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Unambiguous Type Coupling
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Assume a type T1 has a reference to a type T2. If T1 uses the reference to create instances of T2, then T1 must have access to T2 at run time in order to invoke the constructor for T2. The form of type coupling that involves type instantiation is called unambiguous type coupling (UTC). No type substitutions for T2 (such as types derived from T2) are allowed. Unambiguous type coupling is designated by the symbol , subscripted with the letters Tu . There are two common situations in which UTC occurs: between unrelated classes and between related classes. You can consider two classes to be related if one derives directly or indirectly from the other. Figure 1-12 shows an example of UTC between unrelated classes.
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Figure 1-12. Unambiguous type coupling between unrelated classes T1 and T2 In this example, T1 is not related to T2, meaning T1 is not derived in any way from T2. If T1 creates instances of T2 using a constructor call, UTC is introduced. In OO languages like C# and Java, constructors are called using the new operator like this: T2 t2 = new T2(); The new operator implicitly invokes the constructor for T2. At run time, the executable code for T2 has to be available, because it contains, among other things, the constructor code. You can avoid UTC by having T1 use reflection to instantiate T2, but then T1 must identify the T2 class name T2 . If the name is embedded as a literal in T1 s code, then T1 incurs logic coupling to T2. If T1 and T2 are related through inheritance, T1 implicitly calls the constructor for T2 when T1 is being instantiated. Assume T1 is derived from T2, as shown in Figure 1-13. In order to create an instance of T1 at run time, you must build an instance of T2 first. In languages like C# and Java, the constructor code for T1 contains a call to the constructor for the base class T2. This call is made to the unambiguous type T2, and no substitutes (e.g., derived classes) are acceptable.
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CHAPTER 1 COUPLING
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Figure 1-13. Unambiguous type coupling between T1 and T2, due to inheritance
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K-coupling
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K-coupling occurs between two classes Class1 and Class2 when Class2 defines some constant K that Class1 uses explicitly. To show K-coupling on diagrams, I ll use the symbol subscripted with the letter K. Figure 1-14 shows an example.
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Class1 -Class2.MyConstant
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Class2 -MyConstant : int
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Figure 1-14. K-coupling between Class1 and Class2 Listing 1-5 shows a sample C# implementation of Class1 and Class2. Listing 1-5. An Example of Classes That Are K-coupled public class Class1 { int i; public Class1() { i = Class2.MyConstant; } } public class Class2 { public const int MyConstant = 5; } K-coupling is similar to UTC, in that an explicit class name is called out, but now the name of a constant also appears. K-coupling is more benign than unambiguous type coupling, because the compiler can often remove the former but not the latter. To see how, consider what a compiler might do when compiling Class1. When the symbol Class2.MyConstant is found, the compiler needs to have access to Class2 to determine the constant s value. The compiler might then embed the value of MyConstant in the executable code of Class1. At run time, Class1 no longer needs access to Class2, since Class1 already knows the value of Class2.MyConstant.
CHAPTER 1 COUPLING
There are a few caveats with K-coupling. First, the constant appearing in Class1 must be used by value, as shown in the previous listing. In some languages, it is possible to access constants also by reference (i.e., using a pointer), in which case Class1 won t know the constant s value until the pointer is initialized at run time. If Class1 uses a pointer to get the constant s value, then the coupling between Class1 and Class2 is not considered K-coupling. What type of coupling would it be The answer depends on the constant s type. If the type is built in, such as integer or Boolean, then Class1 has no compile-time coupling to Class2. If the constant is of a user-defined type, then Class1 is typecoupled to that user-defined type. Listing 1-6 shows a C++ example in which Class1 uses a pointer to a constant integer contained in Class2. Listing 1-6. A C++ Example Using a Pointer in Class1 to Reference a Constant in Class2 class Class1 { public: const int* myValue; Class1() {} }; class Class2 { public: const int myValue; Class2() : myValue(5) {} }; class MyClass { public: MyClass() { Class1* c1 = new Class1(); Class2* c2 = new Class2(); c1->myValue = &c2->myValue; } }; Class1 doesn t know anything about Class2, so you can compile and run it without the presence of Class2. You must have Class2 present to compile MyClass. As long as Class1.myValue is initialized to point to an integer value somewhere in the system, Class1 works. Of course, in order for Class1 to work correctly, the pointer needs to point not just to any integer value, but to the right value, which is dependent on the system requirements. As mentioned, K-coupling between T1 and T2 doesn t always occur when T1 references a typed constant defined in T2. K-coupling occurs only when certain types are involved. Exactly which types produce K-coupling depends on the programming language and the compiler. Referencing constants of built-in scalar types like Boolean, integer, float, and char generally causes K-coupling. In C# and VB .NET, all value types, which include structures, cause K-coupling. Whether strings qualify for K-coupling depends on how the compiler treats constant strings. Consider this C# example. Assume Class2 defines a constant string like this: public class Class2 { public const string MyString = "whatever"; }
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