这是个有点奇怪的问题。我的目标是理解语言设计决策,并确定在c++中反射的可能性。

为什么c++语言委员会没有在语言中实现反射?在不运行在虚拟机上的语言(如java)中反射是否太困难了? 如果要在c++中实现反射,会遇到什么挑战?

我想反射的用途是众所周知的:编辑器可以更容易地编写,程序代码将更小,可以为单元测试生成模拟等等。但是如果你能评论一下反射的用法就太好了。


当前回答

这基本上是因为它是一个“可选的额外项目”。许多人选择c++而不是Java和c#等语言,这样他们可以更好地控制编译器的输出,例如,一个更小和/或更快的程序。

如果您选择添加反射,有各种可用的解决方案。

其他回答

反射可以在c++中实现,并且已经在c++中实现过。

它不是原生的c++特性,因为它有一个沉重的成本(内存和速度),不应该由语言默认设置——语言是“默认最大性能”导向的。

因为你不应该为你不需要的东西付钱,而且正如你自己所说,编辑器比其他应用程序更需要它,那么它应该只在你需要的地方实现,而不是“强制”到所有的代码中(你不需要对你将在编辑器或其他类似应用程序中使用的所有数据进行反思)。

Reflection requires some metadata about types to be stored somewhere that can be queried. Since C++ compiles to native machine code and undergoes heavy changes due to optimization, high level view of the application is pretty much lost in the process of compilation, consequently, it won't be possible to query them at run time. Java and .NET use a very high level representation in the binary code for virtual machines making this level of reflection possible. In some C++ implementations, however, there is something called Run Time Type Information (RTTI) which can be considered a stripped down version of reflection.

The reason C++ doesn't have reflection is that this would require the compilers to add symbol information to the object files, like what members a class type has, information about the members, about the functions and everything. This essentially would render include files useless, as information shipped by declarations would then be read from those object files (modules then). In C++, a type definition can occur multiple times in a program by including the respective headers (provided that all those definitions are the same), so it would have to be decided where to put the information about that type, just as to name one complication here. The aggressive optimization done by a C++ compiler, which can optimize out dozens of class template instantiations, is another strong point. It's possible, but as C++ is compatible to C, this would become an awkward combination.

c++是一种不需要反射的语言,因为c++是一种可以用来编写具有反射的语言的语言。

根据Alistair Cockburn的说法,在反射环境中不能保证子类型。

反射与潜在类型系统更相关。在c++中,你知道你得到了什么类型,你知道你可以用它做什么。