這幾個月忙於工作, 這個部落格都快被我給荒廢掉了. 昨晚終於花了時間完成了一篇積了一兩個月, 說明 virtual function 在執行時期的 override 過程以及 virtual function 與 constructor/destructor 混搭結果 的文字.

今天我準備了個題目給花了時間看了它的朋友, 當作一次對小弟的該題目說明能力的測驗. 請問, 下面的 code snip 的輸出結果是什麼?
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Background

不知道你有沒有過這樣的經驗 – 想要在父類的 constructor 裏面呼叫被子類 override (或是 implement) 的 (pure) virtual function?
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The issue with boost::decay documentation that I blogged about 10 days ago has been fixed in trunk.

前陣子看了 Hubert 的 一個 boost::exception 的例子, 猛然注意到自己不確定 boost::decay 的作用.

去看了它的 文件, 才發現文件不盡然正確… 已 submit 了一個 ticket 給 Boost, 希望早日修正.

Did you know, for x is an integer, that -~x yields x + 1 and ~-x yields x - 1?

via: Advanced bit manipulation-fu.

elleryq 的 這篇 blog post 談到的 ctor (換成 dtor 也一樣) 呼叫 virtual function 狀況 (或說是 problem) 會是很好的 C++ programmer 面試題目. 可在口試時用以窺探 interviewee 對 C++ object model 的了解.
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Very well put

當你發現回傳的是一個 char *，又沒有要求你特別 free 他，你是不是就該猜測他是回傳 function 內部的一個 static buffer 了呢？

Via: 從 inet_ntoa 看 thread safe 的 API.

When you lie to your compiler, don’t be surprised to get results those differ from what you expect.

In this case, the coder promised his/her compiler the value of “value” won’t be changed, as it was declared const.

Therefore our best friend (yes, the compiler) who trusted the coder, thought “hey, I could cache this value in a register and not fetch it from RAM every time I reference it”, i.e. optimization.

If we change the code in the link slightly, so that function funcC would print *value before the assignment, one would see that the value of *value had actually been modified by function funcA. However, the modification was not visible in main, because the compiler was lied to as the coder explicitly told it that there would be no modification to value.

In the body of function funcC, though value was declared as a pointer to const instance, the compiler was smart enough to know it was lied to. There was a const_cast and a assignment, therefore, the modification to *value was reflected in the output.

It is true that C++ provides programming constructs those consume more CPU cycles than it would when done without higher level of abstractions. But, hey, that’s the cost of abstractions, why pay when they are not needed?

Stop laying out your C++ design like Java. Avoid inheritance when possible. Don’t make every member function and destructor virtual without really thinking. They not only increase your run time overhead, they boost the complexity (read coupling) of your code like nothing else.

C++ does every C-style stuff the same way as C does. Not to mention it is capable of more compile time computation than C. I just fail to see how C++ could be slower than C in run time, seriously.

今天稍早, 跟一位同事聊到 Boost 的 lexical_cast. 打開了 Firefox 請出 Google, 想找到能幫助我介紹 lexical_cast 的資源. 意外地, Google 把我帶到讓我賺了介紹獎金, 現在跟我是同事的 Jeff, 談到 lexical_cast 的 文章.

在這篇文字的討論串, Jeff 寫道:

我覺得如果可以用這樣的寫法，那就太好了：

uint32_t ID = lexical_cast<auto>("1234567");

甚至最好是：

template <class From, class To = auto>
To lexical_cast(From s) {...}

uint32_t ID = lexical_cast("1234567");

Jeff Hung

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