问题描述
使用嵌套的公共 C++ 类和枚举的优缺点是什么?例如，假设您有一个名为 printer 的类，并且该类还存储有关输出托盘的信息，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举/类的好处，但是当涉及到公共枚举/类时，办公室是分开的 - 这似乎更像是一种风格选择.
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，你更喜欢哪个，为什么?
So, which do you prefer and why?
推荐答案
嵌套类
嵌套在类中的类有几个副作用，我通常认为这些是缺陷(如果不是纯粹的反模式).
Nested classes
There are several side effects to classes nested inside classes that I usually consider flaws (if not pure antipatterns).
让我们想象以下代码:
class A
{
public :
class B { /* etc. */ } ;
// etc.
} ;
甚至:
class A
{
public :
class B ;
// etc.
} ;
class A::B
{
public :
// etc.
} ;
所以:
- 特权访问: A::B 对 A 的所有成员(方法、变量、符号等)具有特权访问权限，这削弱了封装性
- A 的范围是符号查找的候选: 来自 B 内部的代码将看到来自 A 的 所有 符号作为符号查找的可能候选，这可能会混淆代码莉>
- forward-declaration: 没有完整的 A 声明就无法前向声明 A::B
- 可扩展性:除非您是 A 的所有者，否则不可能添加另一个类 A::C
- 代码冗长: 将类放入类只会使标头变大.您仍然可以将其分成多个声明，但无法使用类似名称空间的别名、导入或使用.
- 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.
作为结论，除非有异常(例如嵌套类是嵌套类的一个亲密部分......即使如此......)，我认为在普通代码中嵌套类没有意义，因为缺陷的重要性超过了数量级感知到的优势.
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，它闻起来像是在不使用 C++ 命名空间的情况下模拟命名空间的笨拙尝试.
Furthermore, it smells as a clumsy attempt to simulate namespacing without using C++ namespaces.
在专业方面，您隔离此代码，如果是私有的，则使其无法使用但从外部"隔离类...
On the pro-side, you isolate this code, and if private, make it unusable but from the "outside" class...
优点:一切.
缺点:没什么.
事实是枚举项会污染全局范围:
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?
通过将每个枚举放在不同的命名空间/类中可以避免这种冲突:
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 定义了类枚举:
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 ;
正是针对这类问题.
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