双木成林：喋喋不休

好久没写博客了。最近重拾C++，在看Effective Modern C++，看到有关pImpl idiom的相关内容，以前这块就没有搞得很清楚，就自己研究了一下

首先， PImpl Idiom的作用，我看起来主要有两个。一是为了让实现和接口分离，二减少重新编译的时间，当具体实现代码所依赖的头文件改动的时候，类本身的头文件不会变动，而导致所有依赖的库重新编译。

第一次实现的版本如下

#include <memory>
#include <string>
class Person {
public:
    Person(std::string name);
    void set_name(std::string name);
    std::string name();
private:
    struct Impl;
    std::unique_ptr<Impl> pImpl_;
};

编译的时候遇到一个错误：

error: invalid application of ‘sizeof’ to an incomplete type ‘Person::Impl’
static_assert(sizeof(_Tp) > 0, “default_delete can not delete incomplete type”);
^~~~~~~~~~~

原因就是， 没有自定义Person的destructor，系统就会自动加上一个。。而在这个系统自动加上Person的destructor中，会尝试去销毁pImpl这个unique_ptr，而且由于系统采用的是inline的方式插入destructor，所以相当于在头文件中，就包含了上述的那个sizeof(_Tp)的代码。

而与此同时，struct Impl;是一个前置声明语句，编译的时候是不知道他的具体大小的。所以编译就出错了。

解决办法就是，在Person中声明一个desturctor，让编译器不会自动生成默认的deconstructor就行。但是这样同样会有问题，我在这里其实并不需要特殊处理，所以我又想用系统自动生成的代码。怎么办呢？

解决办法就是在实现这个方法的时候，加上一个= default这个c++11新加的方法。

新代码如下：

person.hpp

class Person {
public:
    Person(std::string name);
    virtual ~Person();
    void set_name(std::string name);
    std::string name();
private:
    struct Impl;
    std::unique_ptr<Impl> pImpl_;
};

person.cpp

Person::~Person() = default;

问题完美解决

按照一般的想法，理解一个东西与否，还需要提出一个假设，然后验证这个假设是否和事实一样。那么我在这里假设，如果我直接把这个Person::~Person() = default;的代码移到头文件之中去，让他用inline的方式生成destructor，同样会有编译错误。

代码改成如下：

class Person {
public:
    Person(std::string name);
    virtual ~Person() = default;
    void set_name(std::string name);
    std::string name();
private:
    struct Impl;
    std::unique_ptr<Impl> pImpl_;
};

不出所料果然出现了同样的编译错误， 问题解决。

Thx to http://ubuntuforums.org/showthread.php?t=1266104

The point is

1. Get hold of ldconfig & ldconfig.real from the correct version:

Code: aptitude download libc6

unpack it:

Code: dpkg-deb -x libc6*.deb libc6-unpacked/

copy them out: Code: sudo cp libc6-unpacked/sbin/ldconfig* /sbin/

At this point I did:

Code: sudo apt-get -f install

sudo dpkg-reconfigure libc6

sudo dpkg-reconfigure libc6-i386

sudo apt-get install –reinstall libc6

sudo apt-get install –reinstall libc6-i386

Of which the reinstalls seemed to be the crucial ones.

Now all errors seem to be gone.

I am honored to be with you today at your commencement from one of the finest universities in the world. I never graduated from college. Truth be told, this is the closest I’ve ever gotten to a college graduation. Today I want to tell you three stories from my life. That’s it. No big deal. Just three stories.

The first story is about connecting the dots.

I dropped out of Reed College after the first 6 months, but then stayed around as a drop-in for another 18 months or so before I really quit. So why did I drop out?

It started before I was born. My biological mother was a young, unwed college graduate student, and she decided to put me up for adoption. She felt very strongly that I should be adopted by college graduates, so everything was all set for me to be adopted at birth by a lawyer and his wife. Except that when I popped out they decided at the last minute that they really wanted a girl. So my parents, who were on a waiting list, got a call in the middle of the night asking: “We have an unexpected baby boy; do you want him?” They said: “Of course.” My biological mother later found out that my mother had never graduated from college and that my father had never graduated from high school. She refused to sign the final adoption papers. She only relented a few months later when my parents promised that I would someday go to college.

And 17 years later I did go to college. But I naively chose a college that was almost as expensive as Stanford, and all of my working-class parents’ savings were being spent on my college tuition. After six months, I couldn’t see the value in it. I had no idea what I wanted to do with my life and no idea how college was going to help me figure it out. And here I was spending all of the money my parents had saved their entire life. So I decided to drop out and trust that it would all work out OK. It was pretty scary at the time, but looking back it was one of the best decisions I ever made. The minute I dropped out I could stop taking the required classes that didn’t interest me, and begin dropping in on the ones that looked interesting.

It wasn’t all romantic. I didn’t have a dorm room, so I slept on the floor in friends’ rooms, I returned coke bottles for the 5¢ deposits to buy food with, and I would walk the 7 miles across town every Sunday night to get one good meal a week at the Hare Krishna temple. I loved it. And much of what I stumbled into by following my curiosity and intuition turned out to be priceless later on. Let me give you one example:

Reed College at that time offered perhaps the best calligraphy instruction in the country. Throughout the campus every poster, every label on every drawer, was beautifully hand calligraphed. Because I had dropped out and didn’t have to take the normal classes, I decided to take a calligraphy class to learn how to do this. I learned about serif and san serif typefaces, about varying the amount of space between different letter combinations, about what makes great typography great. It was beautiful, historical, artistically subtle in a way that science can’t capture, and I found it fascinating.

None of this had even a hope of any practical application in my life. But ten years later, when we were designing the first Macintosh computer, it all came back to me. And we designed it all into the Mac. It was the first computer with beautiful typography. If I had never dropped in on that single course in college, the Mac would have never had multiple typefaces or proportionally spaced fonts. And since Windows just copied the Mac, it’s likely that no personal computer would have them. If I had never dropped out, I would have never dropped in on this calligraphy class, and personal computers might not have the wonderful typography that they do. Of course it was impossible to connect the dots looking forward when I was in college. But it was very, very clear looking backwards ten years later.

Again, you can’t connect the dots looking forward; you can only connect them looking backwards. So you have to trust that the dots will somehow connect in your future. You have to trust in something — your gut, destiny, life, karma, whatever. This approach has never let me down, and it has made all the difference in my life.

My second story is about love and loss.

I was lucky — I found what I loved to do early in life. Woz and I started Apple in my parents garage when I was 20. We worked hard, and in 10 years Apple had grown from just the two of us in a garage into a $2 billion company with over 4000 employees. We had just released our finest creation — the Macintosh — a year earlier, and I had just turned 30. And then I got fired. How can you get fired from a company you started? Well, as Apple grew we hired someone who I thought was very talented to run the company with me, and for the first year or so things went well. But then our visions of the future began to diverge and eventually we had a falling out. When we did, our Board of Directors sided with him. So at 30 I was out. And very publicly out. What had been the focus of my entire adult life was gone, and it was devastating.

I really didn’t know what to do for a few months. I felt that I had let the previous generation of entrepreneurs down – that I had dropped the baton as it was being passed to me. I met with David Packard and Bob Noyce and tried to apologize for screwing up so badly. I was a very public failure, and I even thought about running away from the valley. But something slowly began to dawn on me — I still loved what I did. The turn of events at Apple had not changed that one bit. I had been rejected, but I was still in love. And so I decided to start over.

I didn’t see it then, but it turned out that getting fired from Apple was the best thing that could have ever happened to me. The heaviness of being successful was replaced by the lightness of being a beginner again, less sure about everything. It freed me to enter one of the most creative periods of my life.

During the next five years, I started a company named NeXT, another company named Pixar, and fell in love with an amazing woman who would become my wife. Pixar went on to create the worlds first computer animated feature film, Toy Story, and is now the most successful animation studio in the world. In a remarkable turn of events, Apple bought NeXT, I returned to Apple, and the technology we developed at NeXT is at the heart of Apple’s current renaissance. And Laurene and I have a wonderful family together.

I’m pretty sure none of this would have happened if I hadn’t been fired from Apple. It was awful tasting medicine, but I guess the patient needed it. Sometimes life hits you in the head with a brick. Don’t lose faith. I’m convinced that the only thing that kept me going was that I loved what I did. You’ve got to find what you love. And that is as true for your work as it is for your lovers. Your work is going to fill a large part of your life, and the only way to be truly satisfied is to do what you believe is great work. And the only way to do great work is to love what you do. If you haven’t found it yet, keep looking. Don’t settle. As with all matters of the heart, you’ll know when you find it. And, like any great relationship, it just gets better and better as the years roll on. So keep looking until you find it. Don’t settle.

My third story is about death.

When I was 17, I read a quote that went something like: “If you live each day as if it was your last, someday you’ll most certainly be right.” It made an impression on me, and since then, for the past 33 years, I have looked in the mirror every morning and asked myself: “If today were the last day of my life, would I want to do what I am about to do today?” And whenever the answer has been “No” for too many days in a row, I know I need to change something.

Remembering that I’ll be dead soon is the most important tool I’ve ever encountered to help me make the big choices in life. Because almost everything — all external expectations, all pride, all fear of embarrassment or failure – these things just fall away in the face of death, leaving only what is truly important. Remembering that you are going to die is the best way I know to avoid the trap of thinking you have something to lose. You are already naked. There is no reason not to follow your heart.

About a year ago I was diagnosed with cancer. I had a scan at 7:30 in the morning, and it clearly showed a tumor on my pancreas. I didn’t even know what a pancreas was. The doctors told me this was almost certainly a type of cancer that is incurable, and that I should expect to live no longer than three to six months. My doctor advised me to go home and get my affairs in order, which is doctor’s code for prepare to die. It means to try to tell your kids everything you thought you’d have the next 10 years to tell them in just a few months. It means to make sure everything is buttoned up so that it will be as easy as possible for your family. It means to say your goodbyes.

I lived with that diagnosis all day. Later that evening I had a biopsy, where they stuck an endoscope down my throat, through my stomach and into my intestines, put a needle into my pancreas and got a few cells from the tumor. I was sedated, but my wife, who was there, told me that when they viewed the cells under a microscope the doctors started crying because it turned out to be a very rare form of pancreatic cancer that is curable with surgery. I had the surgery and I’m fine now.

This was the closest I’ve been to facing death, and I hope it’s the closest I get for a few more decades. Having lived through it, I can now say this to you with a bit more certainty than when death was a useful but purely intellectual concept:

No one wants to die. Even people who want to go to heaven don’t want to die to get there. And yet death is the destination we all share. No one has ever escaped it. And that is as it should be, because Death is very likely the single best invention of Life. It is Life’s change agent. It clears out the old to make way for the new. Right now the new is you, but someday not too long from now, you will gradually become the old and be cleared away. Sorry to be so dramatic, but it is quite true.

Your time is limited, so don’t waste it living someone else’s life. Don’t be trapped by dogma — which is living with the results of other people’s thinking. Don’t let the noise of others’ opinions drown out your own inner voice. And most important, have the courage to follow your heart and intuition. They somehow already know what you truly want to become. Everything else is secondary.

When I was young, there was an amazing publication called The Whole Earth Catalog, which was one of the bibles of my generation. It was created by a fellow named Stewart Brand not far from here in Menlo Park, and he brought it to life with his poetic touch. This was in the late 1960′s, before personal computers and desktop publishing, so it was all made with typewriters, scissors, and polaroid cameras. It was sort of like Google in paperback form, 35 years before Google came along: it was idealistic, and overflowing with neat tools and great notions.

Stewart and his team put out several issues of The Whole Earth Catalog, and then when it had run its course, they put out a final issue. It was the mid-1970s, and I was your age. On the back cover of their final issue was a photograph of an early morning country road, the kind you might find yourself hitchhiking on if you were so adventurous. Beneath it were the words: “Stay Hungry. Stay Foolish.” It was their farewell message as they signed off. Stay Hungry. Stay Foolish. And I have always wished that for myself. And now, as you graduate to begin anew, I wish that for you.

Stay Hungry. Stay Foolish.

Thank you all very much.

今天在写iOS应用的时候遇到一个问题，使用[NSData dataWithContentOfFile:@"foo"]的时候，总是无法读取正确的文件内容。而使用[NSData dataWithContentOfFile:[[NSBundle mainBundle] pathForResource:@”foo” ofType:@”"]的时候就可以。

经过Google以后知道。当使用相对路径的时候，其实他相对的当前目录并不是程序运行的目录，而是“/”。只有使用[NSBundle mainBundle]来生成的路径才是文件真正的路径。

在此记录一下：在以后的开发中不直接使用任何相对路径，而是使用经过计算以后的绝对路径。

此文来自本人原JavaEye博客，发表于2009年2月26日。原文地址

Django应该算是目前最火的Pyhton的Web框架了把。个人感觉，超级方便的ORM，清晰而功能明确的分层以及Killer级别的后台管理都让习惯了Java那一套庞大复杂的人有了眼前一亮的感觉。

不过用久了就会发现，django整个框架之中有一个极其繁琐的地方，就好像一块整洁的布上有一个污点一样，令人觉得不爽。

那就是他的URL机制。。当然，也有人非常喜欢他的URL机制，比如说十分的灵活。不过我个人总觉得还有改进的地方。

首先我认为必须要利用正则表达式这一点，这个是特色以及精华，并且确实十分方便，在我眼里属于不能更改的部分。那么自然而然改进的地方就是正则表达式和函数的映射了。

提出问题是一方面，解决问题就是更加重要的地方了。采用何种新的方式来替换原来的URL处理呢？我为此困扰了好久。直到有一天发现了limodou大牛的Uliweb框架。被它的简洁明了的URL方式吸引了。决定采用类似Uliweb的方式处理URL。实现的具体细节，就是用Django的中间件。

使用中间件自然有他的理由。研究官网的文档后发现，Django处理一个Request的过程是首先通过中间件，然后再通过默认的URL方式进行的。所以说我们要做的就是在Middleware这个地方把所有Request拦截住，用我们自己的方式完成处理以后直接返回Response。需要注意的是，因为我不舍得放弃Killer级别的Admin，所以必须要支持默认的URL配置方式。那么我们可以简化原来的设计思路，把中间件不能处理的Request统统不管，丢给Django去处理。简直是完美的解决方案。:)。同时也不得不赞一声Django的设计。

接下来的设计就是考虑URL和函数处理的问题了。这个时候应该感谢的就是Python强大的语法了。提供了Decorator这种机制来解决这个问题。那么设计的思路转变成，在Request到达Middleware的时候，Import所有INSTALLED_APPS里面的views.py，目的自然就是执行Decorator。而在Decorator内部，根据到达的Request获得其访问地址，然后和处理它的函数一起生成RegexURLPattern对象，储存在缓存中。然后对每个Request进行处理，符合规则的就调用函数处理，不符合规则的，就交给Django自己去处理了。

以下是一小段中间件代码。比较核心的部分了。其实代码量也不大。大家看看有没有实现的问题。

    def process_request(self,request):
        path=request.path_info  #获得request的地址
        urls=settings.INSTALLED_APPS  #获得当前安装的app的位置
        def myimport(path):
            try:
                __import__('%s.views'%path)  #读取所有app的views。。目的是注册request处理函数
            except:
                pass
        map(myimport,urls)   #执行所有
        callback=expose.getCallback(path)  #获取相应url对应的处理函数
        if callback:
            return callback[0](request)  #执行。。

整体设计大致如此，完成了0.1版本，已经放在我的代码库里面了.http://code.google.com/p/linllx/

目前还存在的问题就是。第一，对于需要命名分组的正则表达式的处理还不是很有头绪。第二，对于404错误的处理究竟要如何进行，目前的实现来看不是很好。请看见本文的牛人们提供一下思路。。

总得来说，Django中间件的开发并不像我想像中的那么难，甚至可以说，还是比较简单的。下一步着手开发一个类似于Java中的Filter的中间件把。。当然，这个还是需要继续完善的。。