Object#singleton_method

(::interned name) -> ::Method

obj.singleton_method(sym) → method

VALUE
rb_obj_singleton_method(VALUE obj, VALUE vid)
{
    VALUE sc = rb_singleton_class_get(obj);
    VALUE klass;
    ID id = rb_check_id(&vid);

    if (NIL_P(sc) ||
        NIL_P(klass = RCLASS_ORIGIN(sc)) ||
        !NIL_P(rb_special_singleton_class(obj))) {
        /* goto undef; */
    }
    else if (! id) {
        VALUE m = mnew_missing_by_name(klass, obj, &vid, FALSE, rb_cMethod);
        if (m) return m;
        /* else goto undef; */
    }
    else {
        VALUE args[2] = {obj, vid};
        VALUE ruby_method = rb_rescue(rb_obj_singleton_method_lookup, (VALUE)args, rb_obj_singleton_method_lookup_fail, Qfalse);
        if (ruby_method) {
            struct METHOD *method = (struct METHOD *)RTYPEDDATA_GET_DATA(ruby_method);
            VALUE lookup_class = RBASIC_CLASS(obj);
            VALUE stop_class = rb_class_superclass(sc);
            VALUE method_class = method->iclass;

            /* Determine if method is in singleton class, or module included in or prepended to it */
            do {
                if (lookup_class == method_class) {
                    return ruby_method;
                }
                lookup_class = RCLASS_SUPER(lookup_class);
            } while (lookup_class && lookup_class != stop_class);
        }
    }

  /* undef: */
    vid = ID2SYM(id);
    rb_name_err_raise("undefined singleton method '%1$s' for '%2$s'",
                      obj, vid);
    UNREACHABLE_RETURN(Qundef);
}

Similar to method, searches singleton method only.

Example

class Demo
  def initialize(n)
    @iv = n
  end
  def hello()
    "Hello, @iv = #{@iv}"
  end
end

k = Demo.new(99)
def k.hi
  "Hi, @iv = #{@iv}"
end
m = k.singleton_method(:hi)
m.call   #=> "Hi, @iv = 99"
m = k.singleton_method(:hello) #=> NameError

Ruby version changes

3.4
- Object#singleton_method now returns methods in modules prepended to or included in the receiver’s singleton class. [Bug #20620]
  
  Example
```
o = Object.new
o.extend(Module.new{def a = 1})
o.singleton_method(:a).call #=> 1
```

See also #singleton_class #singleton_methods #send #to_enum also in 4.1 · 4.0 · 3.3 · 3.2 · 3.1 · 3.0

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