浅谈C++/CLI中引用类型的栈对象
在工作中使用C++/CLI,发现其中一些比较有意思的地方,在这就拿C++/CLI中引用类型的栈对象来跟大家一起学习一下,有写得不对的地方望大家多加指点。这也是本人第一篇文章,申请空间已有多半年了,惭愧.......
对于有ISO C++使用背景的技术人员来说,栈对象再也熟悉不过了,它有一个最大特点是确定性资源清理,过了对象的作用返回就会自动释放其所占用的内存。C++/CLI是.NET平台下的C++语言,当然ISO C++的诸多语法保留下来,栈对象便是其一,但是他的语义发生了变化。
大家都知道.NET平台与win32平台最大的差别是托管内存,对C#了解的同志都知道,在C#里面分有值类型(value type)与引用类型(refrence type),值类型对象均分配在栈中,引用类型的都会分配在托管堆中,换句话说就是对象的类型决定了其分配的位置,栈还是托管堆。栈内存的回收还是与以往一样,是确定性的,托管堆内存的回收是由垃圾回收器来负责。然而在这样的内存模型中,MyClass myClass;这个语句在 C++/CLI中的语义会是什么呢?
众所周知,高级语言得以出现,主要归功于编译器,语义会是怎么样当然也取决于编译器的编译结果。下面讲通过一些简单的代码进行讲解,请留意代码中的注释^_^
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using namespace System;
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public ref class MyClass //自定义引用类型
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{
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public:
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MyClass() //提供无参构造函数,与ISO C++一样,在声明栈对象时如果不传入参数,便会调用无参构造函数。
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{}
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~MyClass() //这便是我们的析构器(Destructor)
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{
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m_isDisposed = true;
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}
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String^ GetString()
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{
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if(m_isDisposed)
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{
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throw gcnew ObjectDisposedException("MyClass is disposed,can not access disposed memery!");
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}
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return "MyClass";
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}
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static MyClass^ GetInstance()
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{
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MyClass result; //声明一个栈对象
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return %result;
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/*返回栈对象的追踪句柄(tracking handle),对ISO C++比较熟悉的
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肯定比我都清楚,在使用该函数的返回值会访问非法内存,因为栈对象被销毁,内存已被回收。然而在C++/CLI并
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不会这样,使用的情况取决于MyClass类型的实现,如果MyClass 实现了Destructor,则编译器会在函数体内给我们加上一些
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代码,调用myClass对象的Destructor,如果MyClass没有实现Destructor,此时返回值可以正常使用。一下代码为通过Reflector查看的结果,
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语言为C#
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public static MyClass GetInstance()
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{
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MyClass class2 = null;
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MyClass class4;
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MyClass modopt(IsConst) class5 = new MyClass();
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try
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{
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class2 = class5;
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class4 = class2;
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}
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fault
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{
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class2.Dispose();
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}
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class2.Dispose();
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return class4;
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}
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我们将Destrutor注释掉,再次查看编译结果,发现截然不同
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public static MyClass GetInstance()
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{
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return new MyClass();
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}
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通过编译结果我们知道没有写Destructor的对象,返回结果可以正常使用。
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当然在实际工作中我们不会去编写这样的代码,做为学习可以,了解程序运行的各种情况。
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要是这样写编译器也会给出这样的警告:
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program1.cpp(91) : warning C4172: returning address of local variable or temporary
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*/
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}
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private:
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Boolean m_isDisposed;
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};
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void main()
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{
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MyClass^ myClass = MyClass::GetInstance() ;
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System::Console::WriteLine(myClass->GetString());
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}
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