Pointers

Pointers in AGS

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Various commands in the new scripting language will require you to use pointers. This section has been split into three separate topics to introduce you to pointers depending on your previous programming experience -- select one of the links below:

Pointers for programming newbies
Pointers for people who know Java or C#
Pointers for people who know C or C++

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Pointers for programming newbies

Pointers can be quite a daunting prospect, and in languages like C and C++ they certainly are; but AGS tries to make things as simple as possible.

Basically, a pointer is a variable that points to something of a particular type. For example, a Character pointer would point to characters. What's the point of all this, I hear you ask?

Well, let's look back at AGS 2.62. If you wanted to reference a particular hotspot, you'd need to remember its number. If you wanted to switch on an object, you'd need to remember what number it was too. And because you could accidentally use an object number where you wanted a hotspot number, mistakes could easily happen and it all got rather messy.

That's where pointers step in -- basically, they allow you to do away with identifying things by number, and in the process provide type checking, so you can't accidentally use a hotspot where you meant to use an object.

Let's look at an example. If you wanted to write a string to a file in 2.62, you'd do this:

int handle = FileOpen("temp.txt", FILE_WRITE);
FileWrite(handle, "Test!");
FileClose(handle);

That's simple enough; but what if you wanted to open the file in one place, and write to it somewhere else? You'd have to make handle a global variable, and then make sure you remembered that it was a file handle and not a hotspot number or anything else. Now, with 2.7 the same code would be:

File *file = File.Open("temp.txt", eFileWrite);
file.WriteString("Test!");
file.Close();

Looks fairly simple, doesn't it. The only slightly confusing part is getting used to declaring the variable as File* rather than int; but that's something you'll get used to quite quickly, and all the examples in the manual should point you in the right direction.

Let's look at another example. Suppose you want a variable that contains the current hotspot that the mouse is over. In 2.62, you might have something like this:

// top of global script
int mouseOverHotspot;

// repeatedly_execute
mouseOverHotspot = GetHotspotAt(mouse.x, mouse.y);

How would you do this in 2.7? Well, quite simply:

// top of global script
Hotspot *mouseOverHotspot;

// repeatedly_execute
mouseOverHotspot = Hotspot.GetAtScreenXY(mouse.x, mouse.y);

But hold on, what if you want to know whether the mouse is over your Door hotspot (say it's hotspot 2). In 2.62, you'd have done:

if (mouseOverHotspot == 2) {
  Display("Mouse over the door");
}

but that's rather messy, because what if you change the door's hotspot number? You'd have to remember to go back and change all the 2's to 3, or whatever. In 2.7, you now just do this (assuming you gave the hotspot a script name of hDoor):

if (mouseOverHotspot == hDoor) {
  Display("Mouse over the door");
}

If you're a fan of numbers for some strange reason, you can still use them like this:

if (mouseOverHotspot == hotspot[2]) {
  Display("Mouse over the door");
}

So, that concludes our introduction to pointers. Hopefully you've got an understanding of what they are and what they do; if there's anything you can't work out, feel free to ask on the Technical forums.

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Pointers for people who know Java or C#

AGS pointers work in a very similar way to object variables in Java and C#. The main difference is that AGS pointers are declared in the C-style manner with an asterisk t represent the pointer. So:

Hotspot *hs;

would declare a variable hs which points to a Hotspot. This would be equivalent to the following in Java or C#:

Hotspot hs;

In AGS, pointers are used to point to various built-in types, such as Hotspots, Inventory Items, Characters and so on. Because AGS does not have a new keyword, you cannot create pointers to custom struct types.

You use pointers in the same way as you would in Java and C#. Various built-in AGS static methods return a pointer to an instance (for example, File.Open, Hotspot.GetAtScreenXY, and so on). You can save this pointer into a pointer variable, and then call its methods as you would in Java or C#. The following examples are all valid:

File *theFile = File.Open("test.dat", eFileWrite);
if (theFile == null) Display("It's null!");
File *file2 = theFile;
if (theFile == file2) Display("They're the same file!");
theFile = null;
file2.WriteInt(10);
file2.Close();

If you attempt to call a method on a null pointer, an error will occur (just like you'd get an exception in Java or C#).

Pointer memory management in AGS is all automatic -- the memory is freed when there are no longer any variables pointing to the instance. Thus, if you have global pointer variables in your global script, it's a good idea to set them to null when you're no longer using them, to allow the memory to be freed.

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Pointers for people who know C or C++

Pointers in AGS are based on the C/C++ syntax, so they are declared with an asterisk. However, in AGS you can only create pointers to built-in AGS types, and not to any custom structs declared in your script.

Pointer members are accessed with the dot operator, and not the -> C-style operator. Because AGS doesn't support features such as pointers-to-pointers and so forth, there is no need for a separate operator.

In AGS, pointer memory management is done automatically based on reference counting (similar to the way COM works), so there is no new or delete keyword. When an object is no longer referenced by any pointer variables, it will be freed. For this reason, if you have any global pointer variables it's advisable to set them to null if you are done with them.

AGS pointers are strongly typed, and you cannot cast between types at will like you can in C and C++. AGS will only allow you to compare and assign pointers of the same type, or of the same base type. There is a special keyword null which all pointers can be set to and compared with, which indicates that they are unassigned.

Because there is no new keyword, you cannot create object instances; rather, they are returned by static member functions in AGS, such as File.Open and Hotspot.GetAtScreenXY. See the examples for the functions to get an idea of how to use them.