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Since a native pointer is not sufficient to refer to a location on the GC heap, another kind of variable is introduced by C++/CLI. It is called a tracking handle, because the GC keeps track of variables of this kind. Instead of an asterisk, a caret (^) is used to define a tracking handle: int^ i = gcnew int(0); In the same way, a handle to the String object can be stored in a local variable: System::String^ str = gcnew System::String(L'a', 10); A tracking handle either refers to an object on the GC heap, or is a variable referring to nothing. To avoid confusions with the value 0, the keyword nullptr has been introduced for this null value.
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CHAPTER 2 MANAGED TYPES, INSTANCES, AND MEMORY
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System::String^ str = nullptr; The keyword nullptr can also be used to check if a tracking handle refers to an object or not: bool bRefersToAnObject = (str != nullptr); As an alternative, you can use this construct: bool bRefersToAnObject = !str; There are significant differences between native pointers and tracking handles. A tracking handle can only be used as a simple handle to an object for example, to call a method of an object. Its binary value must not be used by your code. You cannot perform pointer arithmetic on a tracking handle. Allowing pointer arithmetic would imply that a programmer can control internals of the GC for example, the order in which objects are allocated on the GC heap. Even if a thread creates two objects in two continuous operations, a different thread can create an object that is allocated between the two objects. A garbage collection can also be implemented so that the order of the objects can change during a garbage collection, or the memory for new objects can be allocated so that newer objects are allocated at lower addresses. All this is outside of the programmer s control. Although the concept of tracking handles differs from the concept of native pointers, there are a lot of similarities. As with native pointers, the size of a tracking handle is platform dependent. On a 32-bit CLR, a tracking handle s size is 32 bits; on a 64-bit CLR, it is 64 bits. In fact, both native pointers and tracking handles store addresses in the virtual memory of the process. In the current implementation of the CLR (version 2.0), a tracking handle points to the middle of an object s header. Figure 2-2 shows that an object header is 8 bytes long: 4 bytes for a type identifier, 4 bytes for flags, and some other object-specific data that the CLR uses to provide different services. The actual member variables of the object follow the object header.
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Figure 2-2. A tracking handle referring to a managed object The type identifier shown in Figure 2-2 can be compared to the vtable pointer of a C++ object. However, it is used not only for method dispatching and dynamic casting, but also for certain other services of the runtime environment. For this chapter, it is sufficient to know that an object has an 8-byte object header followed by its fields, as shown in Figure 2-2.
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CHAPTER 2 MANAGED TYPES, INSTANCES, AND MEMORY
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Since a tracking handle has certain similarities to a native pointer, C++/CLI uses the arrow operator, ->, to access an object via a tracking reference: System::String^ strUpper = str->ToUpper();
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There are different object-oriented systems with different definitions of an object. Early object-oriented systems like Smalltalk consider everything to be an object. In such a world, all local variables refer to objects on the heap, like tracking handles in C++/CLI. In contrast to Smalltalk, .NET does not have a purely object-oriented type system. The CLI differentiates between objects and values. Objects can be defined as instances created on .NET s managed heap. As discussed previously, objects always have an 8-byte object header, and they are always accessed in a referenced way. On the one hand, the object header is required by many runtime services. Garbage collection is the most obvious one. Virtual method dispatching is another straightforward service based on the object header. But there are other services, too, which will be discussed in later chapters. On the other hand, an explicit allocation operation per object, plus the 8-byte object header, plus at least 4 bytes for a tracking handle to the object, plus the costs of accessing the object s state indirectly is an overhead that you will not want for every instance. Consider an instance that is supposed to simply act as an iterator variable of a loop: // intAsAValue.cpp // compile with "cl /clr intAsAValue.cpp" int main() { for (int i = 0; i < 10; ++i) System::Console::WriteLine(i); } If the CTS supported only objects on the GC heap and variables referring to these objects, the index variable would have to be instantiated on the GC heap and every access would be done with an extra level of indirection. The following code shows the overhead: // intAsAnObject.cpp // compile with "cl /clr intAsAnObject.cpp" int main() { for (int^ i = gcnew int(0); *i < 10; ++(*i)) System::Console::WriteLine(*i); } This code compiles and executes as expected, but the runtime provides many more services for the variable i than actually needed. For example, there is no need to decouple i s
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