Programming IDL Objects: Why and how to do it

D. M. Zarro (ADNET/GSFC)



1.1 Creating an IDL object
1.2 Putting data into an object
1.3 Getting data out of an object
1.4 Destroying an object


2.1 Modifying object properties
2.2 Adding object methods
2.3 How does inheritance work?


3.1 Using MAP objects
3.2 Image analysis with objects


IDL objects were first introduced in version 5.0. The concept of object oriented programming (OOP) is not new. It is the basis of many modern languages such as C++ and Java. The idea behind OOP is that operations and data go hand in hand. Why? Because if you are handed a piece of data without any way of operating on it, then it would not be very useful to you. Furthermore, even if you were given software to operate on the data, the data may still not be useful without a set of rules to describe how the software interacts with it.

Consequently, the main thrust of OOP techniques is to keep operations and data married together so that they understand each other. This concept is called encapsulation. A valid question to ask is: what is wrong with non-OOP programming techniques in which procedures (or functions) are written to operate on data? For example, one can develop a reader to read a file, a plotter to plot it, and a writer to save it, as in the following pseudocode:

IDL> read,file,data
IDL> plot,data
IDL> write,file,data

The short answer is that there is nothing wrong with using non-OOP techniques. The long answer is that, as data sets become complicated and operations more complex, simple procedures are no longer simple. Procedures often become a series of steps that are called repeatedly or they require enhancements to support different datasets, which were not originally considered. In the OOP world, these procedures are called methods. These methods operate on data in exactly the same way as regular non-OOP procedures but they are special in that they function differently depending on the data in question. This concept is called polymorphism and will be described in more detail in section 2.3

The purpose of this tutorial is not to make the case for using OOP techniques in IDL, but to illustrate situations in which such techniques are more advantageous than non-OOP techniques. We start with demonstrating how to create a very basic IDL data object, followed by how to interact and manipulate it, and finally how to apply it to a real world example of reading and displaying a FITS image. Although not a strong pre-requisite, we assume that the reader is familiar with the use of IDL pointers and structures.

1.1 Creating an IDL object

An IDL object is simply a container in memory space. This container holds:

The descriptions are called properties, and the procedures are called methods. The interesting twist is that both properties and methods can change as the data changes.

The first step in creating an object is deciding a name. The name is important because it should signify something about the the data. For example, if we are developing software to sell different types of cars, such as Fords, Toyotas, and Hondas, then the logical object name is probably car. This generic name is referred to as the object class. There is nothing special or magic about the name, although it should be unique. However, the name does become important later when we develop different objects.

Since we are keeping things simple - and are already thinking in terms of data - we name our object class data. We define this class in a file named that contains the following code:

function data::init



pro data__define


So, what does the above all mean. Let's take it in steps:

  1. CLASS filename: naming the file is an IDL convention for writing a class definition file. The class name has to be part of the file name, followed by double underscore and the word define. If we were defining a car class, then the car class definition file would be named

  2. INIT method: the first line in the file defines a method to initialize the object. It is mandatory for all definition files. The naming convention for this (and all methods) is class name, followed by double colon and the method name (::init). In this example, the init method doesn't do very much. We will modify it later to actually do something useful. For now, it simply returns unity, which signifies that the object is successfully created.

  3. CLASS definition: the second line defines an IDL structure in which the data will be contained. An IDL structure is a variable that can hold different data types such as strings, floats, integers, and even other structures. In this example, it contains a pointer field (called ptr) that we create with the command: ptr_new(). The latter function creates an address in memory where future data will reside. The advantage of using a pointer is that we can dynamically insert data even though we don't know its size ahead of time. In general, the class definition file is where the attributes of the object are defined as structure tags. These attributes are called properties. In our data object example, we have a single property namely ptr.
Having written and saved the above file into the current IDL working directory, we create a data object as follows:

IDL> a=obj_new('data')
IDL> help,a
A               OBJREF    = ObjHeapVar37(DATA)

Creating the data object is called instantiation. The IDL function call to obj_new('data') looks for the file, compiles it, and executes the init and structure definition code that creates the data object. Calling help on the output variable a shows that it is an object of class type data. In the language of OOP, the variable a is an instance of the data class. Currently, this object doesn't do very much other than waiting around for some data to come along and some operations to perform on the data.

1.2 Putting data into an object

As currently defined, we cannot interact with the data object because we have not defined any methods to communicate with it. The next step is to write a method that allows us to insert data into the object. We re-edit the file as follows:

function data::init

;-- allocate memory to pointer when initializing object




pro data::set,value

;-- if data value exists, then insert into pointer location

 if n_elements(value) ne 0 then *(self.ptr)=value



pro data__define


Several things are going on that require explanation:

  1. SELF variable: we have added the following line to data::init,


    What is self? When we are working inside an object, we reference the object using the variable name self. The above line says:

    "Take the structure field named ptr, which we have defined to be a pointer, and allocate new memory to it using the IDL pointer function ptr_new(/allocate)."

    We only need to do this once when the data object is first created.

  2. SET method: we name the method for inserting data set. Any name will do, but following IDL convention we precede it with the class name data::. This method is a procedure that takes the variable data as a command line argument. As is always good practice, we check that the variable value exists before proceeding by calling n_elements(value). If the latter returns a non-zero value, then we insert the value into the pointer value ptr.

  3. Inserting data: if you are confused by the syntax *(self.ptr)=value, don't worry. Here is the simple breakdown. Recall that self is an internal reference to the data object, which is defined to be a structure with a tag (or field) named ptr. We reference this pointer as self.ptr. The latter is not an actual data value but an address to where data is located (or stored). The asterisk symbol references the value of the data stored at the pointer location. When the data object is first initialized, there is no data at this location. The IDL statement *(self.ptr)=value says:

    "Take my input data value and insert (or copy) it to this pointer location."

    In the language of OOP, this action is called setting the object property.

1.3 Getting data out of an object

In addition to inserting data, we need a method for extracting data from the object. We shall call this method data::get, and include it in the file as follows:

function data::init

;-- allocate memory to pointer when initializing object




pro data::set,value

;-- if data value exists, then insert into pointer location

 if n_elements(value) ne 0 then *(self.ptr)=value



function data::get,value

;-- if data value is stored in object pointer, then copy it out

 if n_elements(*(self.ptr)) ne 0 then value=*(self.ptr)




pro data__define


The get method differs from set in that we have defined it to be a function. The choice of function versus procedure is more of a matter of convenience than convention. The function first checks for the existence of a data value at the pointer location self.ptr. If data is present, then the value *(self.ptr) is copied to the output variable value. If there is no data value, then an undefined value is returned.

1.4 Destroying an object

When finished with using an object, it is recommended that the memory allocated to the object be released. All objects should therefore have a method that will take care of cleaning up after themselves. The IDL naming convention for this method is ::cleanup. In the case of the data object, this cleanup method would involve freeing the pointer property of any allocated data. To implement a cleanup method, we include the following lines in the file

function data::cleanup

;-- free memory allocated to pointer when destroying object



The IDL procedure ptr_free flushes the pointer variable self.ptr of any saved data and re-initializes it. This method is not called directly. Instead it is called automatically when the object is destroyed using the IDL obj_destroy procedure as follows:

IDL> obj_destroy,a                             ;-- destroy object
IDL> help,a
A               OBJREF    = ObjHeapVar4        ;-- object is now null


Having defined a data class, we next demonstrate how it can be used in common applications, and how it can be extended to perform different functions.

2.1 Modifying object properties

Because we have used a pointer as its property, the data object can accomodate any data type. For example, let's create a 2-dimensional float array and insert it and extract it as follows:

IDL> image=findgen(512,512)
IDL> a=obj_new('data')        ;-- create object variable a
IDL> a->set,image             ;-- insert image
IDL> image2=a->get()          ;-- extract image
IDL> help,image2
IMAGE2            FLOAT     = Array[512, 512]

This example introduces the arrow syntax for calling methods. The statement a->set,image says:

"Call the method named set on the object variable named a, and pass the argument variable named image."

We apply the same syntax when calling the get method except that we invoke it as a function and return the value into the output variable image2.

Next, let's try inserting and extracting a data structure such as the system variable !d:

IDL> a->set,!d
IDL> var=a->get()
IDL> help,var,/st
** Structure !DEVICE, 17 tags, length=88:
   NAME            STRING    'X'
   X_SIZE          LONG               640
   Y_SIZE          LONG               512
   X_VSIZE         LONG               640
   Y_VSIZE         LONG               512
   X_CH_SIZE       LONG                 6
   Y_CH_SIZE       LONG                12
   X_PX_CM         FLOAT           40.0000
   Y_PX_CM         FLOAT           40.0000
   N_COLORS        LONG          16777216
   TABLE_SIZE      LONG               256
   FILL_DIST       LONG                 1
   WINDOW          LONG                 0
   UNIT            LONG                 0
   FLAGS           LONG            328124
   ORIGIN          LONG      Array[2]
   ZOOM            LONG      Array[2]

In this example, we insert the variable !d into the object and then retrieve it into the variable var. Note that we only create the object variable once, and recycle it as necessary. We can of course create as many data objects as we like and store different data types accordingly. Just remember to give them different variable names.

2.2 Adding object methods

As we have already seen, we can add new methods to an object by editing its class definition file. We can make the data object more useful by giving it the ability to read data from a file and plot the data. In the following example, we open the file and add two methods that we conveniently call data::read and data::plot:

pro data::read,file

 if n_elements(file) ne 1 then return ;-- at least one file name entered
 check=findfile(file,count=count)     ;-- check if file exists
 if count ne 1 then return            ;-- bail if not there
 image=fltarr(512,512)                ;-- assume a 512x512 image file
 openr,lun,file,/get_lun              ;-- open file
 readf,lun,image                      ;-- read image
 free_lun,lun                         ;-- close file
 self->set,image                      ;-- insert image into object



pro data::plot

 value=self->get()                                    ;-- extract data value from object
 dsize=size(value)                                    ;-- determine data dimensions
 if dsize[0] eq 2 then tvscl,congrid(value,512,512)   ;-- if 2-dimensional, CONGRID and TVSCL it


The read method accepts a file name as its argument. As is good practice, we check if the filename argument is entered and use IDL's findfile to test if the file actually exists. We subsequently open the file, read the data, and insert into the object using the set method call: self->set,value. Note that because we are referencing the object itself, we use the self variable name for the object.

The plot method extracts the data value from itself via the get method call: self->get(). It checks that the data is a 2-dimensional image using the IDL size function, and plots it with a call to IDL's tvscl command and congrid, which expands the image to a 512x512 array size. To demonstrate this sequence of steps, let's create an image file image.dat in the current directory and deploy the data object as follows:

IDL> image=findgen(512,512)        ;-- create 512x512 image array
IDL> file='image.dat'
IDL> openw,lun,file,/get_lun       ;-- write image to file
IDL> printf,lun,image
IDL> free_lun,lun

IDL> .run data__define             ;-- recompile class definition file
IDL> a->read,file                  ;-- read image and display it
IDL> a->plot

The call to the plot method produces the following simple image:

Sample data plot

Note that it is not necessary to reinitialize the data object since we are using the same object variable to store the image data. However, it is necessary to recompile the class definition file since we have added new methods to it.

2.3. How does inheritance work?

Looking at the last two lines in the previous example, it doesn't appear from our data object example that we have advanced very far using OOP techniques. In particular, compared to the opening example of reading and plotting data using non-OOP procedures, it seems that a lot of effort was invested in writing object methods that essentially reproduce the same functionality as conventional procedure calls.

The real power of OOP techniques comes not in what we have just done, but in what we are about to do. The data object that we have created is a building block that can be used to develop objects that perform more complicated functions. Consider the following problem:

"I have an image in a FITS file that I would like to read and display. I would also like to remember the name of the FITS file so that I can track it."

The data object that we have created provides a convenient storage facility for FITS image data, but it lacks the required FITS file reader and it doesn't have a way of remembering the file name. We could of course re-edit the data class definition file to add this functionality, but that would involve much more work. The OOP solution to the problem is to define a new class that somehow inherits the functionality of the data class. The following is how to do it.

We define a new class called fits by creating a file named, which contains the lines:

pro fits::read,file

 if n_elements(file) ne 1 then return   ;-- at least one file name entered
 check=findfile(file,count=count)       ;-- check that file exists
 if count eq 0 then return
 image=mrdfits(file)                    ;-- call Astronomy library FITS reader
 self->set,image                        ;-- insert image data into property
 self.filename=file                     ;-- save filename in property 



pro fits__define
 void={fits,filename:'', inherits data} ;-- inherit from data class


Several new concepts are happening here. Let's start from the bottom up.

  1. FITS__DEFINE: the procedure that defines our fits data structure looks very different from the original data structure in It appears to be missing the data pointer property. Actually, the latter is is not missing. In OOP language, it is inherited from the data class via the statement inherits data. In addition to inheriting data's property, the fits class inherits all of its methods: init, cleanup, set, get, read, and plot. Hence, inheritance has saved us from writing a lot of extra code. Note that the fits data structure contains a new property called filename, which is a string data type. This property will be used to store the FITS file name.

  2. FITS::READ: object inheritance imports data's simple read method data::read, which clearly will not work on a FITS file. To overcome this problem, we write a new method that uses the Astronomy library routine to read the FITS file and insert the resulting image data into the object pointer by using the inherited set method. The fits read method thus overrides data's read method. This new method also saves the FITS filename as a property of the fits object once the data is saved.

In OOP language, the fits class is a derived class of data, and a fits object is referred to as child of the parent data object. The following example demonstrates how to use the fits class to create an object to read a Big Bear H-alpha image contained in the file bbso_halph_fl_20040310_173531.fts:

IDL> file='bbso_halph_fl_20040310_173531.fts'
IDL> f=obj_new('fits')                 ;-- create object
IDL> help,f
F             OBJREF    = ObjHeapVar37(FITS)

IDL> f->read,file                      ;-- read file
IDL> f->plot                           ;-- plot data
IDL> image=f->get()                    ;-- extract image
IDL> help,image
IMAGE            INT       = Array[512, 512]

BBSO H-alpha

We create a fits object using the obj_new function, and feed it the FITS file name. After reading, we plot the image, and extract the image data using the get method. As defined, the get method inherited from the data class only allows us to extract the data value from the property pointer. However, we would also like to extract the filename that is associated with the data, which is also a property. To include this functionality, we override the get method in with a new get method in as follows:

function fits::get,filename

 filename=self.filename             ;-- copy filename in variable
 image=self->data::get()            ;-- call DATA's GET method to return data


The above lines illustrate the simplicity and elegance of inheritance. We have added a new output argument filename in which we return the string value of the filename, which is saved in the property self.filename. Since we also wish to return the data value, we include a call to the get method that we have already defined for the data class. There is no need to rewrite the latter. Hence, in the last example, we can execute the following:

IDL> data=f->get(filename)                        
IDL> help,data,filename
DATA            INT       = Array[512, 512]
FILENAME        STRING    = 'bbso_halph_fl_20040310_173531.fts'

Note also that the fits object methods (read and get) retain the same names as their parent data method names. The difference is in their behaviors, which depends upon which data type is being operated on. This ability to behave differently depending upon class (or data type) is called polymorphism.


The example classes in this tutorial demonstrate how objects can be designed, created, and used. If interested in experimenting with these classes, you can download complete definition files via the following links: and Although useful for illustrative purposes, these classes are too simplistic for handling more complicated solar datasets. For example, not all solar datasets conform to the FITS format standard. Variations in the use of header keyword names and values often require the use of special readers. Moreover, different datasets usually require the application of instrument-specific processing algorithms inorder to be useful. This dependence of operations upon the properties of the dataset naturally lends itself to the use of objects as a tool for analyzing solar data.

3.1 Using MAP objects

IDL maps are described in maps.html. An IDL map is a structure data type that stores data and associated identifying information. A map structure is not a true object since it does not contain methods. Nevertheless, it is useful for displaying and, in particular, overlaying solar images from different instruments. The following example illustrates how to to read, process, and display the quicklook SOHO/EIT file efr20040309.072550 using a map and procedures available in the IDL SolarSoft library:

IDL> files='efr20040309.072550'                   
IDL> read_eit,files,index,data                   ;-- read EIT QL file
IDL> eit_prep,index,data=data,outindex,outdata   ;-- prep data
IDL> index2map,outindex,outdata,emap             ;-- create map
IDL> eit_colors,195                              ;-- load 195 color table
IDL> plot_map,emap,/log,grid=20,/limb            ;-- plot map


This example highlights several points. Even though we are primarily interested in reading and plotting an EIT image, we have to know and perform several steps:

  1. read the EIT FITS file using the special reader read_eit.

  2. process the EIT image using the procedure eit_prep, which performs dark current subtraction and flatfielding.

  3. convert the processed image to a map structure emap via the procedure index2map.

  4. plot the map structure emap using the procedure plot_map, which is called with the keywords /limb, /log, and grid=20 inorder to display a log-scaled image with a heliographic grid and limb.
In addition, we have to remember to pass the variables: index, data as arguments to these procedures, and know that our EIT dataset is a 195 A image that has a custom color table (which we load with the procedure eit_colors). That's a lot to remember each time. It would be nice if we could somehow encapsulate the above procedures and data into an object and only have to remember as little as necessary to get the job done.

What we need is a map class that will allow us to define map objects to store data such as EIT images and provide methods to manipulate them. Such a class already exists. It is defined in the file The map class is analogous to our example data class except that it uses a map structure to store data and its corresponding properties (such as pointing).

Now consider the following OOP approach to the same example:

IDL> file='efr20040309.072550'                   
IDL> eobj=obj_new('eit')                         ;-- create an EIT object
IDL> eobj->read,file                             ;-- read EIT file
IDL> eobj->plot                                  ;-- plot EIT image

The above example produces exactly the same plot output as the conventional example, but what is different? Let's take it in steps:

In summary, the object variable eobj object is a map object since the eit class is derived from a map class. The details of the relationship between eit and map classes are not overly important to the average user who is interested in performing basic data analysis. The main point is that, by using an eit map object, the overhead of remembering several instrument-specific procedure names is significantly reduced.

3.2 Image analysis with objects

The eit class has a get method that provides access to the EIT image data and map structure as follows:

IDL> edata=eobj->get(/data)
IDL> help,edata
EDATA           INT       = Array[1024, 1024]

IDL> emap=eobj->get(/map)
IDL> help,/st,emap
** Structure <40e0fb08>, 13 tags, length=2097256, refs=2:
   DATA            INT       Array[1024, 1024]
   XC              FLOAT          -8.15294
   YC              FLOAT           21.0663
   DX              FLOAT           2.63500
   DY              FLOAT           2.63500
   TIME            STRING    ' 9-Mar-2004 07:24:58.402'
   ID              STRING    ' Rocket Science EIT 195'
   ROLL_ANGLE      FLOAT           0.00000
   ROLL_CENTER     FLOAT     Array[2]
   DUR             FLOAT           12.5970
   XUNITS          STRING    'arcsecs'
   YUNITS          STRING    'arcsecs'
   SOHO            BYTE         1

The pointing information contained within a map structure allows us to analyze different images regardless of the image source. By making a map structure a property of a map object, we simplify the steps involved in performing typical image processing operations. We will conclude this tutorial by demonstrating three such operations: rotating an image; correcting for differential solar rotation; and overlaying images.