Item 5: Prefer auto to Explicit Type Declarations

auto saves typing, prevents incorrect or less-performant (eliminate unnecessary temporary objects) usages, and sometimes is a must.

Benefits of using auto:

• You can’t forget to initialize a variable, unlike int x;
• You won’t type long type names, such as typename std::iterator_traits<It>::value_type
• Declare closure-typed variable
  • Closure type is only known to compiler and there is no syntax to declare a closure
  • So in this case, you must use auto (unlike the former two which are just niceties)
• Prevent mistakenly using the incorrect type
  • And be more resilient against interface changes

std::vector<int> v;

// unsigned is incorrect
// The correct type is std::vector<int>::size_type
unsigned s = v.size();

// Deduce the correct type
auto s = v.size();

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

// std::string is incorrect (it should be const std::string).
// Worse, compiler will make temporary copies for each pair,
// which is slower and sometimes is a bug.
for (const std::pair<std::string, int>& p : m) {
  ...
}

// Deduce the correct type std::pair<const std::string, int>
for (const auto& p : m) {
  ...
}