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instance method

Module#const_missing

mod.const_missing(sym) → obj

VALUE
rb_mod_const_missing(VALUE klass, VALUE name)
{
    VALUE ref = GET_EC()->private_const_reference;
    rb_vm_pop_cfunc_frame();
    if (ref) {
	rb_name_err_raise("private constant %2$s::%1$s referenced",
			  ref, name);
    }
    uninitialized_constant(klass, name);

    UNREACHABLE_RETURN(Qnil);
}

Invoked when a reference is made to an undefined constant in mod. It is passed a symbol for the undefined constant, and returns a value to be used for that constant. The following code is an example of the same:

Example

def Foo.const_missing(name)
  name # return the constant name as Symbol
end

Foo::UNDEFINED_CONST    #=> :UNDEFINED_CONST: symbol returned

In the next example when a reference is made to an undefined constant, it attempts to load a file whose name is the lowercase version of the constant (thus class Fred is assumed to be in file fred.rb). If found, it returns the loaded class. It therefore implements an autoload feature similar to Kernel#autoload and Module#autoload.

Example

def Object.const_missing(name)
  @looked_for ||= {}
  str_name = name.to_s
  raise "Class not found: #{name}" if @looked_for[str_name]
  @looked_for[str_name] = 1
  file = str_name.downcase
  require file
  klass = const_get(name)
  return klass if klass
  raise "Class not found: #{name}"
end

See also #const_get #const_set #const_defined? #const_source_location also in 4.1 · 4.0 · 3.4 · 3.3 · 3.2 · 3.0

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