33 lines
1.8 KiB
XML
33 lines
1.8 KiB
XML
<?xml version="1.0"?>
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<clause number="8.8" title="Structs" informative="true">
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<paragraph>The list of similarities between classes and structs is long-structs can implement interfaces, and can have the same kinds of members as classes. Structs differ from classes in several important ways, however: structs are value types rather than reference types, and inheritance is not supported for structs. Struct values are stored "on the stack" or "in-line". Careful programmers can sometimes enhance performance through judicious use of structs. </paragraph>
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<paragraph>For example, the use of a struct rather than a class for a Point can make a large difference in the number of memory allocations performed at run time. The program below creates and initializes an array of 100 points. With Point implemented as a class, 101 separate objects are instantiated-one for the array and one each for the 100 elements. <code_example><![CDATA[
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class Point
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{
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public int x, y;
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public Point(int x, int y) {
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this.x = x;
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this.y = y;
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}
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}
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class Test
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{
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static void Main() {
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Point[] points = new Point[100];
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for (int i = 0; i < 100; i++)
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points[i] = new Point(i, i*i);
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}
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}
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]]></code_example></paragraph>
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<paragraph>If Point is instead implemented as a struct, as in <code_example><![CDATA[
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struct Point
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{
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public int x, y;
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public Point(int x, int y) {
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this.x = x;
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this.y = y;
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}
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}
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]]></code_example>only one object is instantiated-the one for the array. The Point instances are allocated in-line within the array. This optimization can be misused. Using structs instead of classes can also make an application run slower or take up more memory, as passing a struct instance by value causes a copy of that struct to be created. </paragraph>
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</clause>
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