Strange problem with classes

Question

I'm trying to make a simple game engine. I've never used OOP before, so this is probably a simple mistake, but I get this error when trying to create an instance of a class.

invalid conversion from `World*' to `int'
initializing argument 1 of `World::World(int)'

This is the code to create the class.

  World w = new World(100);

And the actual class:

class World {
    int maxParts;
    public:
     GameObject **parts;
     World(int maxParts);
     int addObject(int type);
    private:
     int firstFreeId();
};

World::World(int maxParts)
{
    parts = new GameObject *[maxParts];
}

...

Thanks for any help.

Answer 1

The new operator returns a pointer:

World *w = new World(100);

After having created a World object in this way, you would usually access it using the -> operator:

w->addObject(5);

When you're done with it, don't forget to delete it or you will have a memory leak:

delete w;
w = NULL;

Setting w = NULL is not strictly necessary, but it's good practice because it prevents you from accidentally accessing the object after it's been deleted.

Answer 2

You're missing an asterisk here:

World *w = new World(100);

When you create something with new it gives you a pointer to the new object, so the variable you put the pointer into must be declared with an asterisk.

Answer 3

I would highly recommend starting with C# or java instead of C++ to learn OOP.

Since this was received negatively I have edited the response. I was not trying to be obnoxious. My recommendation stands here is why:

First off I like Dave Hinton's reasons of course (thanks for chiming in), here are some more:

I think if you use .net or the Java sdk you'll have a lot more out of the box libraries and objects to tap into. I know plenty exist in C++ but are a little harder to acquire and piece together I think. I'm talking about much higher level objects than the STL of course. For instance in .net I can use something called a "MailMessage" to construct an email. I wouldn't know off hand how to do that these days with gcc and C++. I guess the emphasis here is on the framework support for Java or C# versus that of C++ (non .net C++ that is).

Next, I think namespaces are slightly easier in java and C# although I know this will probably get lash back.

Above all it boils down to the fact that if you look the proposed answers (and they're correct right?) you'll notice they would never have been an issue in Java or C#:

1. destructor
2. missing asterisk
3. confusion (for beginners, at least was for me) between '.' and '->'

Answer 4

new World(100) dynamically creates a new World object, passing 100 to the constructor, but evaluates to a pointer to the created object.

If you just want to construct a World object you don't need a new expression, you can just construct it directly:

World w(100);

The error you are getting is because you are trying to construct a World object based on the World pointer returned by the new expression. The only World constructor takes an int and you can't convert a pointer type to an int without an explicit cast.

Answer 5

World w = foo; calls the constructor of World with the argument foo (unless foo is a World object already) and then calls the copy constructor of World with the result as an argument. new World(foo) calls the constructor of World with the argument foo, stores the result on the heap and returns a pointer to that.

So World w = new World(100) first creates a World object on the heap, calling the constructor with the arguemnt 100. It then calls the constructor again with the result of new World(100) as the argument (because World w = foo calls the constructor with foo as an argument) which does not work because the constructor does not accept arguments of type World*.

Answer 6

You also need a destructor to clean up your pointer array.

World::~World()
{
    delete[] parts;
}

Note the different form of delete when deleting an array - this is critical.

Or you could use std::vector<GameObject*> and you won't have to worry about it.