C++ – ny reason to use the ‘auto’ keyword in C++03


Note this question was originally posted in 2009, before C++11 was ratified and before the meaning of the auto keyword was drastically changed. The answers provided pertain only to the C++03 meaning of auto — that being a storage class specified — and not the C++11 meaning of auto — that being automatic type deduction. If you are looking for advice about when to use the C++11 auto, this question is not relevant to that question.

For the longest time I thought there was no reason to use the static keyword in C, because variables declared outside of block-scope were implicitly global. Then I discovered that declaring a variable as static within block-scope would give it permanent duration, and declaring it outside of block-scope (in program-scope) would give it file-scope (can only be accessed in that compilation unit).

So this leaves me with only one keyword that I (maybe) don't yet fully understand: The auto keyword. Is there some other meaning to it other than 'local variable?' Anything it does that isn't implicitly done for you wherever you may want to use it? How does an auto variable behave in program scope? What of a static auto variable in file-scope? Does this keyword have any purpose other than just existing for completeness?

Best Solution

In C++11, auto has new meaning: it allows you to automatically deduce the type of a variable.

Why is that ever useful? Let's consider a basic example:

std::list<int> a;
// fill in a
for (auto it = a.begin(); it != a.end(); ++it) {
  // Do stuff here

The auto there creates an iterator of type std::list<int>::iterator.

This can make some seriously complex code much easier to read.

Another example:

int x, y;
auto f = [&]{ x += y; };

There, the auto deduced the type required to store a lambda expression in a variable. Wikipedia has good coverage on the subject.