blocks|key|1780700|text|我不经常使用嵌套类，但我偶尔会用到它们。特别是当我定义了某种数据类型，然后我想定义一个为该数据类型设计的STL+functor。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1780701|例如，考虑一个具有ID号、类型代码和字段名的通用Field类。如果我想通过ID号或名称搜索这些Field的vector，我可能会构造一个函数来执行此操作：|offset|length|style|CODE|1780702|class+Field
{
public:
++unsigned+id_;
++string+name_;
++unsigned+type_;

++class+match+:+public+std::unary_function<bool,+Field>
++{
++public:
++++match(const+string&+name)+:+name_(name),+has_name_(true)+{};
++++match(unsigned+id)+:+id_(id),+has_id_(true)+{};
++++bool+operator()(const+Field&+rhs)+const
++++{
++++++bool+ret+=+true;
++++++if(+ret+&&+has_id_+)+ret+=+id_+==+rhs.id_;
++++++if(+ret+&&+has_name_+)+ret+=+name_+==+rhs.name_;
++++++return+ret;
++++};
++++private:
++++++unsigned+id_;
++++++bool+has_id_;
++++++string+name_;
++++++bool+has_name_;
++};
};|code-block|syntax|javascript|1780703|然后，需要搜索这些Field的代码可以使用Field类本身范围内的match：|1780704|vector<Field>::const_iterator+it+=+find_if(fields.begin(),+fields.end(),+Field::match("FieldName"));|1780705|entityMap^0|0|O|5|1B|5|1H|6|0|0|9|5|L|5|X|5|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@$D|U|E|V|F|G]|$D|W|E|X|F|G]|$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|10|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|11|8|@$D|12|E|13|F|G]|$D|14|E|15|F|G]|$D|16|E|17|F|G]]|9|@]|A|$]]|$1|O|3|P|5|J|7|18|8|@]|9|@]|A|$K|L]]|$1|Q|3|-4|5|6|7|19|8|@]|9|@]|A|$]]]|R|$]]

I don't use nested classes much, but I do use them now and then. Especially when I define some kind of data type, and I then want to define a STL functor designed for that data type.

For example, consider a generic <code>Field</code> class that has an ID number, a type code and a field name. If I want to search a <code>vector</code> of these <code>Field</code>s by either ID number or name, I might construct a functor to do so:

<pre><code>class Field
{
public:
 unsigned id_;
 string name_;
 unsigned type_;

 class match : public std::unary_function&lt;bool, Field&gt;
 {
 public:
 match(const string&amp; name) : name_(name), has_name_(true) {};
 match(unsigned id) : id_(id), has_id_(true) {};
 bool operator()(const Field&amp; rhs) const
 {
 bool ret = true;
 if( ret &amp;&amp; has_id_ ) ret = id_ == rhs.id_;
 if( ret &amp;&amp; has_name_ ) ret = name_ == rhs.name_;
 return ret;
 };
 private:
 unsigned id_;
 bool has_id_;
 string name_;
 bool has_name_;
 };
};
</code></pre>

Then code that needs to search for these <code>Field</code>s can use the <code>match</code> scoped within the <code>Field</code> class itself:

<pre><code>vector&lt;Field&gt;::const_iterator it = find_if(fields.begin(), fields.end(), Field::match("FieldName"));
</code></pre>

blocks|key|1994636|text|One+can+implement+a+Builder+pattern+with+nested+class。特别是在C%2B%2B中，我个人发现它在语义上更清晰。例如：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1994637|class+Product{
++++public:
++++++++class+Builder;
}
class+Product::Builder+{
++++//+Builder+Implementation
}|code-block|syntax|javascript|1994638|而不是：|1994639|class+Product+{}
class+ProductBuilder+{}|1994640|entityMap|0|LINK|mutability|MUTABLE|url|https://en.wikipedia.org/wiki/Builder_pattern#C.2B.2B_Example^0|0|1H|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|Z|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|10|8|@]|9|@]|C|$G|H]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

<a href="https://en.wikipedia.org/wiki/Builder_pattern#C.2B.2B_Example" rel="noreferrer">One can implement a Builder pattern with nested class</a>. Especially in C++, personally I find it semantically cleaner. For example:

<pre><code>class Product{
 public:
 class Builder;
}
class Product::Builder {
 // Builder Implementation
}
</code></pre>

Rather than:

<pre><code>class Product {}
class ProductBuilder {}
</code></pre>

blocks|key|1994742|text|我认为使类成为嵌套类而不仅仅是朋友类的主要目的是能够在派生类中继承嵌套类。友谊不会在C%2B%2B中继承。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1994743|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I think the main purpose of making a class to be nested instead of just a friend class is the ability to inherit nested class within derived one. Friendship is not inherited in C++.

blocks|key|1781047|text|你也可以考虑一流的辅助类型的main函数，在那里你可以启动所有需要的类来工作。比如班级游戏，启动所有其他的职业，比如视窗，英雄，敌人，关卡等等。这样你就可以从main函数中去掉所有的东西了。在这里你可以创建游戏对象，也许还可以做一些与Gemente+it+self无关的外部调用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1781048|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

You also can think about first class ass type of main function, where You initiate all needed classes to work togheter. Like for example class Game, initiate all other classes like windows, heroes, enemy's, levels and so on. This way You can get rid all that stuff from main function it self. Where You can create obiect of Game, and maybe do some extra external call not related to Gemente it self.

Can someone please point me towards some nice resources for understanding and using nested classes? I have some material like Programming Principles and things like this <a href="https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.1.0/com.ibm.zos.v2r1.cbclx01/cplr061.htm" rel="noreferrer">IBM Knowledge Center - Nested Classes</a>

But I'm still having trouble understanding their purpose. Could someone please help me?

Why would one use nested classes in C++?

有没有人可以告诉我一些很好的资源来理解和使用嵌套类？我有一些像编程原理之类的资料，还有像这样的但是我仍然不能理解他们的目的。谁能帮帮我？

问为什么要在C++中使用嵌套类？
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