C ++类引用 [英] C++ Class References

问题描述

来自Delphi，我习惯使用类引用（元类）如下：

  type 
 TClass = TForm类; 
 var 
 x：TClass; 
 f：TForm; 
 begin 
 x：= TForm; 
 f：= x.Create（）; 
 f.ShowModal（）; 
 f.Free; 
 end; 
 

实际上，从TObject派生的每个X类都有一个名为ClassType的方法，返回一个可以使用的TClass创建X的实例。

在C ++中有什么类似的东西吗？

解决方案

很显然，现代的Delphi支持元类，与原来的Smalltalk非常相似。

在C ++中没有类似的东西。 p>

在C ++中模拟该特性的一个主要问题是，运行时动态分配表示类型的值，并能够从这些值创建实例，在C ++中需要静态地知道一个类型的构造函数以实例化。

可能你可以通过使用C ++实现很多相同的高级目标 static polymorphism ，其中包括函数重载和模板机制，而不是元类的极端运行时多态性。

C ++的效果是使用可克隆的范例对象和/或几乎相同的想法多态对象工厂对象。前者是相当不寻常的，后者可以遇到现在和然后（主要是差异是参数化发生的地方：与examplar对象它的对象的状态，而对象工厂它的参数的创建函数）。个人而言，我会保持远离，因为C ++是为静态类型设计的，这个想法是关于C ++模拟一种具有非常不同的特性和编程风格的语言等。

Coming from Delphi, I'm used to using class references (metaclasses) like this:

type
  TClass = class of TForm;
var
  x: TClass;
  f: TForm;
begin
  x := TForm;
  f := x.Create();
  f.ShowModal();
  f.Free;
end;

Actually, every class X derived from TObject have a method called ClassType that returns a TClass that can be used to create instances of X.

Is there anything like that in C++?

解决方案

Apparently modern Delphi supports metaclasses in much the same way as original Smalltalk.

There is nothing like that in C++.

One main problem with emulating that feature in C++, having run-time dynamic assignment of values that represent type, and being able to create instances from such values, is that in C++ it's necessary to statically know the constructors of a type in order to instantiate.

Probably you can achieve much of the same high-level goal by using C++ static polymorphism, which includes function overloading and the template mechanism, instead of extreme runtime polymorphism with metaclasses.

However, one way to emulate the effect with C++, is to use cloneable exemplar-objects, and/or almost the same idea, polymorphic object factory objects. The former is quite unusual, the latter can be encountered now and then (mostly the difference is where the parameterization occurs: with the examplar-object it's that object's state, while with the object factory it's arguments to the creation function). Personally I would stay away from that, because C++ is designed for static typing, and this idea is about cajoling C++ into emulating a language with very different characteristics and programming style etc.

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