C++ pointer to class

Question

Can anyone tell me what the difference is between: ``` Display *disp = new Display(); ``` and ``` Display *disp; disp = new Display(); ``` and ``` Display* disp = new Display(); ``` and ``` Display* disp(new Display()); ```

Accepted Answer

The first case:

```
Display *disp = new Display();
```

Does three things:

- It creates a new variable `disp`, with the type `Display*`, that is, a pointer to an object of type `Display`, and then

- It allocates a new `Display` object on the heap, and

- It sets the `disp` variable to point to the new `Display` object.

In the second case:

```
Display *disp; disp = new GzDisplay();
```

You create a variable `disp` with type `Display*`, and then create an object of a different type, `GzDisplay`, on the heap, and assign its pointer to the `disp` variable.

This will only work if GzDisplay is a subclass of Display. In this case, it looks like an example of polymorphism.

Also, to address your comment, there is no difference between the declarations:

```
Display* disp;
```

and

```
Display *disp;
```

However, because of the way C type rules work, there is a difference between:

```
Display *disp1;
Display* disp2;
```

and

```
Display *disp1, disp2;
```

Because in that last case `disp1` is a pointer to a `Display` object, probably allocated on the heap, while `disp2` is an actual object, probably allocated on the stack. That is, while the pointer is arguably part of the type, the parser will associate it with the variable instead.

C++ pointer to class

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