X hits on this document





39 / 396


class Test { static void Main() { Point[] points = new Point[100]; for (int i = 0; i < 100; i++) points[i] = new Point(i, i); } }

An alternative is to make Point a struct.

struct Point { public int x, y;

public Point(int x, int y) { this.x = x; this.y = y; } }

Now, only one object is instantiated—the one for the array—and the Point instances are stored in-line in the array.

Struct constructors are invoked with the new operator, but that does not imply that memory is being allocated. Instead of dynamically allocating an object and returning a reference to it, a struct constructor simply returns the struct value itself (typically in a temporary location on the stack), and this value is then copied as necessary.

With classes, it is possible for two variables to reference the same object and thus possible for operations on one variable to affect the object referenced by the other variable. With structs, the variables each have their own copy of the data, and it is not possible for operations on one to affect the other. For example, the output produced by the following code fragment depends on whether Point is a class or a struct.

Point a = new Point(10, 10); Point b = a; a.x = 20; Console.WriteLine(b.x);

If Point is a class, the output is 20 because a and b reference the same object. If Point is a struct, the output is 10 because the assignment of a to b creates a copy of the value, and this copy is unaffected by the subsequent assignment to a.x.

The previous example highlights two of the limitations of structs. First, copying an entire struct is typically less efficient than copying an object reference, so assignment and value parameter passing can be more expensive with structs than with reference types. Second, except for ref and out parameters, it is not possible to create references to structs, which rules out their usage in a number of situations.

1.8 Arrays

An array is a data structure that contains a number of variables that are accessed through computed indices. The variables contained in an array, also called the elements of the array, are all of the same type, and this type is called the element type of the array.

Array types are reference types, and the declaration of an array variable simply sets aside space for a reference to an array instance. Actual array instances are created dynamically at runtime using the new operator. The new operation specifies the length of the new array instance, which is then fixed for the lifetime of the instance. The indices of the elements of an array range from 0 to Length - 1. The new operator automatically initializes the elements of an array to their default value, which, for example, is zero for all numeric types and null for all reference types.

26Copyright Microsoft Corporation 1999-2003. All Rights Reserved.

Document info
Document views1436
Page views1436
Page last viewedMon Jan 23 21:49:35 UTC 2017