Arrays and Structures are considered to be complex data types in ColdFusion. In contrast, simple data types are ones that contain a single piece of data, such as an Integer, String, or Boolean value. A complex data type can contain multiple pieces of data, which, in the case of arrays, are usually related. All the data are referenced under a single variable name. You can think of a complex variable as a variable that contains a collection of other variables inside it. An array maps Integers to arbitrarily typed objects (Integers, Strings, Booleans and Objects) while a structure, or associative array, maps arbitrarily typed objects to arbitrarily typed objects.

Arrays and structures are also known as reference types. A reference is an object containing information which refers to data stored elsewhere, as opposed to containing the data itself. References are fundamental to constructing many data structures and in exchanging information between different parts of a program. References increase flexibility in where objects can be stored, how they are allocated, and how they are passed between areas of code. As long as we can access a reference to the data, we can access the data through it, and the data itself need not be moved. They also make sharing of data between different code areas easier; each keeps a reference to it.

Array and structure reference types are pointers to a memory space. Pointers are the most primitive and error-prone but also one of the most powerful and efficient types of references, storing only the address of an object in memory. Because arrays and structures are pointers to a space in memory, it allows for easy copying of that data. However, when copying an array or structure, there is a distinct difference between how the copy is performed. This difference relates to the concepts of shallow and deep copying.

When copying an array, a deep copy is performed by ColdFusion. This means that the data is copied and a reference to the new copy is created in memory. Therefore, following the example below, a change in the value held at position 1 of array b will not over-write the value held in position 1 of array a as they have different memory references:

<cfscript>
	a = arrayNew(1);
	a[1] = 100;
	b = a; //deep copy for Array
	b[1] = 500;
	writeOutput(a[1]); // the returned result is 100
</cfscript>

We can also use the little-known Java function equals() to demonstrate that the two arrays are not equal (the references are different).

<!---
the following returns false
a and b do not refer to the same object
 --->
<cfdump var="#a.equals(b)#">

This is not the case for a structure. When copying a structure, as in the example below, a shallow copy is performed by ColdFusion. This means that a change in b.name will also result in a change of a.name:

<cfscript>
	a = structNew();
	a.name = "Simon Whatley";
	b = a; //shallow copy for Struct
	b.name = "John Doe";
	writeOutput(a.name); // the returned result is John Doe
</cfscript>

Using the equals() method, we can show that the two structures refer to the same object:

<!---
the following returns true
a and b refer to the same object
 --->
<cfdump var="#a.equals(b)#">

However, by using the duplicate() function, a deep copy of a structure can be performed:

<cfscript>
	a = structNew();
	a.name = "Wu Mingshi";
	b = duplicate(a); //deep copy for Struct
	b.name = "Ivan Ivanovic";
	writeOutput(a.name); // the returned result is Wu Mingshi
</cfscript>

The above code results in two structures being created.

Arrays and structures although similar, behave very differently. Both are known as reference types which refer to a pointer in memory. Both have the ability to contain multiple values underneath a single variable name. Both use an index to access an individual value, but the index is numeric for arrays and a arbitrary, text value for structures. Arrays are extremely useful for numeric calculations, tabular data, and data sorting. Structures, by their nature, cannot be sorted by value, only by key name. They are commonly for related data, where order is not important and direct access to an individual element is important. Many of ColdFusion’s variable scopes (such as server, application, variables, session, form etc) can be accessed as structures.