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CHAPTER 6 CLASSES AND STRUCTS
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constructors, a default constructor is created implicitly, just as in classic C++. This constructor does not actually do any real work; the CLR automatically zeroes out any managed object upon creation without an actual constructor call.
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A static constructor or class constructor is a static method in a class that is called prior to when the class is first accessed. A static constructor handles any class-level initialization. In classic C++, if you want code to run when a class is first loaded, for example, when an application starts up, you would probably define a class with a constructor and make that class a static member of another class. The static initialization for the enclosing class will invoke the constructor of the member, as in Listing 6-1. Listing 6-1. Using a Static Initialization // startup_code.cpp #include <stdio.h> class Startup { public: Startup() { // Initialize. printf("Initializing module.\n"); } }; class N { static Startup startup; N() { // Make use of pre-initialized state. } }; Alternatively, you might have a static counter variable that is initialized to zero, and have code in the class constructor that checks the counter to see whether this class has ever been used before. You need to be careful about thread safety in such a function, taking care to ensure that the counter is only modified by atomic operations or locking the entire function. You could then choose to run some initialization code only when the first instance is created. C++/CLI provides language support for this common design pattern in the form of static constructors, as demonstrated in Listing 6-2.
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CHAPTER 6 CLASSES AND STRUCTS
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Listing 6-2. Using a Static Constructor // static_constructor.cpp using namespace System; ref class C { private: static String^ data; static C() { Console::WriteLine("C static constructor called."); data = "Initialized"; } public: C() { Console::WriteLine("C Constructor called."); Console::WriteLine(data); } }; int main() { Console::WriteLine("main method"); C c1; C^ c2 = gcnew C(); } Here is the output for Listing 6-2: C static constructor called. main method C Constructor called. Initialized C Constructor called. Initialized The static constructor should be private and cannot take any arguments, since it is called by the runtime and cannot be called by user code. You cannot define a static destructor; there is no such animal. This makes sense because there is no time in a program when a type is no longer available when it would make sense to call a default destructor.
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Unlike native types, reference types do not automatically get a copy constructor and an assignment operator. They may be created explicitly if required. These functions don t always make sense for reference types, which normally don t represent a value that can be copied or assigned. Value types can be copied and assigned automatically. They behave as if they have copy constructors and assignment operators that copy their values.
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In managed classes, const fields are not seen as constant when invoked using the #using directive. You can initialize constant values that will be seen as constants even when invoked in that way by declaring them with the literal modifier. The literal field so created has the same visibility rules as a static field and is a compile-time constant value that cannot be changed. It is declared as in Listing 6-3. Listing 6-3. Declaring Literals ref class Scrabble { // Literals are constants that can be initialized in the class body. literal int TILE_COUNT = 100; // the number of tiles altogether literal int TILES_IN_HAND = 7; // the number of tiles in each hand // ... }; A literal field is allowed to have an initializer right in the class declaration. The value initialized must be computable at compile time. literal is added as a modifier in the same position that static would appear, that is, after other modifiers (see Listing 6-4) but before the variable name; literal is considered a storage class specifier. Listing 6-4. Initializing a Literal // literal.cpp using namespace System; ref class C { literal String^ name = "Bob"; public:
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