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Managed, Unmanaged, Native: What Kind of Code Is This?(转)

With the release of Visual Studio .NET 2003 (formerly known as Everett) on April 24th, many developers are now willing to consider using the new technology known as managed code. But especially for C++ developers, it can be a bit confusing. That's because C++,

What Is Managed Code?

 Managed Code is what Visual Basic .NET and C# compilers create. It compiles to Intermediate Language (IL), not to machine code that could run directly on your computer. The IL is kept in a file called an assembly, along with metadata that describes the classes, methods, and attributes (such as security requirements) of the code you've created. This assembly is the one-stop-shopping unit of deployment in the .NET world. You copy it to another server to deploy the assembly there—and often that copying is the only step required in the deployment.

Managed code runs in the Common Language Runtime. The runtime offers a wide variety of services to your running code. In the usual course of events, it first loads and verifies the assembly to make sure the IL is okay. Then, just in time, as methods are called, the runtime arranges for them to be compiled to machine code suitable for the machine the assembly is running on, and caches this machine code to be used the next time the method is called. (This is called Just In Time, or JIT compiling, or often just Jitting.)

As the assembly runs, the runtime continues to provide services such as security, memory management, threading, and the like. The application is managed by the runtime.

Visual Basic .NET and C# can produce only managed code. If you're working with those applications, you are making managed code. Visual C++ .NET can produce managed code if you like: When you create a project, select one of the application types whose name starts with .Managed., such as .Managed C++ application..

What Is Unmanaged Code?

Unmanaged code is what you use to make before Visual Studio .NET 2002 was released. Visual Basic 6, Visual C++ 6, heck, even that 15-year old C compiler you may still have kicking around on your hard drive all produced unmanaged code. It compiled directly to machine code that ran on the machine where you compiled it—and on other machines as long as they had the same chip, or nearly the same. It didn't get services such as security or memory management from an invisible runtime; it got them from the operating system. And importantly, it got them from the operating system explicitly, by asking for them, usually by calling an API provided in the Windows SDK. More recent unmanaged applications got operating system services through COM calls.

Unlike the other Microsoft languages in Visual Studio, Visual C++ can create unmanaged applications. When you create a project and select an application type whose name starts with MFC, ATL, or Win32, you're creating an unmanaged application.

This can lead to some confusion: When you create a .Managed C++ application., the build product is an assembly of IL with an .exe extension. When you create an MFC application, the build product is a Windows executable file of native code, also with an .exe extension. The internal layout of the two files is utterly different. You can use the Intermediate Language Disassembler, ildasm, to look inside an assembly and see the metadata and IL. Try pointing ildasm at an unmanaged exe and you'll be told it has no valid CLR (Common Language Runtime) header and can't be disassembled—Same extension, completely different files.

What about Native Code?

The phrase native code is used in two contexts. Many people use it as a synonym for unmanaged code: code built with an older tool, or deliberately chosen in Visual C++, that does not run in the runtime, but instead runs natively on the machine. This might be a complete application, or it might be a COM component or DLL that is being called from managed code using COM Interop or PInvoke, two powerful tools that make sure you can use your old code when you move to the new world. I prefer to say .unmanaged code. for this meaning, because it emphasizes that the code does not get the services of the runtime. For example, Code Access Security in managed code prevents code loaded from another server from performing certain destructive actions. If your application calls out to unmanaged code loaded from another server, you won't get that protection.

The other use of the phrase native code is to describe the output of the JIT compiler, the machine code that actually runs in the runtime. It's managed, but it's not IL, it's machine code. As a result, don't just assume that native = unmanaged.

Does Managed Code Mean Managed Data?

Again with Visual Basic and C#, life is simple because you get no choice. When you declare a class in those languages, instances of it are created on the managed heap, and the garbage collector takes care of lifetime issues. But in Visual C++, you get a choice. Even when you're creating a managed application, you decide class by class whether it's a managed type or an unmanaged type. This is an unmanaged type:

class Foo
{
private:
   int x;
public:
    Foo(): x(0){}
    Foo(int xx): x(xx) {}
};

This is a managed type:

__gc class Bar
{
private:
   int x;
public:
    Bar(): x(0){}
    Bar(int xx): x(xx) {}
};

The only difference is the __gc keyword on the definition of Bar. But it makes a huge difference.

Managed types are garbage collected. They must be created with new, never on the stack. So this line is fine:

Foo f;

But this line is not allowed:

Bar b;

If I do create an instance of Foo on the heap, I must remember to clean it up:

Foo* pf = new Foo(2);
// . . .
delete pf;

The C++ compiler actually uses two heaps, a managed an unmanaged one, and uses operator overloading on new to decide where to allocate memory when you create an instance with new.

If I create an instance of Bar on the heap, I can ignore it. The garbage collector will clean it up some after it becomes clear that no one is using it (no more pointers to it are in scope).

There are restrictions on managed types: They can't use multiple inheritance or inherit from unmanaged types, they can't allow private access with the friend keyword, and they can't implement a copy constructor, to name a few. So, you might not want your classes to be managed classes. But that doesn't mean you don't want your code to be managed code. In Visual C++, you get the choice.


---------------------------------------------------------

About the Author

Kate Gregory is a founding partner of Gregory Consulting Limited (www.gregcons.com). In January 2002, she was appointed MSDN Regional Director for Toronto, Canada. Her experience with C++ stretches back to before Visual C++ existed. She is a well-known speaker and lecturer at colleges and Microsoft events on subjects such as .NET, Visual Studio, XML, UML, C++, Java, and the Internet. Kate and her colleagues at Gregory Consulting specialize in combining software develoment with Web site development to create active sites. They build quality custom and off-the-shelf software components for Web pages and other applications. Kate is the author of numerous books for Que, including Special Edition Using Visual C++ .NET.

posted on 2005-11-21 10:47 梦在天涯 阅读(2130) 评论(11)  编辑 收藏 引用 所属分类: CPlusPlusManage c++ /CLI

评论

# re: Managed, Unmanaged, Native: What Kind of Code Is This?(转) 2006-04-19 12:19 梦在天涯

非托管

在.net 编程环境中,系统的资源分为托管资源和非托管资源。
对于托管的资源的回收工作,是不需要人工干预回收的,而且你也无法干预他们的回收,所能够做的只是了解.net CLR如何做这些操作。也就是说对于您的应用程序创建的大多数对象,可以依靠 .NET Framework 的垃圾回收器隐式地执行所有必要的内存管理任务。

对于非托管资源,您在应用程序中使用完这些非托管资源之后,必须显示的释放他们,例如System.IO.StreamReader的一个文件对象,必须显示的调用对象的Close()方法关闭它,否则会占用系统的内存和资源,而且可能会出现意想不到的错误。

我想说到这里,一定要清楚什么是托管资源,什么是非托管资源了?

最常见的一类非托管资源就是包装操作系统资源的对象,例如文件,窗口或网络连接,对于这类资源虽然垃圾回收器可以跟踪封装非托管资源的对象的生存期,但它不了解具体如何清理这些资源。还好.net Framework提供了Finalize()方法,它允许在垃圾回收器回收该类资源时,适当的清理非托管资源。如果在MSDN Library 中搜索Finalize将会发现很多类似的主题,这里列举几种常见的非托管资源:ApplicationContext,Brush,Component,ComponentDesigner,Container,Context,Cursor,FileStream,Font,Icon,Image,Matrix,Object,OdbcDataReader,OleDBDataReader
,Pen,Regex,Socket,StreamWriter,Timer,Tooltip 等等资源。可能在使用的时候很多都没有注意到!

关于托管资源,就不用说了撒,像简单的int,string,float,DateTime等等,.net中超过80%的资源都是托管资源。

非托管资源如何释放,.NET Framework 提供 Object.Finalize 方法,它允许对象在垃圾回收器回收该对象使用的内存时适当清理其非托管资源。默认情况下,Finalize 方法不执行任何操作。默认情况下,Finalize 方法不执行任何操作。如果您要让垃圾回收器在回收对象的内存之前对对象执行清理操作,您必须在类中重写 Finalize 方法。然而大家都可以发现在实际的编程中根本无法override方法Finalize(),在C#中,可以通过析构函数自动生成 Finalize 方法和对基类的 Finalize 方法的调用。

例如:

~MyClass()

{

// Perform some cleanup operations here.

}

该代码隐式翻译为下面的代码。

protected override void Finalize()

{

try

{

// Perform some cleanup operations here.

}

finally

{

base.Finalize();

}

}

但是,在编程中,并不建议进行override方法Finalize(),因为,实现 Finalize 方法或析构函数对性能可能会有负面影响。一个简单的理由如下:用 Finalize 方法回收对象使用的内存需要至少两次垃圾回收,当垃圾回收器回收时,它只回收没有终结器(Finalize方法)的不可访问的内存,这时他不能回收具有终结器(Finalize方法)的不可以访问的内存。它改为将这些对象的项从终止队列中移除并将他们放置在标记为“准备终止”的对象列表中,该列表中的项指向托管堆中准备被调用其终止代码的对象,下次垃圾回收器进行回收时,就回收并释放了这些内存。
  回复  更多评论   

# re: Managed, Unmanaged, Native: What Kind of Code Is This?(转) 2010-06-20 00:06 DollySANDERS32

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# re: Managed, Unmanaged, Native: What Kind of Code Is This?(转) 2010-06-24 17:26 dissertation writing

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