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代入演算子の不思議

時々見かける、こんな感じのコード。


int a, b, c;
a = b = c = 0;


代入演算子を連続して記述する

と右から順に値が格納されるらしい。

だから、以下のコードはエラーになる。


object x;
fuga f;
f = x = new fuga();


理由は、fuga オブジェクトが object 型の変数 x に格納された値を格納しようとしているから。
つまり、object 型の x の値は、fuga 型の f に格納できないのでエラーになる。

そこで、以下のクラスを用意してみる。


class hoge
{
  private int _b, _c;
  public int A { get; set; }
  public int B {
    get { return _b; }
    set { _b = value + 1; }
  }
  public int C {
    get { return _c; }
    set { _c = value + 1; }
  }
}


そして、以下のようなコードを書いて実行した結果が以下。


var x = new hoge();
x.A = x.B = x.C = 0;
Console.WriteLine(x.A); // 0
Console.WriteLine(x.B); // 1
Console.WriteLine(x.C); // 1
Console.ReadLine();


さて問題です

どうして、上から「2, 2, 1」と表示されないのでしょうか?

投稿日時 : 2009年8月19日 19:12

Feedback

# re: 代入演算子の不思議 2009/08/19 20:05 えムナウ

>どうして、上から「2, 2, 1」と表示されないのでしょうか?
どうして、上から「2, 2, 1」と表示されると考えたのでしょうか?

# re: 代入演算子の不思議 2009/08/20 8:40 irxn

(x = y)という式がxに依らずyの値を返すからですよね。

# re: 代入演算子の不思議 2009/08/20 8:59 うつせみ(虚蝉)

こういう問題好きです。考え方の勉強になりますし。

# re: 代入演算子の不思議 2009/08/20 10:44 通りすがり

irxnさんに補足すれば(x = y)は左辺の型にキャストした右辺を返すのですね。

class Program
{
static void Main(string[] args)
{
Fuga fuga;
(fuga = new FugaEx()).WriteText(); //fuga
Console.WriteLine((fuga = new FugaEx()).GetType());//fugaEX
Console.ReadLine();

}
}

class Fuga
{
public virtual void WriteText()
{
Console.WriteLine("fuga");
}
}

class FugaEx : Fuga
{
public new void WriteText()
{
Console.WriteLine("fugaEx");
}
}

面白いです。

# re: 代入演算子の不思議 2009/08/20 10:56 通りすがり

連投すいません
GetTypeでは左辺でも右辺でもfugaEXが返るので、検証としては妥当ではありませんね。

class Program
{
static void Main(string[] args)
{
Fuga fuga;
(fuga = new FugaEx()).WriteText(); //fuga
(fuga = new FugaEx()).WriteTextNew();//fugaEX
Console.ReadLine();
}
}

class Fuga
{
public void WriteText()
{
Console.WriteLine("fuga");
}

public virtual void WriteTextNew()
{
Console.WriteLine("fuga");
}
}

class FugaEx : Fuga
{
public new void WriteText()
{
Console.WriteLine("fugaEx");
}

public override void WriteTextNew()
{
Console.WriteLine("fugaEx");
}
}
これで大丈夫かな?

# 代入演算子 2009/08/20 12:57 田舎エンジニアのBlog

代入演算子

# 代入演算子 2009/08/20 12:58 田舎エンジニアのBlog

代入演算子

# re: 代入演算子の不思議 2009/08/20 15:50 R・田中一郎

えムナウ さん

>どうして、上から「2, 2, 1」と表示されると考えたのでしょうか?

単純に、右辺の値を左辺に代入すると、そうなると考えることは不自然ではありますまい。

----------------------------------------
irxn さん

>(x = y)という式がxに依らずyの値を返すからですよね。

そういうことですね。
何か盛り上がらなかった・・・orz

----------------------------------------
うつせみ(虚蝉) さん

>こういう問題好きです。考え方の勉強になりますし。

ありがとうございます。
励みになります(T-T)

----------------------------------------
通りすがり さん

>irxnさんに補足すれば(x = y)は左辺の型にキャストした右辺を返すのですね。

そーゆーことですねー

>面白いです。

ありがとうございます(T-T)

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