c# data matrix code Figure 2-3. Objects and values in Visual C#.NET

Generating DataMatrix in Visual C#.NET Figure 2-3. Objects and values

Figure 2-3. Objects and values
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As just discussed, a value type can be instantiated in two different ways: as a value or as an object. The variant instantiated on the managed heap is often called a boxed object. The object acts only as a box for the real value. Like many other .NET languages, C++/CLI supports an implicit conversion from a value V to a tracking handle of type V^. int i = 5; int^ i2 = i;
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CHAPTER 2 MANAGED TYPES, INSTANCES, AND MEMORY
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Since System::Object is the lowest common denominator of all possible instances, a conversion to System::Object^ exists, too. int i = 5; System::Object^ o = i; Conversions like the two mentioned here look quite strange at first sight. What should the result of this cast be Should the value 5 be treated as the pointer to the object This would obviously end up in an invalid pointer, because no managed object will ever end up at the address 0x00000005. Should the address of the value be returned In this case, 5 would be regarded as a part of the object header. The 4 bytes prior to that would be the first part of the object header, and the 4 bytes after that would be considered the actual value. Both solutions would soon end up in chaos. Instead of these nonsense approaches, a new instance of a boxed object is created on the managed heap by the cast operation. Since the new object is a boxed object, this is described as boxing. The compiler automatically emits code that performs a boxing operation whenever a conversion from a value to a reference type occurs. You should be aware that every implicit or explicit cast from a value to a reference type causes a boxing operation. In the following code, two boxing operations occur: // boxing1.cpp // compileWith cl /clr boxing1.cpp using namespace System; int main() { int i = 5; Console::WriteLine(Object::ReferenceEquals(i, i)); } ReferenceEquals is a static helper method of System::Object that expects two arguments of type System::Object^, and returns true if both tracking handles refer to the same object. Although the same value is passed for these two arguments, two boxed objects are created here. Since these are two different objects, ReferenceEquals will return false. Boxing often occurs silently under the hood. In the following code, 1,000,000 boxed objects of the value type System::DateTime from the FCL are created: // boxing2.cpp // compileWith cl /clr boxing2.cpp using namespace System; void f(DateTime^ dt) { Console::WriteLine(dt->ToLongDateString()); }
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int main() { DateTime dt = DateTime::Now; for (int i = 0; i < 1000000; ++i) f(dt); } Every iteration of the for loop calls f passing the dt value. Since f expects an argument of type DateTime^ a tracking handle to DateTime the dt value is boxed. This can be a significant overhead for the memory allocations as well as for the GC. To avoid this overhead, the object can be boxed before the loop starts: DateTime^ dt = DateTime::Now; for (int i = 0; i < 1000000; ++i) f(dt); However, this approach can have side effects. If the f is implemented so that it calls a function on its argument that modifies the state of the boxed value, the same object will be modified by all iterations. If a value is passed instead of a boxed object, every iteration will create a new boxed object. This new boxed object will be modified and the value passed to the function will remain unchanged. To avoid these problems, System::DateTime does not have functions that modify the state of an object. Functions like DateTime::AddDays return a new value with the modified state instead of returning an existing one. Many other value types from the FCL, and other libraries, however, do have functions that can be used to modify the value. It is important to understand that a boxed object and the value used to create it are independent instances.
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