C++ LexicalScope::top_level_p方法代码示例

    Object* const_get(STATE, Symbol* name, ConstantMissingReason* reason, Object* filter, bool replace_autoload) {
      LexicalScope *cur;
      Object* result;

      *reason = vNonExistent;

      CallFrame* frame = state->vm()->get_ruby_frame();

      // Ok, this has to be explained or it will be considered black magic.
      // The scope chain always ends with an entry at the top that contains
      // a parent of nil, and a module of Object. This entry is put in
      // regardless of lexical scoping, it's the fallback scope (the default
      // scope). This is not case when deriving from BasicObject, which is
      // explained later.
      //
      // When looking up a constant, we don't want to consider the fallback
      // scope (ie, Object) initially because we need to lookup up
      // the superclass chain first, because falling back on the default.
      //
      // The rub comes from the fact that if a user explicitly opens up
      // Object in their code, we DO consider it. Like:
      //
      // class Idiot
      //   A = 2
      // end
      //
      // class ::Object
      //   A = 1
      //   class Stupid < Idiot
      //     def foo
      //       p A
      //     end
      //   end
      // end
      //
      // In this code, when A is looked up, Object must be considering during
      // the scope walk, NOT during the superclass walk.
      //
      // So, in this case, foo would print "1", not "2".
      //
      // As indicated above, the fallback scope isn't used when the superclass
      // chain directly rooted from BasicObject. To determine this is the
      // case, we record whether Object is seen when looking up the superclass
      // chain. If Object isn't seen, this means we are directly deriving from
      // BasicObject.

      cur = frame->lexical_scope();
      while(!cur->nil_p()) {
        // Detect the toplevel scope (the default) and get outta dodge.
        if(cur->top_level_p(state)) break;

        result = cur->module()->get_const(state, name, G(sym_private), reason, false, replace_autoload);
        if(*reason == vFound) {
          if(result != filter) return result;
          *reason = vNonExistent;
        }

        cur = cur->parent();
      }

      // Now look up the superclass chain.
      Module *fallback = G(object);

      cur = frame->lexical_scope();
      if(!cur->nil_p()) {
        bool object_seen = false;
        Module* mod = cur->module();
        while(!mod->nil_p()) {
          if(mod == G(object)) {
            object_seen = true;
          }
          if(!object_seen && mod == G(basicobject)) {
            fallback = NULL;
          }

          result = mod->get_const(state, name, G(sym_private), reason, false, replace_autoload);
          if(*reason == vFound) {
            if(result != filter) return result;
            *reason = vNonExistent;
          }

          mod = mod->superclass();
        }
      }

      // Lastly, check the fallback scope (=Object) specifically if needed
      if(fallback) {
        result = fallback->get_const(state, name, G(sym_private), reason, true, replace_autoload);
        if(*reason == vFound) {
          if(result != filter) return result;
          *reason = vNonExistent;
        }
      }

      return cNil;
    }