acct; Account Account Account Account Account in Java

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acct; Account Account Account Account Account
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acct; acct2 = acct; acct3 = "Rena Stern"; acct4( "Anna Engel", 40000 ); acct5 = Account( acct3 );
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Exercise 145
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The parameter of the copy constructor does not strictly need to be const, but it does strictly need to be a reference Why is the following wrong
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Account::Account( const Account rhs );
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The Class Destructor
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One purpose of a constructor is to provide for the automatic acquisition of a resource We saw an example of this in our Account class constructor in which a character array is allocated through application of the new expression, and a unique account number is secured Alternatively, we might wish to set a mutual exclusion lock on an area of shared memory or a critical section of a thread What's missing is a symmetric operation that provides for the automatic deallocation or return of a resource associated with a class object about to end its lifetime A destructor is just such a special class member function It serves as the complement to the constructors of the class A destructor is a special user-defined member function that is invoked automatically whenever an object of its class goes out of scope or whenever the delete expression is applied to a pointer to a class object The destructor is given the name of the class prefixed with a tilde (~) It can neither return a value nor can it take any parameters Because it cannot specify any parameters, it cannot be overloaded Although we can define multiple class constructors, we can provide only a single destructor to be applied to all objects of our class Here is our Account class destructor:
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class Account { public: Account(); explicit Account( const char*, double=00 ); Account( const Account& ); ~Account(); // private: char *_name; unsigned int _acct_nmbr; double _balance; }; inline Account::~Account() { delete [] _name; return_acct_nmbr( _acct_nmbr ); }
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Notice that we did not implement our destructor to reset the values of our data members:
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inline Account::~Account() { // necessary delete [] _name; return_acct_nmbr( _acct_nmbr ); // unnecessary _name = 0; _balance = 00; _acct_nmbr = 0; }
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Although doing so is not wrong, it is unnecessary because the memory associated with the members is about to be reclaimed More generally, consider the following class:
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class Point3d { public: // private: float x, y, z; };
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A constructor is necessary to allow users to initialize the three coordinate members But is a destructor necessary The destructor is not necessary in this case There is no resource deallocation required for a Point3d class object The memory of the three coordinate members is created and destroyed automatically by the compiler at the beginning and end of each object's lifetime In general, if the data members of a class are contained by value, as are our three coordinate members of the Point3d class, no
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destructor is necessary Not every class requires a destructor, even if we have provided one or more constructors for that class Destructors serve primarily to relinquish resources acquired either within the constructor or during the lifetime of the class object, again such as freeing a mutual exclusion lock or deleting memory allocated through operator new A destructor is not limited only to relinquishing resources A destructor, in general, can perform any operation that the class designer wishes to have executed subsequent to the last use of an object of that class For example, a common technique for program performance instrumentation is to define a Timer class The Timer's constructor starts some form of program clock Its destructor stops the clock and in some way displays the results A Timer class object might then be defined conditionally within critical code segments we wish to time, such as the following:
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{ // beginning of critical code segment #ifdef PROFILE Timer t; #endif // critical code segment // destruction of t automatically // displays accumulated time }
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To be sure we understand the behavior of a destructor (and constructor, for that matter), let's walk through an example using the following program fragment:
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(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) ); (12) (13) (14) (15)
#include "Accounth" Account global( "James Joyce" ); int main() { Account local( "Anna Livia Plurabelle", 10000 ); Account &loc_ref = global; Account *pact = 0; { Account local_too( "Stephen Hero" ); pact = new Account( "Stephen Dedalus" } delete pact; }
How many constructors are invoked Four: one for the global object, global, on line (2); one each for the two local objects, local and local_too, on lines (5) and (10) respectively; and one for the heap object allocated on line (11) Neither the declaration of the reference to a class object, loc_ref, on line (6) nor that of the pointer to class object, pact, on line (7) results in a constructor being invoked A reference serves as an alias for an already constructed object In this case, loc_ref serves as an alias for global A pointer, too, only addresses an object that has already been constructed (in this case, the object allocated on the heap on line (11)) or addresses no object at all (line (7)) Similarly, there are four destructors invoked: one for the global object, global, declared on line (2); one each for the two local objects; and one for the heap object deleted on line (14) Unlike constructors, however, there is no associated source code statement to
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