平凡的世界

    In this entry, I show you 4 tips that address frequently asked questions from C++ programmers of all levels of expertise. It's surprising to discover how many experienced programmers are still unaware of the deprecation of the .h notation of standard header files, the proper usage of namespaces, and the rules regarding binding of references to temporary objects, for example. These issues and others will be discussed here.
    在這篇文章里我將談?wù)劯鞣N層次的Ｃ++程序員經(jīng)常問(wèn)及的四個(gè)問(wèn)題。例如我很驚訝地發(fā)現(xiàn)還有很多程序員沒(méi)有意識(shí)到標(biāo)準(zhǔn)頭文件擴(kuò)展名.h的爭(zhēng)議，命名空間的恰當(dāng)用法以及引用臨時(shí)對(duì)象的規(guī)則。這些問(wèn)題及其它將在這里進(jìn)行討論。

    First, we start by explaining the difference between the deprecated “xxx.h” header names and the modern, standard-compliant “xxx” header-naming notation. Next, we explore a few dark corners of C++ which due to compilers' limitations and the somewhat recondite nature of the associated language rules tend(原文為“rulestend”) to confuse many programmers, e.g., the notion of comma-separated expressions and the rules of binding references to rvalues. Finally, we will learn how to invoke a function prior to a program's startup.

    首先我們從解釋受非議的“XXX.h”頭文件名與現(xiàn)代、符合標(biāo)準(zhǔn)的“<XXX>”頭文件名記號(hào)之間的區(qū)別開(kāi)始。接下來(lái)我們探索C++不為人知的角落，由于編譯器的局限性和關(guān)聯(lián)語(yǔ)言規(guī)則某些隱蔽的自然特性迷惑了許多程序員，比如逗號(hào)分隔表達(dá)式的意義與引用型變量的規(guī)則。最后我們將學(xué)習(xí)如何在程序啟動(dòng)前啟動(dòng)一個(gè)函數(shù)。

    Tip 1: “iostream.h” or “iostream”?

    語(yǔ)題1：“iostream.h” or “iostream”?

    Many C++ programmers still use “iostream.h” instead of the newer, standard compliant “iostream” library. What are the differences between the two? First, the .h notation of standard header files was deprecated more than five years ago. Using deprecated features in new code is never a good idea. In terms of functionality, “iostream” contains a set of templatized I/O classes which support both narrow and wide characters, as opposed to “iostream.h” which only supports char-oriented streams. Third, the C++ standard specification of iostream's interface was changed in many subtle aspects. Consequently, the interfaces and implementation of “iostream” differ from those of “iostream.h”. Finally, “iostream” components are declared in namespace std whereas “iostream.h” components are global.

     很多C++程序員還在使用“iostream.h”代替新的符合標(biāo)準(zhǔn)的“iostream”庫(kù)。兩者有什么區(qū)別呢？首先，標(biāo)準(zhǔn)頭文件“.h”擴(kuò)展名在五年前就倍受爭(zhēng)議。在新代碼中使用有爭(zhēng)議的（過(guò)時(shí)的）特性永遠(yuǎn)都不是一個(gè)好主意。從本質(zhì)上看，“iostream”包括一系列支持窄字符與寬字符的模板化（templatized） I/O輸入輸出類(lèi)，而相反地，“iostream.h”只支持字符流。第三，iostream接口的標(biāo)準(zhǔn)C++規(guī)范在許多細(xì)節(jié)方面進(jìn)行了變動(dòng)。因此，“iostream”的接口與實(shí)現(xiàn)同那些“iostream.h”是有區(qū)別的。最后，“iostream”是在std命名空間中定義的而“iostream.h”則是全局的。

    Because of these substantial differences, you cannot mix the two libraries in one program. As a rule, use “iostream” unless you're dealing with legacy code that is only compatible with “iostream.h”.

    由于這些本質(zhì)方面的不同，不能在同一程序中混合使用這兩種庫(kù)。作為一個(gè)規(guī)則，應(yīng)盡量使用“iostream”，除非你要處理的是只能與“iostream.h”兼容的遺留代碼。（感謝autumnm1981）

    Tip 2: Binding a Reference to an R-Value

    話題2：將引用與右值綁定

    （R-Value：右值，與“左值”相對(duì)。例如x=3中，“x”是一個(gè)“左值”，“3”是一個(gè)右值。從本質(zhì)上講，左值是一個(gè)內(nèi)存的地址，右值是一個(gè)實(shí)際的二進(jìn)制值。）

    R-Values and L-Values are a fundamental concept of C++ programming. In essence, an R-Value is an expression that cannot appear on the left-hand side of an assignment expression. By contrast, an L-Value refers to an object (in its wider sense), or a chunk of memory, to which you can write a value. References can be bound to both R-Values and L-Values. However, due to the language's restrictions regarding R-Values, you have to be aware of the restrictions on binding references to R-Values, too.

    右值和左值是C++編程的一個(gè)基本概念。本質(zhì)上來(lái)講右值是一個(gè)不可能出現(xiàn)在等號(hào)左邊的表達(dá)式。相反，左值引用一個(gè)對(duì)象（廣義范圍上的），或者一塊可讀寫(xiě)的內(nèi)存。引用既可以指向右值也可以指向左值。然而，由于語(yǔ)言在處理右值上的限制，你也得在將引用指向右值是慎重考慮。

    Binding a reference to an R-Value is allowed as long as the reference is bound to a const type. The rationale behind this rule is straightforward: you can't change an R-Value, and only a reference to const ensures that the program doesn't modify an R-Value through its reference. In the following example, the function f() takes a reference to const int:

    將引用與右值綁定像引用常量（這里const type應(yīng)為"常量",感謝新奧爾良土鱉 ）一樣也是被允許的。這條原則背后的原理是很顯而易見(jiàn)的：你無(wú)法改變右值，因?yàn)閷?duì)常量的引用確保程序不會(huì)通過(guò)這個(gè)接口改變右值。下面的例子，f()函數(shù)包含一個(gè)對(duì)常整型變量的引用。



    The program passes the R-Value 2 as an argument to f(). At runtime, C++ creates a temporary object of type int with the value 2 and binds it to the reference i. The temporary and its reference exist from the moment f() is invoked until it returns; they are destroyed immediately afterwards. Note that had we declared the reference i without the const qualifier, the function f() could have modified its argument, thereby causing undefined behavior. For this reason, you may only bind references to const objects.

    這段代碼將右值“2”做為函數(shù)f()的一個(gè)參數(shù)。代碼運(yùn)行時(shí)，C++將創(chuàng)建一個(gè)值為2的臨時(shí)整型變量并將其與引用類(lèi)型i綁定。這個(gè)臨時(shí)對(duì)象與它的接口將在 f()運(yùn)行期間一直存到直到函數(shù)f返回。函數(shù)f返回后它們立即被釋放。注意我們沒(méi)有將i聲明為常量類(lèi)型，但是函數(shù)f仍有可能修改它的這個(gè)參數(shù)，這將引起異常。因此最好是將引用與常量類(lèi)型綁定。

    The same rule applies to user-defined objects. You may bind a reference to a temporary object only if it's const:

    同樣的規(guī)則適用于自定義類(lèi)型。只有一個(gè)臨時(shí)對(duì)象為常量時(shí)才可以與引用類(lèi)型綁定。

struct A{};



    Tip 3: Comma-Separated Expressions

    話題3：逗號(hào)表達(dá)式

Comma-separated expressions were inherited from C. It's likely that you use such expressions in for- and while-loops rather often. Yet, the language rules in this regard are far from being intuitive. First, let's see what a comma separated expression is.

    逗號(hào)表達(dá)式是從C語(yǔ)言沿襲下來(lái)的。它就像你經(jīng)常使用的for循環(huán)與while-loop循環(huán)一樣。但是這里面的語(yǔ)法規(guī)則遠(yuǎn)不像看起來(lái)的那樣。首先，讓我們看看什么是逗號(hào)表達(dá)式。

    An expression may consist of one or more sub-expressions separated by commas. For example:

    一個(gè)表達(dá)式可以被逗號(hào)分隔為一個(gè)或若干個(gè)子表達(dá)式。例如：



The if condition contains three expressions separated by commas. C++ ensures that each of the expressions is evaluated and its side effects take place. However, the value of an entire comma-separated expression is only the result of the rightmost expression. Therefore, the if condition above evaluates as true only if cin.good() returns true. Here's another example of a comma expression:

    這條if語(yǔ)句被逗號(hào)分事為三個(gè)表達(dá)式。從C++的角度每個(gè)表達(dá)式都是合法的但是副作用產(chǎn)生了。整個(gè)逗號(hào)表達(dá)式的值是由最右邊的表達(dá)式?jīng)Q定的。于是只有con.good()的返回值是true時(shí)整個(gè)表達(dá)式的值才是true。這里有另一個(gè)關(guān)于逗號(hào)表達(dá)式的例子。



    Tip 4: Calling a Function Before Program's Startup

    在程序啟動(dòng)前調(diào)用函數(shù)

Certain applications need to invoke startup functions that run before the main program starts. For example, polling, billing, and logger functions must be invoked before the actual program begins. The easiest way to achieve this is by calling these functions from a constructor of a global object. Because global objects are conceptually constructed before the program's outset, these functions will run before main() starts. For example:

    有些應(yīng)用需要在主程序啟運(yùn)前運(yùn)行啟動(dòng)函數(shù)。例如投票、支付和登錄函數(shù)必須在實(shí)際種程序啟動(dòng)前運(yùn)行。一個(gè)最簡(jiǎn)單的實(shí)現(xiàn)方法就是在一個(gè)全局對(duì)象的構(gòu)造函數(shù)里調(diào)用這些函數(shù)。因?yàn)槿謱?duì)象在程序的最開(kāi)頭被隱式的創(chuàng)建，這些函數(shù)就可以在main()函數(shù)之前得到運(yùn)行。例如：



    The global object log is constructed before main() starts. During its construction, log invokes the function activate_log(). Thus, when main() starts, it can read data from the log file.

    全局對(duì)象log在main()函數(shù)啟動(dòng)之前被創(chuàng)建。在它的構(gòu)造函數(shù)里，log調(diào)用了active_log()函數(shù)。于是，當(dāng)main()函數(shù)啟動(dòng)時(shí)，它可以從日志文件中讀取數(shù)據(jù)。