C Programming – Concept of ‘auto’ Keyword

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.

It will break if your code contains the auto keyword. In nearly 30 years of C and C++ programming I've never come across any that did. The keyword was introduced in the first C compiler to specify local function variables, but compilers almost immediately became clever enough not to require it, and very little code that uses it will survive today - that's why C++0x chose to recycle it rather than introduce a new keyword which would cause portability problems.

The purpose of the auto keyword in C++0X is to allow the compiler to work out the type of a variable, where this is possible:

vector <int> v;
auto it = v.begin():

the compiler can see that v.begin() must return a vector<int>::iterator and so can create a variable of that type, saving a lot of keyboarding or typedef creation.