Where is the C auto keyword used?

Question

If you'd read the IAQ (Infrequently Asked Questions) list, you'd know that auto is useful primarily to define or declare a vehicle:

auto my_car;

A vehicle that's consistently parked outdoors:

extern auto my_car;

For those who lack any sense of humor and want "just the facts Ma'am": the short answer is that there's never any reason to use auto at all. The only time you're allowed to use auto is with a variable that already has auto storage class, so you're just specifying something that would happen anyway. Attempting to use auto on any variable that doesn't have the auto storage class already will result in the compiler rejecting your code. I suppose if you want to get technical, your implementation doesn't have to be a compiler (but it is) and it can theoretically continue to compile the code after issuing a diagnostic (but it won't).

Small addendum by kaz:

There is also:

static auto my_car;

which requires a diagnostic according to ISO C. This is correct, because it declares that the car is broken down. The diagnostic is free of charge, but turning off the dashboard light will cost you eighty dollars. (Twenty or less, if you purchase your own USB dongle for on-board diagnostics from eBay).

The aforementioned extern auto my_car also requires a diagnostic, and for that reason it is never run through the compiler, other than by city staff tasked with parking enforcement.

If you see a lot of extern static auto ... in any code base, you're in a bad neighborhood; look for a better job immediately, before the whole place turns to Rust.

auto is a modifier like static. It defines the storage class of a variable. However, since the default for local variables is auto, you don't normally need to manually specify it.

This page lists different storage classes in C.

The auto keyword is useless in the C language. It is there because before the C language there existed a B language in which that keyword was necessary for declaring local variables. (B was developed into NB, which became C).

Here is the reference manual for B.

As you can see, the manual is rife with examples in which auto is used. This is so because there is no int keyword. Some kind of keyword is needed to say "this is a declaration of a variable", and that keyword also indicates whether it is a local or external (auto versus extrn). If you do not use one or the other, you have a syntax error. That is to say, x, y; is not a declaration by itself, but auto x, y; is.

Since code bases written in B had to be ported to NB and to C as the language was developed, the newer versions of the language carried some baggage for improved backward compatibility that translated to less work. In the case of auto, the programmers did not have to hunt down every occurrence of auto and remove it.

It's obvious from the manual that the now obsolescent "implicit int" cruft in C (being able to write main() { ... } without any int in front) also comes from B. That's another backward compatibility feature to support B code. Functions do not have a return type specified in B because there are no types. Everything is a word, like in many assembly languages.

Note how a function can just be declared extrn putchar and then the only thing that makes it a function that identifier's use: it is used in a function call expression like putchar(x), and that's what tells the compiler to treat that typeless word as a function pointer.

In C auto is a keyword that indicates a variable is local to a block. Since that's the default for block-scoped variables, it's unnecessary and very rarely used (I don't think I've ever seen it use outside of examples in texts that discuss the keyword). I'd be interested if someone could point out a case where the use of auto was required to get a correct parse or behavior.

However, in the C++11 standard the auto keyword has been 'hijacked' to support type inference, where the type of a variable can be taken from the type of its initializer:

auto someVariable = 1.5;   // someVariable will have type double

Type inference is being added mainly to support declaring variables in templates or returned from template functions where types based on a template parameter (or deduced by the compiler when a template is instantiated) can often be quite painful to declare manually.