Initializer Lists Can Be Mandatory in Java

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Initializer Lists Can Be Mandatory
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An initializer list is required in three common situations
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1 If any data member does not have a zero-parameters constructor, the data member must be initialized in the initializer list 2 Constant data members must be initialized in the initializer list A constant data member can never be assigned to after the object is constructed An example might be the social security number in an Employee class Each Employee has his or her own social security number data member, but presumably the social security number never changes
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Additional C++ Class Features
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3 A data member that is a reference variable (for instance an istream &) must be initialized in the constructor
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Type Conversions
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A type conversion creates a object of a new type
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C++ has rules that allow the mixing of types For instance, if i is an int and - d is a double,d=i is allowed This is known as an implicit type conversion because it is performed without the use of an explicit type conversion operator A temporary tmp is created from i and then is used as the righthand side of the assignment Some languages do not allow implicit type conversion because of the danger of accidental usage and weakening the notion of strong typing However, forcing all type conversions to be explicit tends to load code with conversions, sometimes unnecessarily A type conversion creates a temporary object of a new type In C++ the rules for type conversion follow this general principle: If you can construct an object of type tl by providing an object of another type t2,then a type conversion from t2 to tl is guaranteed to follow the same semantics In the case of the Rational class, any appearance of an IntType object is implicitly converted to a (temporary) Rational when needed, as in the previously cited examples in main (Figure 21 1 , lines 24 and 27) The temporary is created by executing the constructor If you do not want implicit type conversions, declare your one-parameter constructors to be explic it A technical point: In our case, even though a conversion is defined for int to IntType and one is defined from IntType to Rational,transitivity does not hold Thus these two conversions do not imply a third conversion from int to Rational This lack of transitivity is why the type conversion from int to IntType is performed in Figure 21 1 at lines 24 and 27 We could attempt to provide a constructor for Rational that takes an int,which would solve our problems by providing the third type conversion However, if IntType is an int,that approach provides two identical constructors, and the compiler will complain about the ambiguity We can also define a type conversion by overloading operator ( ) For instance, we can specify a type conversion from Rational to int by writing the member function
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operator int ( ) const { return denom == 1 numer
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A constructor defines an automatic type conversion
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Conversions are not transitive
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Conversions can also be defined as member functions, but do not overdo them or ambiguity can result
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int( longDecimal(
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Objects and Classes
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
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const Rational
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Rational::operator=( const Rational &rhs
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if( this
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numer denom
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rhs numer; rhsdenom;
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return *this;
const Rational & Rational::operator+=( const Rational I numer = numer * rhsdenom + rhsnumer * denom; denom = denom * rhsdenom; reduce( ) ; return *this;
Assignment operators (two of five) for the Rational class
Overloading the type conversion operator in this way is not recommended Too many implicit conversions can easily get you in trouble; again, ambiguity can result We present an example of this problem in Section 99
233 Operator Overloading
We examine the operators in the same order given in the class interface Many of the operators, such as the assignment operators, use no new principles Two of them are shown in Figure 215 However, we do have to be careful For example, lines 13 and 14 cannot be interchanged For the corresponding / = operator, we need to use temporaries The next group of operators are the binary arithmetic operators A binary arithmetic operator usually returns an object by value because the result is stored in a temporary It also can be implemented by calling the corresponding assignment operator A simple implementation is provided in Figure 21 6 for the addition operator Note how we use a previously defined operator, an excellent general technique An interesting technical issue here is the return type As usual, we have three choices: We can return by value, by reference, or by constant reference A return by reference is certainly wrong: We cannot allow expressions =c such as (a+b) because a+b is not a named object; the assignment could at best be meaningless
A binary arithmetic operator usually returns an object by value because the result is stored in a temporary- It can be implemented by calling the corresponding assignment operator