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Binding

Objects of class Binding encapsulate the execution context at some particular place in the code and retain this context for future use. The variables, methods, value of self, and possibly an iterator block that can be accessed in this context are all retained. Binding objects can be created using Kernel#binding, and are made available to the callback of Kernel#set_trace_func.

These binding objects can be passed as the second argument of the Kernel#eval method, establishing an environment for the evaluation.

class Demo
  def initialize(n)
    @secret = n
  end
  def get_binding
    return binding()
  end
end

k1 = Demo.new(99)
b1 = k1.get_binding
k2 = Demo.new(-3)
b2 = k2.get_binding

eval("@secret", b1)   #=> 99
eval("@secret", b2)   #=> -3
eval("@secret")       #=> nil

Binding objects have no class-specific methods.

Public Instance Methods

eval(string [, filename [,lineno]]) → obj click to toggle source

Evaluates the Ruby expression(s) in string, in the binding’s context. If the optional filename and lineno parameters are present, they will be used when reporting syntax errors.

def get_binding(param)
  return binding
end
b = get_binding("hello")
b.eval("param")   #=> "hello"
 
               static VALUE
bind_eval(int argc, VALUE *argv, VALUE bindval)
{
    VALUE args[4];

    rb_scan_args(argc, argv, "12", &args[0], &args[2], &args[3]);
    args[1] = bindval;
    return rb_f_eval(argc+1, args, Qnil /* self will be searched in eval */);
}
            
local_variable_defined?(symbol) → obj click to toggle source

Returns a true if a local variable symbol exists.

def foo
  a = 1
  binding.local_variable_defined?(:a) #=> true
  binding.local_variable_defined?(:b) #=> false
end

This method is short version of the following code.

binding.eval("defined?(#{symbol}) == 'local-variable'")
 
               static VALUE
bind_local_variable_defined_p(VALUE bindval, VALUE sym)
{
    ID lid = check_local_id(bindval, &sym);
    const rb_binding_t *bind;

    if (!lid) return Qfalse;

    GetBindingPtr(bindval, bind);
    return get_local_variable_ptr(bind->env, lid) ? Qtrue : Qfalse;
}
            
local_variable_get(symbol) → obj click to toggle source

Returns a value of local variable symbol.

def foo
  a = 1
  binding.local_variable_get(:a) #=> 1
  binding.local_variable_get(:b) #=> NameError
end

This method is short version of the following code.

binding.eval("#{symbol}")
 
               static VALUE
bind_local_variable_get(VALUE bindval, VALUE sym)
{
    ID lid = check_local_id(bindval, &sym);
    const rb_binding_t *bind;
    const VALUE *ptr;

    if (!lid) goto undefined;

    GetBindingPtr(bindval, bind);

    if ((ptr = get_local_variable_ptr(bind->env, lid)) == NULL) {
      undefined:
        rb_name_error_str(sym, "local variable `%"PRIsVALUE"' not defined for %"PRIsVALUE,
                          sym, bindval);
    }

    return *ptr;
}
            
local_variable_set(symbol, obj) → obj click to toggle source

Set local variable named symbol as obj.

def foo
  a = 1
  b = binding
  b.local_variable_set(:a, 2) # set existing local variable `a'
  b.local_variable_set(:b, 3) # create new local variable `b'
                              # `b' exists only in binding.
  b.local_variable_get(:a) #=> 2
  b.local_variable_get(:b) #=> 3
  p a #=> 2
  p b #=> NameError
end

This method is a similar behavior of the following code

binding.eval("#{symbol} = #{obj}")

if obj can be dumped in Ruby code.

 
               static VALUE
bind_local_variable_set(VALUE bindval, VALUE sym, VALUE val)
{
    ID lid = check_local_id(bindval, &sym);
    rb_binding_t *bind;
    VALUE *ptr;

    if (!lid) lid = rb_intern_str(sym);

    GetBindingPtr(bindval, bind);
    if ((ptr = get_local_variable_ptr(bind->env, lid)) == NULL) {
        /* not found. create new env */
        ptr = rb_binding_add_dynavars(bind, 1, &lid);
    }

    *ptr = val;

    return val;
}
            

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