Class: Union

Inherits:
Struct
  • Object
show all
Defined in:
lib/union.rb

Overview

The Union class provides an analogue of a C union. Only one member of a Union object can be set to a non-nil value at a time.

Constant Summary collapse

VERSION =

The version of the union library

'1.3.0'

Instance Method Summary collapse

Constructor Details

#initialize ⇒ Union

Creates and returns a new Union. Unlike Struct::Class.new, this does not take any arguments. You must assign attributes individually.

Union.new('Human', :name, :age)
h = Union::Human.new

h.name = 'Daniel' # => #<struct Union::Human name="Daniel", age=nil>
h.age  = 38       # => #<struct Union::Human name=nil, age=38>

– The semantics of Union.new(arg1, arg2, …) would be non-sensical. Which attribute should be set while the rest are ignored? The first one or the last one? I decided to disallow it altogether.



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# File 'lib/union.rb', line 23

def initialize
  super
  define_union_setters
end

Instance Method Details

#[]=(symbol, value) ⇒ Object

Identical to Struct attribute assignment, except that setting one instance variable sets all other instance variables to nil. Also, you cannot use a numeric index. You must use a string or symbol.

Union.new('Human', :name, :age)
h = Union::Human.new

h[:name] = 'Daniel' # => #<struct Union::Human name="Daniel", age=nil>
h[:age]  = 38       # => #<struct Union::Human name=nil, age=38>



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# File 'lib/union.rb', line 38

def []=(symbol, value)
  # Validate that the symbol is a valid member
  symbol_str = symbol.to_s
  unless members.any? { |m| m.to_s == symbol_str }
    raise ArgumentError, "no member '#{symbol}' in union"
  end

  # Set the value for the specified member
  super(symbol, value)

  # Clear all other members
  members.each { |m| super(m, nil) unless m.to_s == symbol_str }

  value
end

#current_value ⇒ Object

Returns the current value (the value of the non-nil member) Returns nil if no member is set

Union.new('Human', :name, :age)
h = Union::Human.new
h.name = 'Daniel'
h.current_value # => 'Daniel'



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# File 'lib/union.rb', line 74

def current_value
  member = which_member
  member ? self[member] : nil
end

#which_member ⇒ Object

Returns the name of the currently set member (the one with a non-nil value) Returns nil if no member is set

Union.new('Human', :name, :age)
h = Union::Human.new
h.name = 'Daniel'
h.which_member # => :name



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# File 'lib/union.rb', line 62

def which_member
  members.find { |member| !self[member].nil? }
end