A constant is a type of variable that is set once at the start of the game and then never changes. In fact, constant values cannot be changed after they have been declared. This makes them ideal for holding values that are used throughout the game to identify special data.

In the GameMaker Language there are two types of user-defined constant: macros and enums, both of which are explained below. Also note that any value that is always the same is classed as a constant, regardless of the data type, for example, a string or the number 3.

NOTE: The GameMaker Language also has a number of built-in constant values that are used to identify specific things. These are outlined on the appropriate pages for the runtime functions that require them in the GML Reference section.



While not exactly variables, macros are similar to them in how they are used, ie: they are named values that you can use throughout your code to replace hard-coded values. Basically, a macro is a named variable that holds a constant single value of any data type. You can define your own macros using the Script Editor and then use them in your code and DnD™ as if they were regular variables, with the one difference being that they can't be changed in the game.

The syntax structure for a macro is as follows:

#macro <variable> <value>

For example say you define the following macro "total_weapons" (note the preceding "#" and the lack of a colon ";" at the end):

#macro total_weapons 10

You would then call this in your code like this:

if ++pos == total_weapons
    pos = 0;

Note that you would not be able to change the constant value, so code like this will cause the game to crash:

total_weapons = 11;

You can define a macro anywhere in your code or in a script and it will be pre-compiled and included in your game as if it was there from the start, but we recommend that you create a dedicated script asset and define all your macros in there. It will be easier to organise and debug later!

If you need the value of a macro to change at run-time then you should probably make it a global variable, since these can be changed from code during a game, unless you set the macro to be a runtime  function. By setting the macro to a function it means that this function will be called every time you use the macro. For example:

#macro col make_colour_hsv(irandom(255), 255, 255)

You would then call this macro something like this:

image_blend = col;

Using this code will make the image blend a different colour every time the macro is used. It is worth noting that you can also split macros over multiple lines using the \ character to show where the line breaks. An example would be something like:

#macro hello show_debug_message("Hello" + \
string(player_name) + \
", how are you today?");

This is purely cosmetic, in that splitting a macro like this will have no effect over the result of the final macro when used, and is simply to provide support for multi-line text on macros that have longer lines of code.

One very important feature of macros is that they can be defined for use with specific Configurations (configs), meaning you can have the same macro name but give it different values based on the currently selected config. For example, say you have a configuration for Android Ads and another for iOS Ads, then you could define a single macro to hold the required app ID value:

#macro ad_id "";
#macro Android:ad_id "com.yoyogames.googlegame"
#macro iOS:ad_id "com.yoyogames.appstoregame"

As you can see, you give the config name first then a colon : and then the macro name and value. Note that you cannot have any white-space between the colon : and either the config name nor the macro name otherwise you will get an error.



An enum is an "enumerator", and it essentially permits you to create your own limited data type with a list of constant values, and they have the following structure:

enum <variable>
    <constant> [= <value>],
    <constant> [= <value>],
    // etc...

In the following example, we create an enum for the colours of the rainbow and assign it various constants and default values:

enum rainbowcolours

The enum entries can only be integer numbers or expressions with previous enums that evaluate to an integer number, and by default are numbered from 0 upwards, so our example given above would default to red = 0, orange = 1, yellow = 2, etc...

You can also assign values to the enum variables at the time of creation:

enum enum_test
    val = 10;

enum rainbowcolours
    red = 5,
    orange = 5 * 2,
    yellow = 15,
    green = 20,
    blue = 25,
    indigo = 30,
    violet = 35 * enum_test.val

Notice in the above example we use another enum to create an expression for "violet". This only works if the enum being referenced was created before the enum that is using it in an expression, but it will not work for variables or functions, since the enum value must be able to be evaluated as a constant when the project is Compiling. Also note that all enum values evaluate to integer values, and when you create your own you should be aware that only integer values are permitted for enums to work. This value can be any integer number that a floating point double precision number can represent, including negative values.

To later access the value within a given enum type, you can use the point "." method, like this:

variable = <enum_name>.<enum_variable>;

As an example, let's use the "rainbowcolours" enum that we created in the code above:

colour_value = *;

The colour_value variable would now hold the value 100 (20 * 5).

Note that you cannot modify the values for any enum constant after it has been created, much the same as you can't modify macros after they have been created.

NOTE: Enum values are stored as int64s, so running is_real() on them will return false.


Built-In Constants

The following table shows a list of the built-in constants that can be returned by some functions and operations in your projects:

Constant Description
pointer_null This constant indicates that the pointer is not pointing to anything meaningful (the same as NULL in C++ or null in C#)
pointer_invalid This constant simply means that the value is not a valid pointer
undefined This constant is returned when a function has to return something but has no appropriate or "correct" value to return
NaN This constant that can be returned when the compiler cannot evaluate the results of an operation as a number - for example, 0 / 0 cannot be defined as a real number, and is therefore represented by NaN
infinity This constant  refers to a number that is considered infinite, such as the result you would get when dividing any floating point value by zero, eg: 1.0/0.
true This constant represents the value 1, which is what GameMaker Studio will evaluate as a boolean "true" (note that any value equal to or greater than 1 will evaluate as true).
false This constant represents the value 0, which is what GameMaker Studio will evaluate as a boolean "false" (note that any value less than or equal to 0 will evaluate as false).
pi This constant represents the value of pi: 3.141592653589793280 etc... although the exact value will depend on various factors like the OS or the platform being targeted.

See Equality Table for information on equality comparisons for a few of the constants listed above.