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There are occasions in our programs (as with dat04) when we need a class object but do not as yet know what the initial values should be Perhaps these values can only be determined later Yet we still need to provide some initial value, if only to indicate that no value has as yet been selected In a sense, it is sometimes necessary to initialize a class object to indicate that it is not yet initialized Most classes provide a special default constructor that requires no initial values to be specified Typically a default constructor initializes the class object such that we can subsequently recognize it as being uninitialized Is our Data class required to provide a constructor As it happens, it is not because all its data members are public A mechanism inherited from the C language supports an explicit initialization list similar to that used to initialize an array For example:
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int main() { // local1ival = 0; local1ptr = 0 Data local1 = { 0, 0 }; // local2ival = 1024; // local2ptr = "Anna Livia Plurabelle" Data local2 = { 1024, "Anna Livia Plurabelle" }; // }
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The values are resolved positionally based on the declaration order of the data members The following, for example, is a compiletime error because ival is declared before ptr:
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// error: ival = "Anna Livia Plurabelle" // ptr = 1024 Data local2 = { "Anna Livia Plurabelle" , 1024 };
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Two primary drawbacks to the explicit initialization list are that it can only be applied to class objects for which all the data members are public (that is, the explicit initialization list does not support the use of data encapsulation and abstract data types these are absent in the C language, from which this form of initialization is derived) and that it requires the explicit intervention of the programmer, adding to the possibility of accident (forgetting to provide the initialization list) or error (somehow mixing up or missetting the order of initialization) Given these drawbacks, can the use of an explicit initialization list in lieu of a constructor ever be justified In practice, yes For some applications it is more efficient to use an explicit initialization list to initialize large data structures with constant values For example, perhaps we are building a color palette or dumping into program text large amounts of constant values such as the control vertices and knot values of a complex geometric model In these cases the explicit initialization can be accomplished at load time, saving the startup cost of a constructor, even one defined as inline, particularly for global objects1">[1]
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See [LIPPMAN96a] for a more detailed discussion with examples and rough performance timings
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In general, however, the preferred class initialization mechanism is the constructor, which is guaranteed to be applied automatically by the compiler to each class object prior to the first use of that object In the next section, we look at the class constructor in detail
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The Class Constructor
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The constructor is identified by providing it with the same name as the class To declare a default constructor, we write2">[2]
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Normally we would declare name to be of type string We declare it to be a C-style character string to postpone considering initialization issues of class data members until Section 144
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class Account { public: // default constructor Account(); // private: char *_name; unsigned int _acct_nmbr; double _balance; };
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The only syntactic constraint on a constructor is that it must not specify a return type, even that of void For example, both of the following declarations are errors:
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// errors: constructor must not specify a return value void Account::Account() { } Account* Account::Account( const char *pc ) { }
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There is no constraint on the number of constructors we may declare for a class, provided the parameter list of each constructor is unique How can we know which or how many constructors to define Minimally, we need to allow the user to provide an initial value for each data member that needs to be set An account number, for example, may either be set or generated automatically to guarantee its uniqueness For our purposes, let's say that it is generated automatically This leaves us with the need to allow for the initialization of the two members _name and _balance:
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Account( const char *name, double open_balance );
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An Account object being initialized with this constructor can be defined as follows:
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Account newAcct( "Mikey Metz", 0 );
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Alternatively, if there are many accounts that begin with an opening balance of zero, a user may request only to specify a name, and to have the constructor initialize _balance to zero automatically One solution is to provide a second constructor of the form
Account( const char *name );
An alternate solution is to provide a default value of zero to the two-para-meter constructor:
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