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Changes between Version 3 and Version 4 of Properties

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01/16/14 11:36:33 (4 years ago)
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hopscc
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  • Properties

    v3 v4  
    33== Introduction == 
    44 
    5 Properties provide a way to get and set the values that characterize an object. A business application could have a Customer class with properties .name, .address and .id. A video game could have a !GameObject class with properties .position, .velocity and .health. 
     5Properties provide a way to get and set values that characterize an object. These values can be private fields or entirely code generated or synthesized. These give class members that provide a mechanism to read, write, or compute the value of a real or virtual private field. 
     6Properties can be used as if they are public data members, but the private field access can be mediated by code 
     7 
     8Properties combine aspects of both fields and methods. To the user of an object, a property appears to be a field, accessing the property requires the same syntax. To the implementer of a class, a property is one or two code blocks, representing a get accessor and/or a set accessor. The code block for the get accessor is executed when the property is read; the code block for the set accessor is executed when the property is assigned a new value. A property without a set accessor is considered read-only. A property without a get accessor is considered write-only. A property that has both accessors is read-write. 
     9 
     10Cobra provides some shortcut syntax for the most common case - property access to a private backgrounding variable and for marking readonly ('''get''') and writeonly ('''set''') properties clearly. 
     11 
     12A class properties can be indexed as well as single variable access. 
    613 
    714== Syntax == 
    8  
    9 to-do 
     15{{{ 
     16pro NAME from var [ is ACCESSMODIFIER ]           (1) 
     17pro NAME from VARNAME [ is ACCESSMODIFIER ]       (2)  
     18pro NAME from var [ as TYPE ] [ is ACCESSMODIFIER ] [ = INITEXPR ] (3) 
     19 
     20pro NAME [as RETURNTYPE] [is ACCESSMODIFIER ] (4) 
     21    [ATTRIBUTE] 
     22    [DOCSTRING] 
     23    [ get 
     24       GETBLOCK ]  
     25    [ set 
     26       SETBLOCK ] 
     27 
     28# shortcut: readonly properties 
     29get NAME from var                          (1) 
     30get NAME  from VARNAME                     (2)  
     31get NAME as RETURNTYPE is ACCESSMODIFIER   (3) 
     32get NAME [as RETURNTYPE] [is ACCESSMODIFIER ] (4) 
     33    [ATTRIBUTE] 
     34    [DOCSTRING] 
     35    GETBLOCK  
     36 
     37# shortcut: write-only properties 
     38set NAME from var                          (1) 
     39set NAME  from VARNAME                     (2)  
     40set NAME as RETURNTYPE is ACCESSMODIFIER   (3) 
     41set NAME [as RETURNTYPE] [is ACCESSMODIFIER ] (4) 
     42    [ATTRIBUTE] 
     43    [DOCSTRING] 
     44    SETBLOCK  
     45}}} 
     46 
     47   1. declare a property name mapping onto an '''existing''' declared (possibly private) backing variable named similarly to the property name.[[BR]] 
     48      Either the backing is named the same as the property name or has a leading single or double underscore prefix [[BR]] 
     49      The properties type is the same as the backing variable, the access modifiers are defaulted or optionally specified. 
     50   2. declare a property name mapping onto  the '''existing''' declared named backing variable (VARNAME).[[BR]] 
     51      The properties type is the same as the backing variable, the access modifiers are defaulted or optionally specified. 
     52   3. declare a property name mapping onto an '''implicitly''' created private backing variable.[[BR]]  
     53      Either the Type or an initialising expression must be given to type the backing variable,[[BR]]  
     54      Access modifier may be explicitly specified or defaults. 
     55   4. Full Property form: [[BR]] 
     56     must specify property type unless want type dynamic, optionally specify access modifiers and get and set mediating code.[[BR]] 
     57    If property is declared abstract or is inside an interface then no body (get set blocks ) needed[[BR]] 
     58    otherwise there must be one or both of the set-block or get-block clauses[[BR]] 
     59    If no Type specified then property is of type '''dynamic'''[[BR]] 
     60    In  a SETBLOCK, any assigned value is accessed through a synthesized variable 'value'.  
     61 
     62 
     63 For all the above  
     64   "is ACCESSMODIFIER" can be on the same line or the next. (see AccessModifiers )[[BR]] 
     65   wiki:Attributes and a wiki:Docstring can be optionally provided ( indented on following lines). 
     66 
     67 
     68A (single) indexed property can also be specified by providing a paramlist enclosed in square brackets for the property name. 
    1069 
    1170== Compared to Methods == 
     
    1675 * Methods are about actions and computations; properties are about inspection or characteristics. 
    1776 
     77 
     78== Examples == 
     79 
     80{{{ 
     81#!cobra 
     82# The vars are already declared so use "... from var": 
     83 
     84class Person1 
     85    var _name as String 
     86    var _age as int 
     87 
     88    cue init(name as String) 
     89        base.init 
     90        _name = name 
     91 
     92    pro name from var 
     93        """ 
     94        The "from var" indicates that the type of the property, as well 
     95        as its value, is taken from the class variable with the matching 
     96        name (underscore + property name or underscore + underscore + property name).  
     97        When you later want to add code for this property, just chop off the "from var" and write 
     98        the full property (pro foo as Type; get ...; set ...) 
     99        """ 
     100 
     101    pro age from var 
     102 
     103 
     104# If the underlying variable name is different than the property name, 
     105# you can specify that name instead of "var": 
     106 
     107class Person2 
     108 
     109    var _name as String 
     110 
     111    cue init(name as String) 
     112        base.init 
     113        _name = name 
     114 
     115    pro nombre from _name 
     116 
     117 
     118 
     119# properties and their backing vars are declared in one line: 
     120class Person3 
     121 
     122    cue init(name as String) 
     123        base.init 
     124        _name = name 
     125 
     126    pro name from var as String 
     127    pro age from var as int 
     128 
     129 
     130# Give an initial value to the declaration to infer the data type. 
     131class Person3 
     132 
     133    pro name from var = ''  # inferred as String 
     134    pro age from var = 0    # inferred as int 
     135 
     136 
     137# Full property form mapping to backing variable 
     138class Person4 
     139    var _name as String 
     140 
     141    pro name as String 
     142        """ 
     143        The name of the person. Setting the name trims preceding and 
     144        trailing whitespace from the value. 
     145        """ 
     146        get 
     147            return _name 
     148        set 
     149            # `value` is an implicit argument in all property setters: 
     150           _name = value.trim 
     151 
     152 
     153 
     154# Properties can be read-only: 
     155class Person6 
     156 
     157    cue init(name as String) 
     158        base.init 
     159        _name = name 
     160 
     161    get name from var as String 
     162 
     163    get lowerName as String 
     164        return _name.toLower 
     165 
     166 
     167# Properties can be write-only: 
     168class Person7 
     169 
     170    cue init(name as String) 
     171        base.init 
     172        _name = name 
     173 
     174    pro name from var as String 
     175 
     176    set internalName from var as String 
     177 
     178    def toString as String is override 
     179        return 'Person7(name=[.name]. internal=[_internalName])' 
     180 
     181 
     182#indexer 
     183class Test 
     184 
     185    pro [i as int] as int 
     186        get 
     187            return i*i 
     188        set 
     189            pass 
     190 
     191    def main is shared 
     192        Test().check 
     193        t = Test() 
     194        assert t[3] == 9 
     195 
     196 
     197    def check 
     198        assert this[1]==1 
     199        assert this[2]==4 
     200        assert this[-3]==9 
     201        this[1] = 5 
     202        this[2] = 6 
     203        assert this[1]==1 
     204        assert this[2]==4 
     205}}} 
     206 
     207 
    18208== See also == 
    19209 
     210 * wiki:Methods 
    20211 * LanguageTopics