The Member Initialization List in Java

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The Member Initialization List
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Let's modify our Account class by redeclaring its _name member to be of type string:
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#include <string> class Account { public: // private: unsigned int _acct_nmbr; double _balance; string _name; };
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We'll need to modify our constructors as well This involves two issues: (1) maintaining compatibility with the original interface while accommodating the new type and (2) initializing a member class object with an associated set of constructors The original two-parameter Account constructor
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Account( const char*, double = 00 );
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is insufficient for our new string class type For example,
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string new_client( "Steve Hall" ); Account new_acct( new_client, 25000 );
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fails because there is no implicit conversion of a string object to char* Writing
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Account new_acct( new_clientc_str(), 25000 );
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works, but is likely to prove confusing to users One solution is simply to add a new constructor of the form
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Account( string, double = 00 );
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Now when we write
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Account new_acct( new_client, 25000 );
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this instance is invoked, whereas older code such as
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Account *open_new_account( const char *nm ) { Account *pact = new Account( nm ); // return pact; }
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continues to invoke the original two-parameter instance Because the string class provides for the conversion of char* into a string object (class conversions are discussed in the next chapter), we can also simply replace the original two-parameter constructor with the new instance taking a first parameter of type string In this case, when we write
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Account myAcct( "Tinkerbell" );
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"Tinkerbell" is converted to a temporary string object That object is then passed to the two-parameter constructor taking a first parameter of type string
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The design trade-off is between a proliferation of Account constructors versus a somewhat less efficient handling of char* arguments due to the creation of the string temporary Our design choice is to provide two versions of the two-parameter constructor Our revised set of Account constructors is the following:
#include <string> class Account { public: Account(); Account( const char*, double=00 ); Account( const string&, double=00 ); Account( const Account& ); // private: // };
The next issue is how to properly initialize a class data member that has an associated set of constructors This breaks down into the following three subcategories: 1 How do we invoke its default constructor We'll need to do this within the default Account constructor 2 How do we invoke its copy constructor We'll need to do this within the Account copy constructor and the two-parameter Account constructor taking a string as its first parameter 3 More generally, how do we pass arguments to the constructor of a member class object We'll need to do this within the twoparameter Account constructor taking char* as its first parameter The solution is the member initialization list (briefly introduced in Section 142) Class data members can be initialized explicitly through the member initialization list, a commaseparated list of member/name argument pairs For example, here is our reimplementation of our two-parameter constructor using the member initialization list (_name, recall, is now a member class object of type string):
inline Account:: Account( const char* name, double opening_bal ) : _name( name ), _balance( opening_bal ) { _acct_nmbr = get_unique_acct_nmbr(); }
The member initialization list follows the signature of the constructor and is set off by a colon The member name is specified, followed by the initial values enclosed within parentheses, similar to the syntax of a function call If the member is a class object, the initial values become the arguments that are passed to the appropriate constructor, which is then applied to the member class object In our example, name is passed to the string constructor applied to _name _balance is initialized with the parameter opening_bal Similarly, here is our other two-parameter Account constructor:
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