Accelerated C++

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  16.1

  Universe (Hello, world.)

  16.1 Use the abstractions you have In particular, if we design our own abstractions well, we should be able to use them to solve problems that we had not considered when we designed them. (更多)

  16.1 Use the abstractions you have

  In particular, if we design our own abstractions well, we should be able to use them to solve problems that we had not considered when we designed them.

  (收起)

  2011-08-03 22:20:35   回应

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  15.1

  Universe (Hello, world.)

  

  这一张讲的是设计，有人对我说过，写代码不值钱，设计才值钱。不过值钱的东西往往不容易理解，像这章就是。将现实问题分解，然后再重新构成软件，本身就是一个费脑筋的过程。要把这个过程给不了解的人讲清楚，就更费脑筋了。这一章的讲解只能说合格，没有前面的章节那么容易理解。 15.1.1 Using inheritance to model the structure We'll call the derived classes String_Pic, for pictures created from strings that our u... (更多)

  这一张讲的是设计，有人对我说过，写代码不值钱，设计才值钱。不过值钱的东西往往不容易理解，像这章就是。将现实问题分解，然后再重新构成软件，本身就是一个费脑筋的过程。要把这个过程给不了解的人讲清楚，就更费脑筋了。这一章的讲解只能说合格，没有前面的章节那么容易理解。

  15.1.1 Using inheritance to model the structure

  We'll call the derived classes String_Pic, for pictures created from strings that our user gives us; Frame_Pic, for a picture created by framing another picture; and HCat_Pic and VCat_Pic, for pictures that are the result of concatenating two other pictures horizontally or vertically respectively. By relating these classes through inheritance, we can use virtual functions to write code that doesn't always need to know the precise kind of picture on which it is operating. That way, our users can still use any of our operations without knowing which kind of picture is being manipulated. We will derive each of these classes from a common base class, which we shall call Pic_base, resulting in the following inheritance hierarchy:

  
  

  String_Pic 创建 strings 形成的图案（picture）；Frame_Pic 创建装饰 strings 的框（frame）；HCat_Pic 和 VCat_Pic 水平或垂直的组合不同的图案（pictures）。

  The next question to resolve is whether to make the inheritance hierarchy visible to our users. There seems to be little reason to do so. None of our operations deals with specific kinds of pictures; instead, they all deal with the abstract notion of a picture. So, there is no need to expose the hierarchy. Moreover, because we intend to use a reference- counting strategy, our users will find it more convenient if we hide the inheritance and associated reference counting. Instead of making our users deal directly with Pic_base and its associated derived classes, we'll define a picture-specific interface class. Our users will access that class, freeing them from having to be aware of any of our implementation's details. In particular, using an interface class will hide the inheritance hierarchy, along with the fact that our class relies on Ptr. Apparently, then we'll need to define six classes: the interface class, the base class for our inheritance hierarchy, and the four derived classes. We'll call the interface class Picture. Internally, Picture will use a Ptr to manage its data.

  为了隐藏细节，提供了接口类（interface class），具体实现类不提供 public interface，后面为了实现时能互相调用成员函数（member function），用了友元（friend）。这符合高内聚、低耦合的原则。

  So far, the Picture class is pretty simple: The only operation is to create a Picture from a vector of strings. We use a default argument (§7.3/127) to make that vector optional. If a user constructs a Picture with no argument, then the compiler will supply vector<string>() as an argument automatically, which yields a vector<string> with no elements. Therefore, the effect of the default argument is to allow us to use a definition such as

  Picture p;       // an empty Picture
  

  to create a Picture with no rows.

  先想做什么，再想怎么做。 (收起)

  2011-07-31 09:07:20   回应

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  15.2

  Universe (Hello, world.)

  15.2.3 Padding the output Static members (both functions and static data members, which we can also define) are useful in that they let us minimize the names that are defined globally. Our pad function is a good example. We can imagine many abstractions that have the notion of padding. In this book we talked about padding in the context of writing a formatted report of student grades, as well a... (更多)

  15.2.3 Padding the output

  Static members (both functions and static data members, which we can also define) are useful in that they let us minimize the names that are defined globally. Our pad function is a good example. We can imagine many abstractions that have the notion of padding. In this book we talked about padding in the context of writing a formatted report of student grades, as well as in the context of writing Pictures. If the Picture class were to define pad as a global function, then we would not also be able to define a pad function for Student_info, or vice versa. By making pad a static member, we allow for the fact that other abstractions in our program might have the notion of padding. As long as each class defines what pad means only in the context of the class, these mutually independent notions of padding can coexist within our program.

  关于 static function 的用途。

  A member of a derived class (such as Frame_Pic) can access the protected members of the base-class parts of objects of its own class (such as Frame_Pic), or of other classes derived from it, but it cannot access the protected members of base-class objects that stand alone—that is, that are not part of a derived-class object. Therefore, member functions of class Frame_Pic, which is derived from class Pic_base, can access protected members of the Pic_base parts of Frame_Pic objects, or objects of classes derived from Frame_Pic, but they cannot access protected members of stand-alone Pic_base objects directly.

  Protected members 的访问限制。 (收起)

  2011-07-29 17:21:50   回应

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  第24页

  Universe (Hello, world.)

  电子版，没有页数，以章节号为准。 2.4 Writing a row The int type is sufficient for rows because the number of rows depends only on the value of pad, which we control. Every C++ implementation is required to allow every int variable to take on values up to at least 32767, which is plenty. Nevertheless, whenever we define a variable that contains the size of a particular data structure, it is a g... (更多)

  电子版，没有页数，以章节号为准。

  2.4 Writing a row

  The int type is sufficient for rows because the number of rows depends only on the value of pad, which we control. Every C++ implementation is required to allow every int variable to take on values up to at least 32767, which is plenty. Nevertheless, whenever we define a variable that contains the size of a particular data structure, it is a good habit to use the type that the library defines as being appropriate for that specific purpose.

  相当细致的type选择，用

  const std::string::size_type cols = greeting.size() + pad * 2 + 2;
  

  作为例子，详细说明为什么不直接用int，这是很容易被忽略的地方。 (收起)

  2011-03-30 16:00:35   回应

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  第25页

  Universe (Hello, world.)

  电子版，没有页数，以章节号为准。 2.5.1 Abbreviating repeated uses of std:: C++ offers a way of saying that a particular name should always be interpreted as coming from a particular namespace. For example, by writing/代码内容已省略/ we can say that we intend to use the name cout to mean std::cout exclusively, and that we do not intend to define anything named ... (更多)

  电子版，没有页数，以章节号为准。

  2.5.1 Abbreviating repeated uses of std::

  C++ offers a way of saying that a particular name should always be interpreted as coming from a particular namespace. For example, by writing

  using std::cout;
  

  we can say that we intend to use the name cout to mean std::cout exclusively, and that we do not intend to define anything named cout ourselves. Once we have done so, we can say cout instead of std::cout.

  相当合理的一种用法，我以前还一直不知道，惭愧。

  Logically enough, such a declaration is called a using-declaration. The name that it mentions behaves similarly to other names. For example, if a using-declaration appears within braces, the name that it defines has its given meaning from where it is defined to the closing brace.

  这种用法可能知道的人多些，但是在Google C++ Style Guide中直接禁止掉了 http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml?showone=Namespaces#Namespaces 理由是 This pollutes the namespace.

  个人认为，在只有几个classes的小项目中可以用。中型以上的项目应该遵照Google C++ Style Guide，因为name的数量到达一定程度后，产生 names collide 的概率会非常大。 (收起)

  2011-03-30 21:23:52   回应

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  第26页

  Universe (Hello, world.)

  电子版，没有页数，以章节号为准。 2.6 Counting 这一节很全面的讲解了for循环为什么应该从0而不是1开始。 (更多)

  电子版，没有页数，以章节号为准。

  2.6 Counting

  这一节很全面的讲解了for循环为什么应该从0而不是1开始。 (收起)

  2011-03-30 22:26:33   回应

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• 

  16.1

  Universe (Hello, world.)

  16.1 Use the abstractions you have In particular, if we design our own abstractions well, we should be able to use them to solve problems that we had not considered when we designed them. (更多)

  16.1 Use the abstractions you have

  In particular, if we design our own abstractions well, we should be able to use them to solve problems that we had not considered when we designed them.

  (收起)

  2011-08-03 22:20:35   回应

  收藏

  推荐
• 

  15.1

  Universe (Hello, world.)

  

  这一张讲的是设计，有人对我说过，写代码不值钱，设计才值钱。不过值钱的东西往往不容易理解，像这章就是。将现实问题分解，然后再重新构成软件，本身就是一个费脑筋的过程。要把这个过程给不了解的人讲清楚，就更费脑筋了。这一章的讲解只能说合格，没有前面的章节那么容易理解。 15.1.1 Using inheritance to model the structure We'll call the derived classes String_Pic, for pictures created from strings that our u... (更多)

  这一张讲的是设计，有人对我说过，写代码不值钱，设计才值钱。不过值钱的东西往往不容易理解，像这章就是。将现实问题分解，然后再重新构成软件，本身就是一个费脑筋的过程。要把这个过程给不了解的人讲清楚，就更费脑筋了。这一章的讲解只能说合格，没有前面的章节那么容易理解。

  15.1.1 Using inheritance to model the structure

  We'll call the derived classes String_Pic, for pictures created from strings that our user gives us; Frame_Pic, for a picture created by framing another picture; and HCat_Pic and VCat_Pic, for pictures that are the result of concatenating two other pictures horizontally or vertically respectively. By relating these classes through inheritance, we can use virtual functions to write code that doesn't always need to know the precise kind of picture on which it is operating. That way, our users can still use any of our operations without knowing which kind of picture is being manipulated. We will derive each of these classes from a common base class, which we shall call Pic_base, resulting in the following inheritance hierarchy:

  
  

  String_Pic 创建 strings 形成的图案（picture）；Frame_Pic 创建装饰 strings 的框（frame）；HCat_Pic 和 VCat_Pic 水平或垂直的组合不同的图案（pictures）。

  The next question to resolve is whether to make the inheritance hierarchy visible to our users. There seems to be little reason to do so. None of our operations deals with specific kinds of pictures; instead, they all deal with the abstract notion of a picture. So, there is no need to expose the hierarchy. Moreover, because we intend to use a reference- counting strategy, our users will find it more convenient if we hide the inheritance and associated reference counting. Instead of making our users deal directly with Pic_base and its associated derived classes, we'll define a picture-specific interface class. Our users will access that class, freeing them from having to be aware of any of our implementation's details. In particular, using an interface class will hide the inheritance hierarchy, along with the fact that our class relies on Ptr. Apparently, then we'll need to define six classes: the interface class, the base class for our inheritance hierarchy, and the four derived classes. We'll call the interface class Picture. Internally, Picture will use a Ptr to manage its data.

  为了隐藏细节，提供了接口类（interface class），具体实现类不提供 public interface，后面为了实现时能互相调用成员函数（member function），用了友元（friend）。这符合高内聚、低耦合的原则。

  So far, the Picture class is pretty simple: The only operation is to create a Picture from a vector of strings. We use a default argument (§7.3/127) to make that vector optional. If a user constructs a Picture with no argument, then the compiler will supply vector<string>() as an argument automatically, which yields a vector<string> with no elements. Therefore, the effect of the default argument is to allow us to use a definition such as

  Picture p;       // an empty Picture
  

  to create a Picture with no rows.

  先想做什么，再想怎么做。 (收起)

  2011-07-31 09:07:20   回应

  收藏

  推荐
• 

  15.2

  Universe (Hello, world.)

  15.2.3 Padding the output Static members (both functions and static data members, which we can also define) are useful in that they let us minimize the names that are defined globally. Our pad function is a good example. We can imagine many abstractions that have the notion of padding. In this book we talked about padding in the context of writing a formatted report of student grades, as well a... (更多)

  15.2.3 Padding the output

  Static members (both functions and static data members, which we can also define) are useful in that they let us minimize the names that are defined globally. Our pad function is a good example. We can imagine many abstractions that have the notion of padding. In this book we talked about padding in the context of writing a formatted report of student grades, as well as in the context of writing Pictures. If the Picture class were to define pad as a global function, then we would not also be able to define a pad function for Student_info, or vice versa. By making pad a static member, we allow for the fact that other abstractions in our program might have the notion of padding. As long as each class defines what pad means only in the context of the class, these mutually independent notions of padding can coexist within our program.

  关于 static function 的用途。

  A member of a derived class (such as Frame_Pic) can access the protected members of the base-class parts of objects of its own class (such as Frame_Pic), or of other classes derived from it, but it cannot access the protected members of base-class objects that stand alone—that is, that are not part of a derived-class object. Therefore, member functions of class Frame_Pic, which is derived from class Pic_base, can access protected members of the Pic_base parts of Frame_Pic objects, or objects of classes derived from Frame_Pic, but they cannot access protected members of stand-alone Pic_base objects directly.

  Protected members 的访问限制。 (收起)

  2011-07-29 17:21:50   回应

  收藏

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