The code:
int main(void)
{
auto a=1;
return 0;
}
gets compiled without errors by the MS Visual Studio 2012 compiler, when the file has the .c extension. I have always thought that when you use the .c extension, compilation should be according to the C syntax, and not C++. Moreover, as far as I know auto without a type is allowed only in C++ since C++11, where it means that the type is deduced from the initializer.
Does that mean that my compiler isn't sticking to C, or is the code actually correct in C-language?
Answer
auto is an old C keyword that means "local scope". auto a is the same as auto int a, and because local scope is the default for a variable declared inside a function, it's also the same as int a in this example.
This keyword is actually a leftover from C's predecessor B, where there were no base types: everything was int, pointer to int, array of int.(*) Declarations would be either auto or extrn [sic]. C inherited the "everything is int" as a default rule, so you could declare integers with
auto a;
extern b;
static c;
ISO C got rid of this, but many compilers still accept it for backward compatibility. If it seems unfamiliar, then you should realise that a related rule is at work in
unsigned d; // actually unsigned int
which is still common in modern code.
C++11 reused the keyword, which few if any C++ programmers were using with the original meaning, for its type inference. This is mostly safe because the "everything is int" rule from C had already been dropped in C++98; the only thing that breaks is auto T a, which no-one was using anyway. (Somewhere in his papers on the history of the language, Stroustrup comments on this, but I can't find the exact reference right now.)
(*) String handling in B was interesting: you'd use arrays of int and pack multiple characters in each member. B was actually BCPL with different syntax.