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C++ Coding Standards Item 5 : Give One entity one cohesive responsibility
Summary
Focus on one thing at a time: Prefer to give each entity (variable, class, function, namespace, module, library) one well-defined responsibility. As an entity grows, its scope of responsibility naturally increases, but its responsibility should not diverge.
同一時間只關注一件事情：最好給每一個實體（variable, class, function, namespace, module, library）一個明確的職責。當實體增長（擴展，怎麼翻譯好一點？）的時候，它的職責範圍自然的擴大了，但是它的職責不應該有不同的發展方向。
Discussion
A good business idea, they say, can be explained in one sentence. Similarly, each program entity should have one clear purpose.
一個好的商業計劃可以用一句話說明。類似的，一個好的程序實體應該有一個清楚地目的。
這條規則主要講有很多功能（職責）的實體，它們通常很難設計，用起來也很困難，所以，還是讓我們的實體（class，function……）目的單一些，單純一點好。
Prefer to build higher-level abstractions from smaller lower-level abstractions. Avoid collecting several low-level abstractions into a larger low-level conglomerate. Implementing a complex behavior out of several simple ones is easier than the reverse.
最好在比較小的低級別的抽象基礎上建立高級別的抽象。避免把許多小的低級別抽象聚合到一個大的低級別的抽象中。相對而言，通過幾個簡單的實體實現一個複雜的行爲，要比通過一個實體來實現簡單的多。（這句話好難講！！）
Examples
Example 1: realloc. In Standard C, realloc is an infamous example of bad design. It has to do too many things: allocate memory if passed NULL, free it if passed a zero size, reallocate it in place if it can, or move memory around if it cannot. It is not easily extensible. It is widely viewed as a shortsighted design failure.
Example 1 講的是標準C語言中的realloc函數，realloc必須要做太多的事情了：傳入NULL的話就分配內存，傳入0值得話就釋放內存，如果在那個地方（程序執行點）可以realloc的話，就正常的工作，否則就要做內存移動……。而且不能很容易的擴展。基本上都認爲它(realloc)的設計是很失敗的。
Example 2: basic_string. In Standard C++, std::basic_string is an equally infamous example of monolithic class design. Too many "nice-to-have" features were added to a bloated class that tries to be a container but isn't quite, is undecided on iteration vs. indexing, and gratuitously duplicates many standard algorithms while leaving little space for extensibility. (See Item 44's Example.)
Example 2：basic_string.又一個在單一功能的類的設計中的反面例子，它的“罪過”我就不逐句翻譯了，大家讀原文也能清楚。
這章講述的內容比較簡單，以前我總是注意不到，或者說在刻意的違反，總是希望一個類可以做這個也可以做那個，呵呵，記得我曾經把幾個功能函數全部寫在一個 CPP文件中，無論從程序設計角度還是文學欣賞的角度看，那個文件都極度的不具備可讀性，所以它也從我的歷史中永久的消失了……
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