Inheritance in .NET framework

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While Standard C++ still supports access declarations, they are deprecated This means that they are allowed for now, but they might not be supported in the future Instead, the standard suggests achieving the same effect by applying the using keyword
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An element of ambiguity can be introduced into a C++ program when multiple base classes are inherited For example, consider this incorrect program:
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// This program contains an error and will not compile #include <iostream> using namespace std; class base { public: int i; }; // derived1 inherits base class derived1 : public base { public: int j; }; // derived2 inherits base class derived2 : public base { public: int k; }; /* derived3 inherits both derived1 and derived2 This means that there are two copies of base in derived3! */ class derived3 : public derived1, public derived2 { public: int sum; }; int main() {
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derived3 ob; obi = 10; // this is ambiguous, which i obj = 20; obk = 30; // i ambiguous here, too obsum = obi + obj + obk; // also ambiguous, which i cout << obi << " "; cout << obj << " " << obk << " "; cout << obsum; return 0; }
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As the comments in the program indicate, both derived1 and derived2 inherit base However, derived3 inherits both derived1 and derived2 This means that there are two copies of base present in an object of type derived3 Therefore, in an expression like
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obi = 10;
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which i is being referred to, the one in derived1 or the one in derived2 Because there are two copies of base present in object ob, there are two obis! As you can see, the statement is inherently ambiguous There are two ways to remedy the preceding program The first is to apply the scope resolution operator to i and manually select one i For example, this version of the program does compile and run as expected:
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// This program uses explicit scope resolution to select i #include <iostream> using namespace std; class base { public: int i; }; // derived1 inherits base
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Inheritance
class derived1 : public: int j; };
public base {
// derived2 inherits base class derived2 : public base { public: int k; }; /* derived3 inherits both derived1 and derived2 This means that there are two copies of base in derived3! */ class derived3 : public derived1, public derived2 { public: int sum; }; int main() { derived3 ob; obderived1::i = 10; // scope resolved, use derived1's i obj = 20; obk = 30; // scope resolved obsum = obderived1::i + obj + obk; // also resolved here cout << obderived1::i << " "; cout << obj << " " << obk << " "; cout << obsum; return 0; } C++
As you can see, because the :: was applied, the program has manually selected derived1's version of base However, this solution raises a deeper issue: What if only one copy of base is actually required Is there some way to prevent two copies from
C++: The Complete Reference
being included in derived3 The answer, as you probably have guessed, is yes This solution is achieved using virtual base classes When two or more objects are derived from a common base class, you can prevent multiple copies of the base class from being present in an object derived from those objects by declaring the base class as virtual when it is inherited You accomplish this by preceding the base class' name with the keyword virtual when it is inherited For example, here is another version of the example program in which derived3 contains only one copy of base:
// This program uses virtual base classes #include <iostream> using namespace std; class base { public: int i; }; // derived1 inherits base as virtual class derived1 : virtual public base { public: int j; }; // derived2 inherits base as virtual class derived2 : virtual public base { public: int k; }; /* derived3 inherits both derived1 and derived2 This time, there is only one copy of base class */ class derived3 : public derived1, public derived2 { public: int sum; }; int main() { derived3 ob; obi = 10; // now unambiguous
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