Following a conversation with a friend regarding how ColdFusion handles arrays and structures in ‘the background’, I was interested to find out what Java classes each were mapped to. This was a relatively simple case of using the functions getClass(), getSuperClass() and getName() to parse out the name of the Java classes.

<!--- Arrays (repeat up the hierarchy) --->
<cfdump var="#arrayNew(1).getClass().getName()#">
<cfdump var="#arrayNew(1).getClass().getSuperClass().getName()#">
<!--- Structures (repeat up the hierarchy) --->
<cfdump var="#structNew().getClass().getName()#">
<cfdump var="#structNew().getClass().getSuperClass().getName()#">

What became immediately apparent was that both were handled differently. In the case of a ColdFusion array, the parent class was the java.util.Vector, which seemed obvious. The Vector class implements a growable array of objects. Like an array, it contains components that can be accessed using an integer index. However, the size of a Vector can grow or shrink as needed to accommodate adding and removing items after the Vector has been created.

ColdFusion structures, however, are handled entirely differently, with the parent class being a coldfusion.runtime class. In fact, as we can see below, the code to handle a ColdFusion structure is entirely written for ColdFusion until the root class of java.lang.Object. Clearly ColdFusion does some complex wizardry to handle the complex nature of structues (associative arrays).

The class hierarchy for an Adobe ColdFusion array:

java.lang.Object
	java.util.AbstractCollection
		java.util.AbstractList
			java.util.Vector
				coldfusion.runtime.Array

The class hierarchy for an Adobe ColdFusion structure:

java.lang.Object
	coldfusion.util.CaseInsensitiveMap
		coldfusion.util.FastHashtable
			coldfusion.runtime.Struct

Being an inquisitive person, I applied the same code to Railo and BlueDragon, with equally interesting results.

The Java class hierarchy behind a Railo array:

java.lang.Object
	railo.runtime.type.ArrayImpl

The Java class hierarchy behind a Railo structure:

java.lang.Object
	railo.runtime.type.StructImpl

The Java class hierarchy behind a BlueDragon array:

java.lang.Object
	com.naryx.tagfusion.cfm.engine.cfData
		com.naryx.tagfusion.cfm.engine.cfJavaObjectData
			com.naryx.tagfusion.cfm.engine.cfArrayData
				com.naryx.tagfusion.cfm.engine.cfArrayListData

The Java class hierarchy behind a BlueDragon structure:

java.lang.Object
	com.naryx.tagfusion.cfm.engine.cfData
		com.naryx.tagfusion.cfm.engine.cfJavaObjectData
			com.naryx.tagfusion.cfm.engine.cfStructData

Adendum: Ben Nadel has a good little post on ColdFusion Data Types from Different Sources and How ColdFusion Sees Them.

In an earlier post I eluded to the implicit creation of arrays in ColdFusion 8. Well, the same can be said of structures.

A structure, also known as an associative array, is a complex data type composed of a collection of keys and a collection of values, where each key is associated with one value (a key-value pair). The operation of finding the value associated with a key is called a lookup or indexing, and this is the most important operation supported by a structure. The relationship between a key and its value is sometimes called a mapping or binding. For example, if the value associated with the key “Age” is 29 and “City” is “London”, we say that our structure maps “Age” to 29 and “City” to “London”.

Using structures, you can call the array element you need using a string rather than a number, which is often easier to remember. The downside is that these aren’t as useful in a loop because they do not use numbers as the index value.

We can think of an address book as a good example of a structure. The classic way of creating and assigning key-values pairs to a structure, in earlier versions of ColdFusion, would be as follows:

<cfscript>
strPerson = structNew();
strPerson.firstName = "Jean";
strPerson.lastName = "Dupont";
strPerson.city = "Paris";
</cfscript>

Or an alternative method uses array-notation to create the necessary key-value pairs:

<cfscript>
strPerson = structNew();
strPerson["Firstname"] = "Hans";
strPerson["Lastname"] = "Mustermann";
strPerson["Country"] = "Germany";
</cfscript>

NB. When using the array-notation, the key names keep their case. However, running the following code results in the value “France” being overwritten with “Germany”, even though the key name is a different case. This serves to highlight that ColdFusion is not case-sensitive.

<cfscript>
strPerson = structNew();
strPerson["Country"] = "France";
strPerson["COUNTRY"] = "Germany";
</cfscript>

Implicit Structures

With the introduction of implicit structures in ColdFusion 8, the creation of structures is greatly simplified. For example, rather than having to use the structNew() function, we can now simply do the following:

Using strings for values:

<cfscript>
myStruct = {firstname="Simon", lastname="Whatley", city="London"};
</cfscript>

Using integers for values:

<cfscript>
myStruct = {account_no=12345678, sort_code=123456};
</cfscript>

Using integers as keys:

This example most closely represents an array since arrays have numeric keys.

<cfscript>
myStruct = {10001="John", 10002="Doe", 10003="New York"};
</cfscript>

The integer could represent the unique identifier of an object, for example, user ID or order ID. Therefore, if we had nested structures like below, 10001 would be the ID of Simon Whatley, whilst 10002 would be the ID of John Doe.

<cfscript>
myStruct1 = {firstname="Simon", lastname="Whatley", city="London"};
myStruct2 = {firstname="John", lastname="Doe", city="New York"};
myStruct3 = {10001=myStruct1, 10002=myStruct2};
</cfscript>

Mixed data types:

It is possible to mix the data types in an structure. For example, we can use an Integer, String and Array as elements within an array, with no problems. Since we need to know the key name before accessing the value, it is also likely we will know the type of the value and will be able to handle it accordingly. However, never assume this is always the case, so type checking is necessary when retrieving the data.

The example below demonstrates the ability to add arrays to structures.

<cfscript>
myArray1 = [1,2,3];
myArray2 = ["One","Two","Three"];
myStruct = {array1=myArray1, array2=myArray2};
</cfscript>

Structures, by their nature, cannot be sorted by value, only by the key name. They are best for related data, where order is not important and direct access to an individual element is important. Many of ColdFusion’s variable scopes can be accessed as structures, for example, Server, Application, Session and Variables etc.

Words of Caution

Implicit structures do have their limitations. For example, you cannot nest implicit struct, or indeed array, creation.

<cfscript>
myStruct1 = {
	myStruct2 = {
		firstName = "Jean",
		lastName = "Dupont",
		country = "France"
	},
	myStruct3 = {
		firstName = "Juan",
		lastName = "Pablo",
		country = "Spain"
	}
}
</cfscript>

The above will throw the following parsing error:

coldfusion.compiler.ParseException: 
Invalid CFML construct found on line 3 at column 10.

UPDATE: The recent ColdFusion Update now includes the ability to nest implicit structures.

To get around this problem, you can create each structure individually and then use the structure as the value in a key-value pair (as seen in the nested structure example above).

A (possible) strength of ColdFusion is that you can add key-value pairs as many times as is necessary. This is the same for explicit and implicit structure creation. However, the following code and screenshot serves to demonstrate that whether you explicitly or implicitly create a structure, if you duplicate a key, the last key-value pair in the sequence is the one that is represented in the structure:

<cfscript>
myStruct = {
	firstName = "Jean",
	lastName = "Dupont",
	country = "France",
	country = "Germany"
};
</cfscript>