validateName(vehicle.model);
vehicle.displayVehicle();
vehicles.push_back(/* Somehow add 'vehicle' */);
Thanks for any help, I understand this might be the wrong way to do this, I was just struggling to find a solution and this is currently where I am at.
My class is set up to have functions to gather user input and assign that to different variables, I do not use the constructor for this. Is that wrong?
A constructor shouldn't do lots of things, and never prompt for user input… but, at the same time, it should not be possible to have a "half-ready" object at any point. I suggest you gather all your user input first, then pass it into the constructor so your object is ready to go in one step.
(I have not shown this in the following example.)
This is where I am stuck, adding the class this way will cause it to be called and therefor the constructor called as well. What I would like to do is just add a class that I have already created and defined beforehand, for example..
Your vector contains unique_ptr<Vehicle>s, so you'll want to start off with one:
std::unique_ptr<Vehicle> vehicle = std::make_unique<Vehicle>();
vehicle->getMake();
vehicle->validateName(vehicle->make);
vehicle->getModel();
vehicle->validateName(vehicle->model);
vehicle->displayVehicle();
vehicles.push_back(std::move(vehicle));
Notice that all the vehicle. are now vehicle->, because vehicle is no longer an actual Vehicle but instead a pointer to one.
std::move is needed because unique_ptrs are not copyable, only moveable.
Taking my advice from the start of the answer, this gets somewhat simpler:
std::string make = GetMake();
ValidateName(make);
std::string model = GetModel();
ValidateName(model);
vehicles.push_back(std::make_unique<Vehicle>(make, model));
vehicles.back()->displayVehicle();
I've moved the displayVehicle call to the end, because it shouldn't make a difference to your output, but allows us to do the "construct Vehicle" and "put it into vehicles" steps simultaneously.
This approach assumes that you have altered your constructor to take make and model, and set up free function GetMake(), GetModel() and ValidateName() to do those tasks.
just a very simple question regarding inheritance in c++.
let's say I have a few classes.
Class A inherits from class B and from class C.
I want to make class D inherit from class A , but the functionality of class C is breaking my code.
Is it possible to somehow exclude class C when I inheriting from class A in class D?
edit:
#Quentin
I'm using SFML and class A inherits from the sf::NonCopyable class. Class A is the SceneNode class on which the hierarchy for all entities/objects in the game world is based. I was making a "TileEngine" class that produces instances of "TileLayer" objects and I wanted the TileLayers to inherit from SceneNode so that I can pass drawing calls onto them through the hierarchy but since they're non copyable I can't fit them into a container and iterate through them in the TileEngine class.
But I think you're right, it doesn't truly break the code. I think I'll just need to add a few variables and come up with a book keeping system to make it work.
I was just curious if what I asked was possible since it'd be an easy solution and I don't know all the ins and outs of using inheritance yet, so even though it seemed unlikely I decided to check. Thx for the replies I think I'll be able to adapt the code on my own.
Nope.
Your A is both a B and a C.
If D cannot be a C, then it cannot be an A either.
Maybe use composition instead?
Update based on your specific case: There are a couple of ways that you can sort this out.
First off, does a SceneNode really need to be non-copyable, and if so, why? If this is a pure design decision, it is now apparent that it was the wrong one, since you're now in need of a copyable SceneNode. If the decision is technical (for example, there is bookkeeping data that is hard to clone correctly), you can try solving that problem. Failing that...
Could your SceneNode be movable instead? Move semantics are generally simpler than copy semantics to implement, and standard containers are perfectly happy with movable-only values. But even in that case...
Could your SceneNode be a simple interface instead? You only mention being able to call a drawing function. This does not sound related to any copying business, so maybe an interface with a pure virtual draw function is all you need. Otherwise...
If you really can't budge these requirements (at which point I would be surprised, but let's pretend), you can simply use a container of std::unique_ptr<TileLayer>. These don't require anything from their pointee, and can be stored in containers at will.
And then there's a whole 'nother batch of techniques that could fit you case. Don't forget that OOP and inheritance are just one way to crack that nut, but C++ offers many more tools and techniques besides it. But first, make it work :)
(This question is more for the people who have access to the book, It's hard to put it into context otherwise)
I've been reading through the GoF's 'Design Patterns' book and there's a sentence that confuses me a little, under 'Creational Patterns->Prototype->Sample code' (page 124).
Near the bottom of the page, there is the implemententation for BombedWall, which as I understand is a concrete prototype, as it inherits from Wall, and redefines the Clone() virtual function. BombedWall also defines another method, HasBomb(), unknown to any clients using the regular Wall interface.
The only way that BombedWall is stored in MazePrototypeFactory (the Prototype client) is as a Wall* (returned from BombedWall::Clone), and the only way to get to HasBomb() afterwards, as far as I understand, is to perform a downcast on that Wall* to a BombedWall* (dynamic or static, depending on whether I know the type), and then I can access the HasBomb() method.
This all seemed fine to me; but then later the author says (same page, last sentence, 2nd last paragraph):
"Clients should never have to downcast the return value of Clone to
the desired type"
What? Then how am I supposed to get to HasBomb()?
I must be missing something...
I gave an answer and totally rewrote it now :)
Basically, the MazePrototypeFactory only knows about the base classes it can use. It doesn't know anything about any of the subclasses you are going to make, but it still should be able to put any possible subclass out there into the maze.
The pattern basically ensures MazeFactory will get a pointer of a type that it understands, Wall, than cause the MazeFactory to need to be modified to be able to produce objects of all the subclasses.
MazeFactory is the client referred to on p 124. It doesn't need to know about HasBomb in order to build the maze.
My guess is that this method only exists to indicate that BombedWall is a different class with extended public interface. Hovewer, this interface is not used in the context of the sample: the maze building algorithm does not differentiate types of walls, while other subsystems (for example, rendering engine) may do so.
Okay so here's a real mess of a question - I don't even entirely know what to search for.
I asked a question here, related to a game's entity handling system: Initiating a derived class with specific variable values
So far, that's working out great for me, but for one thing. I want to have mobs not just collide with eachother, but interact.
How can I look up a specific instance of the derived class, by coordinates?
For example, find the baseObject:Enemy() located at 22,22 and get the value of "nType" from within it
What comes to mind is putting some kind of pointer to an instance in an array, and moving it when said entity moves... but how do I make a derived class add a pointer to itself to an array? and how do I then pull something from that instance's variables?
Whew. Hope this makes sense.
Well, I think a solution to this problem is to use delegates/event passing.
Let's say you have a mob, as someone said earlier you could have a "mob watcher" object. The idea is that when a GameEntity is part of a mob, that entity subscribes to the "mob watcher". If that entity leaves the mob, it will let the "mob watcher" know about it (unsubscribe).
So, when you need to know, who's composing a mob, you could just ask the mob watcher for the "mob list", if you need to search by Entity position you could then work yourself through the list of entities composing the mob, and find the one in the "position of interest".
If your mobs are gigantic you could add some kind of spatial hash feature to your mob watcher, so you could easily filter and ask for "guys in the mob which are located in gameGrid[10][13]".
If you use event passing, it's quite cool because if when you want to forward messages between the mob, sending an event to the mob watcher could be used to then forward the aforementioned event to the subscribed entities.
If you use delegates, it works in a similar way.
Observer Pattern
Delegate Pattern
Well, the specific question you're asking is easy to answer: the pseudo-variable this is available in all member functions; it points to the current object.
The "looking up by coordinates" part is trickier; you might consider using an octree structure to organize your objects (Google is your friend there.)
You will need to make a new object whose job is to keep track of where the "mobs" are. According to your question, this new object will need to contain at least a std::vector which contains a pointer to each "mob" in the world, and also a member function like:
mob* findEnemy( int x, int y );
Unfortunately, what you asked for in your question is probably not what you want. You probably want to get all of the "mobs" near a point:
std::vector<mob*> findEnemiesNearPoint( int x, int y );
...but returning a std::vector can be very slow, so you probably want an array instead, but the array must be sized carefully, and now we're far far beyond the amount of information provided in your question so far.