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Similarly, the intention of the programmer is that the derived class instance of foo(), if invoked without an argument, should be passed a default argument of 2048 For example:
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derived d; base *pb = &d; // derived::foo( int ) is invoked -// intention is that it should return 2048 pb->foo();
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As it happens, this is not the semantic behavior of the C++ virtual mechanism For example, here is a small program exercising our class hierarchy:
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int main() { derived *pd = new derived; base *pb = pd; int val = pb->foo(); cout "main() : val through base: " val endl; val = pd->foo(); cout "main() : val through derived: " val endl; }
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When compiled and executed, the program generates the following output:
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derived::foo() -- ival: 1024 main() : val through base: 1024 derived::foo() -- ival: 2048 main() : val through derived: 2048
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In both invocations, the derived class instance of foo() is invoked correctly This is because the actual instance of foo() invoked is determined at run-time based on the actual class type addressed by both pd and pb The default argument to be passed to foo(), however, is not determined at run-time; rather, it is determined at compile-time and is based on the type of the object through which the function is being invoked When foo() is invoked through pb, the default argument is determined by the declaration of base::foo(), which is 1024 When foo() is invoked through pd, the default argument is determined by the declaration of derived::foo(), which is 2048 If a derived class instance, when invoked through a base class pointer or reference, is passed the default argument specified by the base class, then why specify a default argument in a derived class instance We may wish to have a different default argument based not on the specific subtype implementation of foo() that is invoked, but rather on the type of the pointer or reference through which the function is invoked For example, the values 1024 and 2048 may represent image sizes If we wish to produce a less detailed image, we invoke foo() through base If we wish a finer resolution, we
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invoke foo() through derived But what if we really wish for the actual default argument passed to foo() to be based on the actual instance of the function invoked Unfortunately, the virtual mechanism does not support this directly One programming solution is to specify a default argument that can be recognized as indicating that no value has been passed by the user The intended default argument is instead declared local to the function and is used if no explicit value is passed in For example:
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void base:: foo( int ival = base_default_value ) { int real_default_value = 1024; if ( ival == base_default_value ) ival = real_default_value; // }
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where base_default_value is an agreed-upon value for the entire hierarchy that, if present, indicates the user has not provided an explicit value The derived class instance is implemented in a similar manner:
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void derived:: foo( int ival = base_default_value ) { int real_default_value = 2048; if ( ival == base_default_value ) ival = real_default_value; // }
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Virtual Destructors In the following function we apply the delete expression as follows:
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void doit_and_bedone ( vector< Query* > *pvec ) { // for ( ; it != end_it; ++it ) { Query *pq = *it; // delete pq; } }
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In order for the function to execute correctly, the destructor of the dynamic type addressed by pq must be invoked when the delete
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expression is applied In order for that to happen we must declare the Query class destructor as virtual:
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class Query { public: virtual ~Query() { delete _solution; } // };
The destructor of each subsequently derived class is now treated automatically as virtual doit_and_bedone() executes correctly The behavior of the destructor under inheritance is as follows: The destructor of the derived class is invoked first In the case of pq, it is a virtual function call On completion, the destructor of the immediate base class is invoked statically inline expanded if declared as such For example, if pq addresses an AndQuery object,
delete pq;
invokes the AndQuery destructor through the virtual mechanism That done, the BinaryQuery destructor is invoked statically Following that, the Query destructor is also statically invoked Given the following class hierarchy
class Query { public: // protected: virtual ~Query(); // }; class NotQuery : public Query { public: ~NotQuery(); // };
the access level of the NotQuery destructor is public when invoked through a NotQuery object, but is protected when invoked through a Query pointer or reference That is, the virtual function assumes the access level of the class type through which it is being invoked Thus:y
int main() { Query *pq = new NotQuery; // illegal: destructor is protected delete pq; }
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