c# data matrix barcode OBJECT S EMAN TICS IN C+ +/CLI in Visual C#

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CHAPTER 4 OBJECT S EMAN TICS IN C+ +/CLI
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The compiler output of Listing 4-9 is as shown here: string_array_stack_semantics.cpp string_array_stack_semantics.cpp(7) cannot use this type here without a string_array_stack_semantics.cpp(8) cannot use this type here without a with [ Type=int ]
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: error C3149: 'System::String' : top-level '^' : error C3149: 'cli::array<Type>' : top-level '^'
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There is a risk of misusing these semantics, especially if you use the % operator to get the underlying handle to your stack semantics variable. You must be careful that there are no handles to the stack object that are retained after the function terminates. If you do retain a handle to the object and then try to access the object, you may silently access a destroyed object. The same dangers exist in calling delete on managed objects. You should try to use delete only when you can be sure that there are no others holding handles to the object you are deleting.
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The Unary % Operator and Tracking References
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Suppose you d like to use stack semantics, but you still have a function that takes a handle type. Let s say we have to call a method Report in the PlantQuery function, and that method takes a handle to the DBConnection object. Now that we re using stack semantics, we don t have a handle type, we have a bare object. Listing 4-10 is the function we d like to call. Listing 4-10. A Method Requiring a Handle void Report(DBConnection^ connection) { // Log information about this connection. // ... } In order to call this method, you need to pass a handle, not the instance variable, as the connection parameter. You ll have to use the unary % operator to convert the instance variable to a handle, for example, to pass the variable to a function that takes a handle (see Listing 4-11). The % operator is like the address-of operator for managed types that returns a handle to the object, just as the address-of operator (&) in classic C++ returns a pointer to the object. The address-of operator (&) is used for primitive types, such as int, although you can still assign to a tracking reference. The % operator is used instead of the address-of operator for instances of reference and value types.
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CHAPTER 4 OBJECT SE MAN TICS IN C++ /CLI
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Listing 4-11. Using the % Operator public ref class PlantData { public: static Recordset^ PlantQuery(String^ search) { DBConnection connection; Report(%connection); return connection.Query( search); } }; You can certainly see that the % operator is the managed analog of the & operator for native types. The analogy extends also to the other use of the & symbol to represent a reference. Rather like a tracking handle, you can use % to declare a tracking reference. Like a handle, a tracking reference is updated whenever the garbage collector moves the object it is referencing. Tracking references are somewhat more limited in use than native references. They can be used in function arguments and declared on the stack, but they cannot be declared as a member of a class. They can be used to refer to handles, value types, or value type members, but they cannot be used to refer to objects of reference type directly (as opposed to through a handle). The declaration and assignment to a variable might look like this: int i = 110; int% iref = i; R r; R% r_ref = r; Just like a classic C++ reference, the tracking reference is another reference to the existing object, so if you change the value of the object through the reference and access the object through another means (such as the variable i itself in the foregoing example), the value is changed. There is still only one value. Figure 4-1 shows what s happening in memory.
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