Classy Cars<\/h2>\n\n\n\n
Start with a base class for an automobile. It has the property for the color, and a method drive().<\/p>\n\n\n\n
class Automobile;\n string color;\n task drive();\n ...\n endtask\nendclass<\/pre>\n\n\n\nThis looks handy but sometimes you need to carry something big like a desk \u2013 you need a pickup truck! An important measure of a pickup is the size of its bed.<\/p>\n\n\n\n
class Pickup extends Automobile;\n int bed_size;\nendclass<\/pre>\n\n\n\nNotice that a Pickup is an Automobile. Everything you can do with an Automobile, like set the color, and call drive(), you can also do with a Pickup.<\/p>\n\n\n\n
Mishandling<\/h2>\n\n\n\n
Declare some class variables. <\/p>\n\n\n\n
Automobile a; \/\/ Declare an Automobile class variable\nPickup p; \/\/ And a Pickup variable<\/pre>\n\n\n\nCan the variable a<\/strong> point to a Pickup object?<\/p>\n\n\n\n
p = new(); \/\/ Construct a Pickup object\na = p; \/\/ Legal? Yes!<\/pre>\n\n\n\nYes! What can you do with an Automobile object? You can call drive() and look at the color property. Those members exist in the Pickup object.<\/p>\n\n\n\n
a.drive(); \/\/ Yes \u2013 legal\na.color = \"red\"; \/\/ Yes \u2013 legal<\/pre>\n\n\n\nCan the variable p<\/strong> point to an Automobile object?<\/p>\n\n\n\n
a = new(); \/\/ Construct an Automobile object\np = a; \/\/ ERROR: will not compile<\/pre>\n\n\n\nThe assignment won\u2019t even compile as an extended class handle can never point to a base object. If that was allowed, you could then have the following code that tries to access the bed_size<\/strong> property in an Automobile object. The code will compile, but there is no such property.<\/p>\n\n\n\n
p.bed_size = 64; \/\/ ERROR: No bed_size in Automobile object<\/pre>\n\n\n\nRelationship between class variable<\/h2>\n\n\n\n
Here is a diagram of the two classes, and the variables that point to their objects.<\/p>\n\n\n\n
Classes and variables<\/figcaption><\/figure>\n\n\n\n You can see that an Automobile variable can point to both Automobile and Pickup object, but a Pickup variable can only point to a Pickup object. The range of values for the base class variable is larger than the extended variable\u2019s range. That didn\u2019t make sense to me until I made the above Venn diagram.<\/p>\n\n\n\n
Here are the rules for assigning between base and derived class variables. <\/p>\n\n\n\n
Rules for assigning base and extended class variables<\/figcaption><\/figure>\n\n\n\n Summary<\/h2>\n\n\n\n
This post introduces the concept of the legal values for a class variable, for both a base and extended object. Stay tuned next time when I lose my car in the parking lot, and one more rule.<\/p>\n\n\n\n
Try this code with your favorite simulator. Lean forward, make mistakes, get messy!<\/p>\n","protected":false},"excerpt":{"rendered":"
Introduction Good OOP style says you should start your project with a common base class (or several). When you want…<\/p>\n","protected":false},"author":72251,"featured_media":16329,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"spanish_translation":"","french_translation":"","german_translation":"","italian_translation":"","polish_translation":"","japanese_translation":"","chinese_translation":"","footnotes":""},"categories":[1,982,10],"tags":[300,951,382,383,950,952,953,515,585,594,612,614,648,751],"industry":[],"product":[],"coauthors":[942],"class_list":["post-16324","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-news","category-systemverilog","category-tips-tricks","tag-cast","tag-base-class","tag-class","tag-class-handles","tag-class-variable","tag-derived-class","tag-extended-class","tag-handle","tag-member","tag-method","tag-object-oriented-programming","tag-oop","tag-property","tag-systemverilog"],"featured_image_url":"https:\/\/blogs.sw.siemens.com\/wp-content\/uploads\/sites\/54\/2021\/07\/4_1_venn.png","_links":{"self":[{"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/posts\/16324","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/users\/72251"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/comments?post=16324"}],"version-history":[{"count":4,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/posts\/16324\/revisions"}],"predecessor-version":[{"id":16385,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/posts\/16324\/revisions\/16385"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/media\/16329"}],"wp:attachment":[{"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/media?parent=16324"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/categories?post=16324"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/tags?post=16324"},{"taxonomy":"industry","embeddable":true,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/industry?post=16324"},{"taxonomy":"product","embeddable":true,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/product?post=16324"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/blogs.sw.siemens.com\/verificationhorizons\/wp-json\/wp\/v2\/coauthors?post=16324"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
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