Effective Modern C++

Chapter 2. auto

In concept, auto is as simple as simple can be, but it’s more subtle than it looks. Using it saves typing, sure, but it also prevents correctness and performance issues that can bedevil manual type declarations. Furthermore, some of auto’s type deduction results, while dutifully conforming to the prescribed algorithm, are, from the perspective of a programmer, just wrong. When that’s the case, it’s important to know how to guide auto to the right answer, because falling back on manual type declarations is an alternative that’s often best avoided.

This brief chapter covers all of auto’s ins and outs.

Item 5: Prefer auto to explicit type declarations.

Ah, the simple joy of

int x;

Wait. Damn. I forgot to initialize x, so its value is indeterminate. Maybe. It might actually be initialized to zero. Depends on the context. Sigh.

Never mind. Let’s move on to the simple joy of declaring a local variable to be initialized by dereferencing an iterator:

template<typename It>    // algorithm to dwim ("do what I mean")
void dwim(It b, It e)    // for all elements in range from
{                        // b to e
  for (; b != e; ++b) {
    typename std::iterator_traits<It>::value_type
      currValue = *b;
    …
  }
}

Ugh. “typename std::iterator_traits<It>::value_type” to express the type of the value pointed to by an iterator? Really? I must have blocked out the memory of how much fun that is. Damn. Wait—didn’t I already say that?

Okay, simple joy (take three): the delight of declaring a local variable whose type is ...