Chapter 2 : auto

Item 5: Prefer auto to explicit type declarations.

  • Use auto whenever possible (helps you from explicitly adding the types)
  • Also, it helps avoid few errors
    1. auto doesn’t support uninitialized variables. Unlike int x where x is uninitialized, auto x; is not valid! It has to be either auto x{1} or auto x = 1; (1 is just some random value in this example)
    2. It helps to easily take get the value type of iterator objects. Replace typename std::iterator_traits<It>::value_type currValue = *b; -> auto currValue = *b
    3. For entities like lambda, their type is only known to compiler so how on earth can you make a variable for it using explicit type? auto to the rescue 😉
  • Also helps avoid subtle bugs (example: unsigned len = vec.size(), return type of vec.size() is std::vector<T>::size_type, however since unsigned is platform dependent it can lead to bugs if it doesn’t match std::vector<T>::size_type) (Refer example on page 40 which talks about vec.size() )
  • Can avoid temporary object creation (Refer example on page 40 which talks about unordered map)
  • For people worried about auto decreasing the readability should note that tons of succesful language have type inference where user doesn’t need to explicitly specify the types. Also IDE and editor can/should help reveal the type.

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

  • For some edge-cases, auto may deduce type which you don’t actually want, in that case you should use the explicitly type initializer idiom. Eg. auto x = static_cast<float>(get_double_eps()), this approach is better than implicitly converting to float using float x = get_double_eps(). Also this will help you to get around the functions or interface which return proxy object (as auto will deduce the type to be proxy object and not the type it is acting as proxy for). Proxy objects are those which behave like a certain type but are not of that type. std::vector::operator[] returns proxy object (as reference to bits is not valid). (Refer to Item 6 in book for gory details 😄 )
  • Prefer explicit cast over implicit cast!