為什么C++（來自C++之父的觀點）

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By Bjarne Stroustrup

http://www.research.att.com/~bs/

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劉未鵬(pongba)

C++的羅浮宮(http://blog.csdn.net/pongba)

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注：9月份的時候寫了一篇“Why C++”（翻譯版見這里）。后來我把文章發(fā)給Bjarne，問問他的意見。Bjarne友好的對文章作了詳細的評點并發(fā)回給我，我問他能不能把他的評點版發(fā)到blog上，他說原先發(fā)給我的評點比較隨意，答應給我單獨寫一篇友情評注，也就是下面這篇。

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在后來的郵件交流中，我又問了Bjarne一些C++問題，并得知Bjarne的新書將會在夏季出版，在我們的交流中我曾問起他所說的正確的學習C++的方式到底是什么，他說他的新書將會是最好的答案！

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此外Bjarne還答應接受一次訪談，這個我已經在上次一篇blog里面提到了，引用如下：

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“最近和Bjarne Stroustrup的通信中，他答應接受一次訪談。
為了讓訪談能夠代表更多人的意見而不是我個人的感覺，在這里邀請大家提出自己最想問的問題。
你最想問什么？
BTW.?由于我會對問題進行篩選，然后再公布給大家投票。所以大家的問題最好是深思熟慮的，重要的，有理有據的:-)
問題后如果能附上問題的背景（即你在什么樣的經歷中發(fā)現這個問題很關鍵）那是最好不過！
問題可以留言在blog上，但更建議發(fā)到TopLanguage上，我專門開了一則帖子，這樣大家可以互相討論:-)”

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以下是Bjarne的原文（抱歉，我沒有時間全文翻譯出來，如果哪位同學有興趣譯出來的話可以先跟我說一聲——這是為了避免多人在互相不知道的情況下重復勞動。另外我會在翻譯版上署上你的名字和鏈接（如果有的話））：

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Dear “pongba”,

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You sent me a long and thoughtful note about C++ and its use – or lack of use or misuse – in your environment. I answered with a set of disjointed comments on your note that unfortunately were not suitable for a wider audience, such as your blog. This is the slightly revised set of those notes – made a little bit more coherent, but still not of the quality of a proper paper – that you can post (incl. this little explanatory note):

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C and C++

You worry that many people prefer to use C over C++ and that there may even be a move away from C++ towards C. You are being unnecessarily defensive. Don’t forget that?for major applications, the move has been from C to C++ for a very long time. If people “frown and turn away,” they are displaying ignorance. Don’t be defensive: Challenge the unsupported myths!?My applications page (http://www.research.att.com/~bs/applications.html) is a good place to start.?If there really is a drift from C++ to C –?which I have never seen in the?US?or?Europe?– then it speaks badly on the education of the “drifters”.?Organizations caring about quality code have consistently drifted the other way.

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People often say that C is simpler than C++. They consider that obvious (just look at the thickness of the C standard compared with the C++ standard or K&R2 compared to TC++PL3.?The question is “simpler for what?”?I don’t know of any program that can be simpler when written in C than in C++; I don’t think such a program is possible.?C++ provides better notational support and better type checking for the things that C does well. Improved type checking is important and its importance increases with the size of the program and the demands of correctness:?C simply leaves you in the lurch and doesn’t provide facilities to express even a simple concept as “buffer” directly and without overheads.?Don’t fall into the trap of equating C++ with OOP or GP. Those are powerful techniques?where appropriate.?Just because you don’t need OOP or GP for a project doesn’t imply that C is better for that.?My claim is that it is not.

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C is?not?a simple language and programmers have no problems misusing it. Please explain the rules for conversion from unsigned short to int or try to teach a novice how to deal with pointers to pointers.?Note the need for workarounds for just about any modern or large-program technique.??How many dark corners are there in C? Examples:?explicit memory management, standards conversion and promotions, multidimensional array addressing, pointers to pointers, and macros.?In the absence of C++ features, you get to use them all directly.?How many necessary modern programming techniques do you have to apply through workarounds in C?

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Complexity will go somewhere: if not the language then the application code.

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It is sometimes claimed that C is more efficient than C++.?There is absolutely no evidence that C is more efficient than C for any reasonable definition of “efficient”. There arefundamental reasons that C cannot be more efficient than C++:?They share a machine model and a set of basic facilities to use it. For lots of details related to C++ performance see the?ISO C++ committee’s Technical Report on performance?(link on my C++ page:http://www.research.att.com/~bs/C++.html).

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But yes, the simplicity of C and its efficiency as compared to C++ are popular myths;?in places they are so popular that people feel that no evidence is needed for their support. Unfortunately, such myths are not harmless;?they damage the performance of individuals and organizations that buy into them. For?a more detailed and precise statement of my opinion of C’s relationship to C++, see my papers on that topic:http://www.research.att.com/~bs/bs_faq.html#merge?. See?also my paper on learning C++: “Learning Standard C++ as a new language (link on my publications page).

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It is really hard to think of an area where C is better than C++ today. It would have to involve lack of decent C++ compilers and/or tools?(such as CNU C++ or Microsoft C++ 10 years ago – many people still express strong opinions on C++ based on experience with such compilers) or it has to involve a lot of existing code that happens to make C++ hard to use (e.g. header files with lots of C++ keywords used as identifiers).

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C++ techniques

C++ != OOP. Trust the programmer to choose which language facilities are appropriate – even your colleagues. Using C out of fear that someone will use an inappropriate language feature is bizarre:?our systems are extremely complex and depriving the programmer of even type checking out of fear of complexity is either misguided or disingenuous.

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Yes, we could build a better string than?std::string, but?std::string?is still better than *average* C string manipulation:?gets(s)?– enough said.?Optimize only where needed; go for correctness and simplicity everywhere.

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There is no reason why code expressed using abstractions should be less efficient than equivalent hand written code (in C or C++) using lower-level features (pointers, char arrays, etc.).?The most performance-critical C++ code tends to use templates (and classes): e.g., high-performance computation and serious embedded systems.?See the ISO C++ Standard Committee’s Technical Report on Performance and the JSF++ coding standard (available from my C++ page).

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Obviously, a programmer?can overuse abstraction mechanisms.?Examples are designs that represent every idea as a class with lots of virtual functions in a single huge hierarchy and designs that represent every possible design decision as a template parameter.?Good design is not just deferring all design decisions!?Conversely,?it is easy to underabstract.?For example, try writing a set of mathematical functions for multi-dimensional arrays expressed directly as C/C++ multidimensional arrays.?The result is something that passes array bounds as separate arguments – leading to bugs and run-time efficiencies.

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C++0x

Some of the features coming in C++0x are exciting (e.g., concepts, uniform initialization, and the machine model), but we wont be able to rely on C++0x for another couple of years. C++0x shows where we are going, but if you want to defend C++, do it with minimal reference to C++0x.?C++98 is a well-proven tool; C++0x will simply be better.

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There are technical embarrassments in C++, such as the need for a space between > and > in “vector<list<int>>”.??I was the one who named such technical problems “embarrassments” and listed several to encourage the standards committee to address them.?No serious person considers any language in major real-world use perfect; they all have weaknesses that embarrass their expert users that should – if possible – be addressed.

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Have a look at my papers written for HOPL (The ACM History of Programming Languages conference) to get a more nuanced idea of?how and why C++ evolved the way it did?(links on my publications page:?http://www.research.att.com/~bs/papers.html.?The second paper covers the design criteria and ideas for C++0x.

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You said:

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The bottom line:?keep simple things simple?(but remember that simplicity can be achieved by using high-level libraries); use abstractions when necessary (and even then, make spare use of it; follow good design principles and?established good practices).

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That’s sound advice, and the reference is good. Unfortunately,?it is not easy to know when abstraction is beneficial and how much is “just right”.?I would be a bit more aggressive about the use of classes and templates than you appear to recommend, but then I may be assuming a bit more experience.?For a discussion of the use of abstraction see Bill Venner’s interview with me on that topic (http://www.artima.com/intv/goldilocks.html) that he entitled “The C++ Style Sweet Spot”.

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注：Bjarne在最后一段提到的訪談也是最技術的一篇，非常值得一讀，這里推薦榮耀先生的翻譯：

The C++ Style Sweet Spot?(Part 1: C-style, object orientation, invariants, simple interfaces). Interview by Bill Venners for Artima.com. September 2003.

Modern C++ Style?(Part 2: Multiple inheritance, Abstract classes, multi-paradigm programming, and "resource acquisition is initialization"). Interview by Bill Venners for Artima.com. November 2003.

Abstraction and Efficiency?(Part 3: Raising the level of abstraction, "programming is understanding", and the difference between premature and prudent optimization). Interview by Bill Venners for Artima.com. February 2004.

Elegance and other design ideals?(Part 4: Aspects of software design, the importance of libraries, protecting programmers from themselves, the dangers of premature abstraction, and the essence of elegance). Interview by Bill Venners for Artima.com. February 2004.

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1.?最近Feedburner抓取feed似乎出問題了，然而還有近200位同學通過Feedburner訂閱了C++的羅浮宮（本來用FB的時候沒預料到“墻”的問題，sigh~），所以，麻煩用FB訂閱的同學轉為用下面的feedsky訂閱:-)

http://feed.feedsky.com/pongba

又或者你也可以直接訂閱原始rss（因feedsky的服務似乎也似乎時有問題）：

http://blog.csdn.net/pongba/rss.aspx

2. C++的羅浮宮已改為全文輸出。

3. As Always，歡迎加入我們的討論組——TopLanguage（http://groups.google.com/group/pongba），自從8月份建立以來，我們已經吸引了三百位同學，二千五百多條討論，想知道我們討論些什么，參見TopLanguage討論精選[一]，精選[二]，精選[三]，加入前請看這里。

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