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Item 6. Use the explicity typed initializer idiom when auto deduces undesired types

Test Codes

$ g++ -std=c++14 -DBOOST_SUPPORT main.cc && ./a.out 
-- reports by boost::type_index:
boost::typeindex::type_id_with_cvr<decltype(explicitly_b1)>().pretty_name(): bool
boost::typeindex::type_id_with_cvr<decltype(auto_b1)>().pretty_name(): std::__1::__bit_reference<std::__1::vector<bool, std::__1::allocator<bool> >, true>
boost::typeindex::type_id_with_cvr<decltype(forced_auto_b1)>().pretty_name(): bool

Notes

"Proxy" design pattern / proxy class

  • proxy class: a class that exists for the purpose of emulating and augmenting the behavior of some other type.
    • std::vector<bool>::reference is an example of a proxy class.
    • The utility of proxy classes is well-established. In fact, the design pattern "Proxy" is one of the most longstanding members of the software design patterns Pantheon.
  • Some proxy classes are designed to be apparent to clients.
    • That's the case for std::shared_ptr and std::unique_ptr, for example.
  • Other proxy classes are designed to act more or less invisibly.
    • std::vector<bool>::reference is an example of such "invisible" proxies, as is its std::bitset compatriot, std::bitset::reference.

General Rule: "invisible" proxy classes don't play well with auto

  • avoid code of this form
    • "invisible" proxy types can cause auto to deduce the "wrong" type for an initializing expression.
  // avoid code of this form
  auto someVar = expression of "invisible" proxy class type

  /********* example **********/

  std::vector<bool> features(const Widget& w);  // implemented in somewhere

  Widget w;
  //...
  bool highPriority1 = features(w)[5]; // correct
  auto highPriority2 = features(w)[5]; // wrong! highPriority2 's type won't be bool!

  • how can you recognize when proxy objects are in use?

    • by document
      • Although "invisible" proxy classes are designed to fly beneath programmer radar in day-to-day use, libraries using them often document that they do so.
    • by header files
      • Where documentation comes up short, header files fill the gap.
      • It's rarely possible for source code to fully cloak proxy objects.
    • misc.
      • Paying careful attention to the interfaces you're using can often reveal the existence of proxy classes.
      • In practice, many developers discover the use of proxy classes only when they try to track down mystifying compilation problems or debug incorrect unit test results.

  • solution: the explicity typed initializer idiom

    • auto itself isn't the problem. The problem is that auto isn't deducing the type you want it to deduce.
  // solution: the explicity typed initializer idiom, 
  //    i.e. force a different type deduction
  auto highPriority = static_case<bool>(features(w)[5]);  // correct. highPriority 's type forced to bool!

References