問題描述

C++ 或 Java 中的類型轉(zhuǎn)換和類型轉(zhuǎn)換有什么區(qū)別?

What is the difference between typecasting and typeconversion in C++ or Java ?

推薦答案

類型 casting 將變量引用的值(內(nèi)存塊)視為不同于聲明變量的類型.

Type casting is treating a value (block of memory) referenced by a variable as being of a different type than the type the variable is declared as.

類型 conversion 實際上是在執(zhí)行該值的轉(zhuǎn)換.

Type conversion is actually performing a conversion of that value.

在許多語言中，一些 類型轉(zhuǎn)換(通常是數(shù)字類型)確實會導(dǎo)致轉(zhuǎn)換(這會因語言而異)，但大多數(shù)情況下只是將此 X 視為 Y".

In many languages, some casts (usually numeric ones) do result in conversions (this will vary quite a bit by language), but mostly it's just "treat this X as a Y".

與人類語言的大多數(shù)方面一樣，不幸的是，這些術(shù)語在不同社區(qū)的使用方式略有不同，主要是沿語言線.例如，請參閱下面 James 對 C++ 的評論 —那里的cast"一詞的含義比上面的定義要廣泛得多，后者更多地是在 C 或 Java 模型中.為了讓事情變得有趣，Java 語言規(guī)范實際上包含了各種種類 類型，包括 強制轉(zhuǎn)換.但以上是一個很好的經(jīng)驗法則.

Like most aspects of human language, unfortunately the terms are used slightly differently in different communities, mostly along language lines. For instance, see James' comment below about C++ — the word "cast" there has a much broader meaning than the above definition, which is more in the C or Java mold. And just to make things fun, the Java Language Spec actually gets into various kinds of casts, including casting conversions. But the above is a good rule of thumb.

但舉個簡單的例子:

在 Java 中，在泛型之前，在處理地圖時必須進(jìn)行大量的類型轉(zhuǎn)換并不罕見:

In Java, prior to generics it wasn't unusual to have to do a lot of typecasting when dealing with maps:

Map m = new HashMap();
m.put("one", "uno");

// This would give a compiler error, because although we know
// we'll get a String back, all the compiler knows is that it's
// an Object
String italian = m.get("one");

// This works, we're telling the compiler "trust me, it's a String"
String italian = (String)m.get("one");

幸運的是，添加 generics 解決了這個問題，因為以這種方式進(jìn)行投射往往是一種存在維護(hù)問題的脆弱流程.

Fortunately, the addition of generics addressed this, as casting in this way tends to be a fragile process with maintenance issues.

相比之下，如果你有一個數(shù)字字符串，你會轉(zhuǎn)換:

In contrast, you'd convert if you had a String of digits:

String s = "1234";

...并且需要知道這些數(shù)字以十進(jìn)制表示的數(shù)字:

...and needed to know what number those digits represented in decimal:

// Wrong (cast)
int n = (int)s;

// Right (conversion)
int n = Integer.parseInt(s, 10);

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