设计模式问题 [英] Design pattern question
问题描述
您好，
我正在努力解决以下困扰我的问题。
我有一个具体的课程，Parser说，提供一些基本的解析
功能（比如逐字逐行阅读等）
然后我有第二个Parser派生类，比如说HtmlParser，<除了提供解析器功能外，还提供额外的功能，例如按标签读取标签，属性等等。
什么我想要实现的是工厂。根据收到的内容类型，它会返回正确的
类，所以我称之为工厂
，如factory.create（" html"）;等等
我面临的问题是派生的解析器将包含更多
方法，这些方法在基类中不可用。
那么如何在不依赖if / else /
切换结构的情况下实现通用行为？到目前为止，我看到的唯一方法是动态投射工厂返回的
解析器指针，但我觉得这打败了工厂的整个目的
。
另一个明显的方法是将所有方法放在基础
类中，但其中一些没有任何意义..比如阅读
来自通用文本的标签..
任何提示，我可以找到一些指针？谢谢。
Hello,
I''m trying to solve the following issue which is puzzling me.
I have a concrete class, say Parser, which provides some basic parsing
functionality (such as reading word by word, line by line, etc)
Then I have a second Parser-derived class, say HtmlParser, which
besides providing the Parser functionality, provides extra features,
such as read tag by tag, attributes, etc..
What I''d like to achieve is a "factory" which would return the proper
class depending on the content type received, so I''d call this factory
like factory.create("html");, etc
The problem I''m facing is that the derived parser will contain more
methods which are not available in the base class.
So how can I achieve a generic behavior without relying on if/else/
switch constructs? So far the only way I see is dynamic casting the
Parser pointer returned by the factory, but I feel this defeats the
whole purpose of a factory.
The other obvious approach would be putting all methods in the base
class, but some of them wouldn''t have any sense.. such as reading a
tag from a generic text..
Any hint to where I can find some pointers? Thank you.
推荐答案
al * *****@gmail.com 写道：
al******@gmail.com wrote:
我正在努力解决以下困扰我的问题。
我有一个具体的类，比如Parser，它提供了一些基本的解析
功能（比如逐字逐行读，等等）
然后我有第二个Parser派生类，比如HtmlParser，除了提供Parser功能外，它还提供额外的功能，
，例如按标签读取标签，属性等。 。
我想要达到的是工厂。根据收到的内容类型，它会返回正确的
类，所以我称之为工厂
，如factory.create（" html"）;等等
我面临的问题是派生的解析器将包含更多
方法，这些方法在基类中不可用。
那么如何在不依赖if / else /
切换结构的情况下实现通用行为？到目前为止，我看到的唯一方法是动态投射工厂返回的
解析器指针，但我觉得这打败了工厂的整个目的
。
另一个明显的方法是将所有方法放在基础
类中，但其中一些没有任何意义..比如阅读
来自通用文本的标签..
任何提示，我可以找到一些指针？谢谢。
I''m trying to solve the following issue which is puzzling me.
I have a concrete class, say Parser, which provides some basic parsing
functionality (such as reading word by word, line by line, etc)
Then I have a second Parser-derived class, say HtmlParser, which
besides providing the Parser functionality, provides extra features,
such as read tag by tag, attributes, etc..
What I''d like to achieve is a "factory" which would return the proper
class depending on the content type received, so I''d call this factory
like factory.create("html");, etc
The problem I''m facing is that the derived parser will contain more
methods which are not available in the base class.
So how can I achieve a generic behavior without relying on if/else/
switch constructs? So far the only way I see is dynamic casting the
Parser pointer returned by the factory, but I feel this defeats the
whole purpose of a factory.
The other obvious approach would be putting all methods in the base
class, but some of them wouldn''t have any sense.. such as reading a
tag from a generic text..
Any hint to where I can find some pointers? Thank you.
首先编写使用解析器的类。以这样的方式写它
无论文本的格式是什么它都会起作用。然后你的问题
将被解决。
更抽象地思考。
Write the class that uses the parser first. Write it in such a way that
it will work no matter what format the text is in. Then your problem
will be solved.
Think more abstractly.
al ****** @ gmail.com 写道：
您好，
我正在努力解决以下问题令我困惑的是。
我有一个具体的类，比如Parser，它提供了一些基本的解析
功能（比如逐字逐行读，等等）
然后我有第二个Parser派生类，比如说HtmlParser，除了提供Parser功能之外还有
，提供额外的功能，
如按标签，属性等读取标签。
我想要实现的是工厂。根据收到的内容类型，它会返回正确的
类，所以我称之为工厂
，如factory.create（" html"）;等等
我面临的问题是派生的解析器将包含更多
方法，这些方法在基类中不可用。
那么如何在不依赖if / else /
切换结构的情况下实现通用行为？到目前为止，我看到的唯一方法是动态投射工厂返回的
解析器指针，但我觉得这打败了工厂的整个目的
。
另一个明显的方法是将所有方法放在基础
类中，但其中一些没有任何意义..比如阅读
来自通用文本的标签..
任何提示，我可以找到一些指针？谢谢。
Hello,
I''m trying to solve the following issue which is puzzling me.
I have a concrete class, say Parser, which provides some basic parsing
functionality (such as reading word by word, line by line, etc)
Then I have a second Parser-derived class, say HtmlParser, which
besides providing the Parser functionality, provides extra features,
such as read tag by tag, attributes, etc..
What I''d like to achieve is a "factory" which would return the proper
class depending on the content type received, so I''d call this factory
like factory.create("html");, etc
The problem I''m facing is that the derived parser will contain more
methods which are not available in the base class.
So how can I achieve a generic behavior without relying on if/else/
switch constructs? So far the only way I see is dynamic casting the
Parser pointer returned by the factory, but I feel this defeats the
whole purpose of a factory.
The other obvious approach would be putting all methods in the base
class, but some of them wouldn''t have any sense.. such as reading a
tag from a generic text..
Any hint to where I can find some pointers? Thank you.
好了这里有几个问题：
1）根据你的描述，你正在编写一个词法分析器（扫描仪）
和tokenizer），不要将解析器与词法分析器混淆。从这里开始
：
http://en.wikipedia.org/wiki/Semanti...ter_science%29
确切了解您的需求并定义您的问题。你想要一个词法分析器
还是一个解析器？
2）当并行层次结构存在时，因子方法效果最好，你可以
将您的层次结构表示为词法分析器层次结构和令牌层次结构。
然后lexer类可以通过协方差创建一个令牌类和
虚拟方法：
class lexer {
public：
虚拟标记* create_token（const attr& at）{
返回新标记（at）;
}
virtual~lexer（）{};
};
class html：virtual public lexer {
public：
html_token * create_token（const attr& at）{
返回新的html_token（at）;
}
}
};
班级代币{
public：
令牌（const attr& at）{//构造一个令牌
}
};
class html_token：virtual公共令牌{
pubilc：
html_token（const attr& at）{//构建一个html toke n $ / b $ b}
};
3）if / else / switch构造没有任何问题。事实上，IMO
是唯一可以在领域内动态初始化对象的方法
的C ++。您可以使用特定于平台的功能，即动态库和
名称解析，以促进更通用的解决方案，但这不会与C ++有任何关系。
Ok there are a couple issues revealed here:
1) From your description, you are writing a lexical analyzer (scanner
and tokenizer), do not confuse a parser with a lexical analyzer. Start
from here:
http://en.wikipedia.org/wiki/Semanti...ter_science%29
Know exactly what you want and define your problem. Do you want a lexer
or a parser?
2) A factor method works best when parallel hierarchies exists, you can
formulate your hierarchies into a lexer hierarchy and a token hierarchy.
Then a lexer class can create a token class through covariance and
virtual methods:
class lexer{
public:
virtual token * create_token(const attr & at){
return new token(at);
}
virtual ~lexer(){};
};
class html : virtual public lexer {
public:
html_token * create_token(const attr & at){
return new html_token(at);
}
}
};
class token {
public:
token(const attr & at){ // construct a token
}
};
class html_token : virtual public token{
pubilc:
html_token(const attr & at){ // construct a html token
}
};
3) There is nothing wrong with if/else/switch constructs. In fact IMO
that''s only way to can initialize objects dynamically within the realm
of C++. You can use platform specific feature, i.e. dynamic library and
name resolution, to facilitate a more generic solution but that does not
have anything to do with C++.
非常感谢你的回答..
Hi, thanks a lot for your answer..
好的，这里有几个问题：
1）从您的描述中，您正在编写词法分析器（扫描仪
和tokenizer），不要将解析器与词法分析器混淆。从这里开始
： http：// en.wikipedia.org/wiki/Semanti...ter_science%29
确切了解您的需求并定义您的问题。你想要一个词法分析器
还是一个解析器？
Ok there are a couple issues revealed here:
1) From your description, you are writing a lexical analyzer (scanner
and tokenizer), do not confuse a parser with a lexical analyzer. Start
from here:http://en.wikipedia.org/wiki/Semanti...ter_science%29
Know exactly what you want and define your problem. Do you want a lexer
or a parser?
是的，谢谢你让我知道。我对这些过程的分离没有足够的了解。
实际上，似乎我需要它们两个......
这个想法基本上能够解释一个http头，还有另外一些协议，比如说rtsp举个例子。
两个都有相似之处，但那里有是每个
协议特有的东西。
如果我说在大多数情况下分析和解析是在b / b中完成的，我错了同一个地方？
我的想法就是能够获得一个数据流，然后构建一个包含不同标题的
数据结构，
及其字段和属性..从我的想法来看，我不认为我需要制作一个lex分析器，然后是一个解析器..我能做到
obth当时的东西？
Yes, thanks for letting me know. I didn''t have enough knowledge on the
separation of these processes.
Actually, it seems I need both of them..
The idea is basically being able to interpret a http header, and also
other protocols, say rtsp to put an example.
Both have similarities, but there are some things specific to each
protocol.
Am I wrong if I say that in most cases the analyzing and parsing is
done in the same place?
My idea is simply being able to get a stream of data and then build a
data structure holding the different headers,
with its fields and attributes.. From what I think, I don''t think I
need to make a lex analyzer and then a parser.. I could do
obth things at the time?
2）当并行层次结构存在时，因子方法效果最好，你可以用b $ b来表示你的层次结构成词法分析器层次结构和令牌层次结构。
然后lexer类可以通过协方差创建一个令牌类和
虚拟方法：
class lexer {
public：
虚拟标记* create_token（const attr& at）{
返回新令牌（at）;
}
virtual~lexer（）{};};
class html：虚拟公共词法分析器{
public：
html_token * create_token（const attr& at）{
返回新的html_token（at）;
}
}
};
class token {
public：
token（const attr& at）{//构造一个令牌
}};
class html_token：虚拟公共令牌{
pubilc：
html_token（const attr& at）{//构造一个html令牌
}};
3）if / else / switch构造没有任何问题。事实上，IMO
是唯一可以在领域内动态初始化对象的方法
的C ++。您可以使用特定于平台的功能，即动态库和
名称解析，以促进更通用的解决方案，但这不会与C ++有任何关系。
2) A factor method works best when parallel hierarchies exists, you can
formulate your hierarchies into a lexer hierarchy and a token hierarchy.
Then a lexer class can create a token class through covariance and
virtual methods:
class lexer{
public:
virtual token * create_token(const attr & at){
return new token(at);
}
virtual ~lexer(){};};
class html : virtual public lexer {
public:
html_token * create_token(const attr & at){
return new html_token(at);
}
}
};
class token {
public:
token(const attr & at){ // construct a token
}};
class html_token : virtual public token{
pubilc:
html_token(const attr & at){ // construct a html token
}};
3) There is nothing wrong with if/else/switch constructs. In fact IMO
that''s only way to can initialize objects dynamically within the realm
of C++. You can use platform specific feature, i.e. dynamic library and
name resolution, to facilitate a more generic solution but that does not
have anything to do with C++.
谢谢。我绝对需要更多地考虑这个问题，或者学习现有的
示例，看看他们是如何做到的。
Thanks. I definetly need to give this more thought, or learn existing
examples to see how they do this.
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