問題描述

使用嵌套的公共 C++ 類和枚舉的優(yōu)缺點是什么?例如，假設(shè)您有一個名為 printer 的類，并且該類還存儲有關(guān)輸出托盤的信息，您可以:

What are the pros and cons of using nested public C++ classes and enumerations? For example, suppose you have a class called printer, and this class also stores information on output trays, you could have:

class printer
{
public:
    std::string name_;

    enum TYPE
    {
        TYPE_LOCAL,
        TYPE_NETWORK,
    };

    class output_tray
    {
        ...
    };
    ...
};

printer prn;
printer::TYPE type;
printer::output_tray tray;

或者:

class printer
{
public:
    std::string name_;
    ...
};

enum PRINTER_TYPE
{
    PRINTER_TYPE_LOCAL,
    PRINTER_TYPE_NETWORK,
};

class output_tray
{
    ...
};

printer prn;
PRINTER_TYPE type;
output_tray tray;

我可以看到嵌套私有枚舉/類的好處，但是當(dāng)涉及到公共枚舉/類時，辦公室是分開的 - 這似乎更像是一種風(fēng)格選擇.

I can see the benefits of nesting private enums/classes, but when it comes to public ones, the office is split - it seems to be more of a style choice.

那么，你更喜歡哪個，為什么?

So, which do you prefer and why?

推薦答案

嵌套類

嵌套在類中的類有幾個副作用，我通常認為這些是缺陷(如果不是純粹的反模式).

Nested classes

There are several side effects to classes nested inside classes that I usually consider flaws (if not pure antipatterns).

讓我們想象以下代碼:

class A
{
   public :
      class B { /* etc. */ } ;

   // etc.
} ;

甚至:

class A
{
   public :
      class B ;

   // etc.
} ;

class A::B
{
   public :

   // etc.
} ;

所以:

• 特權(quán)訪問: A::B 對 A 的所有成員(方法、變量、符號等)具有特權(quán)訪問權(quán)限，這削弱了封裝性
• A 的范圍是符號查找的候選: 來自 B 內(nèi)部的代碼將看到來自 A 的 所有 符號作為符號查找的可能候選，這可能會混淆代碼莉>
• forward-declaration: 沒有完整的 A 聲明就無法前向聲明 A::B
• 可擴展性:除非您是 A 的所有者，否則不可能添加另一個類 A::C
• 代碼冗長: 將類放入類只會使標(biāo)頭變大.您仍然可以將其分成多個聲明，但無法使用類似名稱空間的別名、導(dǎo)入或使用.

• Privilegied Access: A::B has privilegied access to all members of A (methods, variables, symbols, etc.), which weakens encapsulation
• A's scope is candidate for symbol lookup: code from inside B will see all symbols from A as possible candidates for a symbol lookup, which can confuse the code
• forward-declaration: There is no way to forward-declare A::B without giving a full declaration of A
• Extensibility: It is impossible to add another class A::C unless you are owner of A
• Code verbosity: putting classes into classes only makes headers larger. You can still separate this into multiple declarations, but there's no way to use namespace-like aliases, imports or usings.

作為結(jié)論，除非有異常(例如嵌套類是嵌套類的一個親密部分......即使如此......)，我認為在普通代碼中嵌套類沒有意義，因為缺陷的重要性超過了數(shù)量級感知到的優(yōu)勢.

As a conclusion, unless exceptions (e.g. the nested class is an intimate part of the nesting class... And even then...), I see no point in nested classes in normal code, as the flaws outweights by magnitudes the perceived advantages.

此外，它聞起來像是在不使用 C++ 命名空間的情況下模擬命名空間的笨拙嘗試.

Furthermore, it smells as a clumsy attempt to simulate namespacing without using C++ namespaces.

在專業(yè)方面，您隔離此代碼，如果是私有的，則使其無法使用但從外部"隔離類...

On the pro-side, you isolate this code, and if private, make it unusable but from the "outside" class...

優(yōu)點:一切.

缺點:沒什么.

事實是枚舉項會污染全局范圍:

The fact is enum items will pollute the global scope:

// collision
enum Value { empty = 7, undefined, defined } ;
enum Glass { empty = 42, half, full } ;

// empty is from Value or Glass?

通過將每個枚舉放在不同的命名空間/類中可以避免這種沖突:

Ony by putting each enum in a different namespace/class will enable you to avoid this collision:

namespace Value { enum type { empty = 7, undefined, defined } ; }
namespace Glass { enum type { empty = 42, half, full } ; }

// Value::type e = Value::empty ;
// Glass::type f = Glass::empty ;

注意 C++0x 定義了類枚舉:

Note that C++0x defined the class enum:

enum class Value { empty, undefined, defined } ;
enum class Glass { empty, half, full } ;

// Value e = Value::empty ;
// Glass f = Glass::empty ;

正是針對這類問題.

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