第7章高水準プログラミング言語

We have learned that some of the data types of a language are primitive types, meaning that data of that type can be directly represented in the registers and memory locations of the machine. However, languages usually contain complex data types as well.

A complex data type is one consisting of multiple primitive types used as its building blocks which is why we also call them composite types. These multiple types may be of the same type, as in a complex data type known as an array, or they may consist of collections of different data types in one construct, such as a C# class.

An array is a typical composite type that is used as a data container. A great way to visualize an array is as a shelf unit, a connected structure where we can place items on each element. An instance of an array in this case could be a bookshelf that is meant to contain books (a book would be another composite type).

An array is a named variable that references a block of contiguous memory locations, and each “shelf” of the array is an element, which occupies exactly as many of the contiguous bytes as it takes to accommodate a value of the data type being stored. In the simplest type of array structure, an indexed array, the shelves are numbered with an index, starting at zero, or the lowest memory location. In a strongly typed language, all elements of an array must be of the same data type which means that every element will be of a uniform length in bytes.

Figure 7.10 illustrates an array in any number of HLLs including Java, C, and C#. We start off with the array declaration, which gives the data type of each of its elements, names the array variable numbers, indicates it is an array with the opening and closing square brackets ([]), and assigns five values to it with what is known as an array initializer (values separated by commas placed between curly braces). We can see that the length of the array is 5, the indexes run from 0 to 4, and each of the elements are contiguous in memory and are 4 bytes in length. The following statement assigns an element of the array to a variable:

Strings, which are another composite data type, are arrays of characters in most HLLs. In object-oriented languages, they are quite a bit more complex. They are implemented in some HLLs as an array which holds individual characters as its elements. The OOP languages usually implement strings as objects with built-in functions (methods). Here is an example of a string in Java:

In our study of variables and data types so far, we have examined the concept of a variable being a name-value pair. With primitive types, the memory location referenced by the name stores the actual value of the variable directly at that spot.

In the case of complex data types, things are not quite so simple. Let us take the example of a string. We can store a name in the string such as “Jimmy” to start. Now let us say that we change the value to “Johnathan” at some later time. The memory required to store the value has now changed and perhaps it will no longer fit in the original location. We have learned that the C language has a primitive data type known as a pointer. It is used for that reason.

Pointers are variables that hold actual computer memory addresses (references). They match the word size of the machine, which is typically 64 bits. We call a variable that holds memory addresses a reference variable. There is no such thing in Java or C. Therefore, when we create an array or a string in these languages, the value that is actually stored in the variable is the memory address of the place where the complex object exists. So, in the case of our string example, if we change the name, we can just change the value in its variable to be a different memory address—refer to Figure 7.3.