item-5-prefer-auto-to-explicit-type-declarations

Item 5. Prefer auto to explicit type declarations

Test Codes

$ # TODO: 

Notes

• What's a std::function object?

  • std::function is a template in the C++11 standard library that generalizes the idea of a function pointer.
  • Whereas function pointers can point only to functions, however, std::function objects can refer to any callable object, i.e., to anything that can be invoked like a function.

• std::function vs auto

  • using std::function is not the same as using auto.
    • An auto-declared variable holding a closure has the same type as the closure, and as such it uses only as much memory as the closure requires.
    • The type of a std::function-declared variable holding a closure is an instantiation of the std::function template, and that has a fixed size for any given signature.
      • The size may not be adequate for the closure it's asked to store, and when that's the case, the std::function constructor will allocate heap memory to store the closure.
      • The result is that the std::function object typically uses more memory than the auto-declared object.
    • Thanks to implementation details that restrict inlining and yield indirect function calls, invoking a closure via a std::function object is almost certain to be slower than calling it via an auto-declared object.
  • In another words, the std::function approach is generally bigger and slower than the auto approach, and it may yield out of memory exceptions, too.

• The advantages of auto

  • avoidance of uninitialized variables
  • avoidance of verbose variable declarations
  • ability to directly hold closures
  • ability to avoid the problems related to "type shortcuts"
    • see example in below value type of std::unordered_map
  • auto types automatically change if the type of their initializing expression changes, and that means that some refactorings are facilitated by the use of auto.
    • The fact of the matter is that writing types explicitly often does little more than introduce opportunities for subtle errors, either in correctness or efficiency or both.

• value type of std::unodered_map

  std::unordered_map<std::string, int> m;

  // "type shortcut" problem
  // the value_type of std::unordered_map<std::string, int> is 
  //   std::pair<const std::string, int>, so the p in below codes is 
  //   a reference of temp object, not the element of m directly. 
  // use `const auto& p` instead.
  for (const std::pair<std::string, int>& p : m){
      // do something
  }

References

• Stackoverflow - C++11 lambda implementation and memory model