downcast

WordNet

directed downward; "a downcast glance"
a ventilation shaft through which air enters a mine

PrepTutorEJDIC

〈人が〉意気消沈した,しおれた,ふさぎこんだ / 伏し目の,下に向けた

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/11/20 19:30:49」(JST)

wiki en

[Wiki en表示]

In object-oriented programming, downcasting or type refinement is the act of casting a reference of a base class to one of its derived classes.

In many programming languages, it is possible to check through RTTI whether the type of the referenced object is indeed the one being cast to or a derived type of it, and thus issue an error if it is not the case.

In other words, when a variable of the base class (parent class) has a value of the derived class (child class), downcasting is possible.

For example in Java:

public class Parent{}
public class Child extends Parent(){}
 
public static void main(String args[]){
    Parent parent = new Child();              // Parent is parent class of Child, parent variable holding value of type Child
    Child child = (Child)parent;              // This is possible since parent object is currently holding value of Child class
}

As you see, the downcasting is useful if we know the type of the value referenced by the Parent variable.

Most often, you will encounter a need to do downcasting when passing a value in a parameter. So, in another example, let's say we have method objectToString that takes Object which we assume the parameter myObject is String type.

public String objectToString(Object myObject){
    return (String)myObject; //This will only work when the myObject currently holding value is string.
}
 
public static void main(String args[]){
    String result = objectToString("My String"); //This will work since we passed in String, so myObject has value of String.
    Object iFail = new Object();
    result = objectToString(iFail);              //This will fail since we passed in Object which does not have value of String.
}

The danger of downcasting in this approach is that it's not compile time check, but rather it is run time check. Downcasting myObject to String ('(String)myObject') was possible in compile time because there are times that myObject is String type, so only in run time we can figure out whether the parameter passed in is logical.

In C++, run-time type checking is implemented through dynamic_cast. Compile-time downcasting is implemented by static_cast, but this operation performs no type check. If it is used improperly, it could produce undefined behavior.

Useful source introducing concept of downcasting: "Upcasting, downcasting" by Sinipull

Criticism

Many people advocate avoiding downcasting, since according to the LSP, an OOP design that requires it is flawed. Some languages, such as OCaml, disallow downcasting altogether.

A popular example of a badly considered design is containers of top types, like the Java containers before Java generics were introduced, which requires downcasting of the contained objects so that they can be used again.

External links

Downcasting is a Code Smell by Jeremy D. Miller
A downcasting tragedy by Jimmy Bogard
Prefer polymorphism over instanceof and downcasting by Bill Venners
Downcasting in C# by Scott Lysle
Multiple downcasting techniques
Upcasting, downcasting by Sinipull

English Journal

[Multi-centered stratified clinical studies for psychological and sleeping status in patients with chronic constipation in China].

Zhu LM, Fang XC, Liu S, Zhang J, Li ZS, Hu PJ, Gao J, Xin HW, Ke MY.SourceDepartment of Gastroenterology, Chinese Academy of Medical Sciences, Beijing, China.
Zhonghua yi xue za zhi.Zhonghua Yi Xue Za Zhi.2012 Aug 28;92(32):2243-6.
OBJECTIVE: To survey the emotional and sleeping status of patients with chronic constipation (CC) and analyze the relationship between psychological status and constipated symptoms.METHODS: From January 2009 to April 2010, 5 centers and 25 stratified hospitals were selected as the representatives of
PMID 23158481

Emotional expression on a profile: feature height, mouth angle, and tilt.

Bhasin S, Kennedy JM, Niemeier M.SourceUniversity of Toronto, Scarborough, Ontario, Canada.
Attention, perception & psychophysics.Atten Percept Psychophys.2010 Jan;72(1):187-92. doi: 10.3758/APP.72.1.187.
Profiles may vary in expression with changes in the height of the profile's features, the mouth's angle at the profile contour, and head tilt. In three experiments, we varied these factors. Features low on the profile, with mouths at an obtuse angle to the profile line and head tilted 15 degrees for
PMID 20045888

Prevalence of major depressive disorder in the general population of South Korea.

Ohayon MM, Hong SC.SourceStanford Sleep Epidemiology Research Center, School of Medicine, Stanford University, 3430 W. Bayshore Road, Suite 102, Palo Alto, Stanford, CA 94303, USA. mahayon@stanford.edu
Journal of psychiatric research.J Psychiatr Res.2006 Feb;40(1):30-6.
INTRODUCTION: Previous epidemiological studies have reported a high prevalence of major depressive disorder (MDD) in North America and Western Europe. However, little information exists on MDD in Asian countries. This study investigates the prevalence of MDD and its characteristics in the general po
PMID 15878179

[Sleep in adolescents and its disorders. A survey in schools].

Bailly D, Bailly-Lambin I, Querleu D, Beuscart R, Collinet C.SourceFédération de Psychiatrie de l'Enfant et de l'Adolescent, Hôpital Sainte-Marguerite, 270, boulevard Sainte Marguerite, Faculté de Médecine de Marseille, 13009 Marseille.
L'Encéphale.Encephale.2004 Jul-Aug;30(4):352-9.
OBJECTIVES: Numerous studies emphasized the high prevalence of sleep problems in adolescents. However, it remains to be seen whether these problems are developmental or if they are related to pathological conditions. In order to try to answer this question, we conducted an epidemiological study inve
PMID 15538310

Japanese Journal

台所菜園に栽培する大蒜の芽

秦斐斐,馬剛,張嵐翠,徐会連
日本作物学会講演会要旨集 225(0), 276-276, 2008
NAID 130004611800

不変情報を用いたリファクタリング支援

片岡欣夫,楠本真二,井上克郎
情報処理学会論文誌 46(5), 1211-1221, 2005-05-15
… ムの不変情報を利用しているところに特徴がある．具体的には，プログラム中のある地点で一定の不変情報のパターンが成立するとき，"Remove Parameter"，"Eliminate Return Value"，"Separate Query from Modifier"，"Encapsulate Downcast" のいずれかのリファクタリングパターンが適用可能であるという関係を利用する．さらに，これらのリファクタリングパターンに対応した不変情報パターンマッチングツールを開発した．最後に，提案手法 …
NAID 170000007554

不変情報を用いたリファクタリング支援

片岡欣夫,楠本真二,井上克郎,カタオカヨシオ,クスモトシンジ,イノウエカツロウ,Kataoka Yoshio,Kusumoto Shinji,Inoue Katsuro
情報処理学会論文誌 46(5), 1211-1221, 2005-05-15
… グラムの不変情報を利用しているところに特徴がある.具体的には, プログラム中のある地点で一定の不変情報のパターンが成立するとき, "Remove Parameter", "Eliminate Return Value", "Separate Query from Modifier", "Encapsulate Downcast"のいずれかのリファクタリングパターンが適用可能であるという関係を利用する.さらに, これらのリファクタリングパターンに対応した不変情報パターンマッチングツールを開発した.最後に, 提案手法を実 …
NAID 120005478147

不変情報を用いたリファクタリング支援

片岡欣夫,楠本真二,井上克郎
情報処理学会論文誌 = Transactions of Information Processing Society of Japan 46(5), 1211-1221, 2005-05-15
… グラムの不変情報を利用しているところに特徴がある.具体的には, プログラム中のある地点で一定の不変情報のパターンが成立するとき, "Remove Parameter", "Eliminate Return Value", "Separate Query from Modifier", "Encapsulate Downcast"のいずれかのリファクタリングパターンが適用可能であるという関係を利用する.さらに, これらのリファクタリングパターンに対応した不変情報パターンマッチングツールを開発した.最後に, 提案手法を実 …
NAID 110002768627

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