Re: Compiler didn't error on function with no return statement

[David Brown <david dot brown at hesbynett dot no>]

On 21/06/13 16:40, Andy Falanga (afalanga) wrote:
> > Who said it's not legal C++?
>
> I inferred that.  Obviously, I shouldn't have.  David said that it
> was legal C and so I made the jump into not legal C++.
>
> > And you *do* get a warning with -Wreturn-type, but you don't get
> > an error because that would reject legal programs:
>
> My understanding was in error.  That is why I thought that I should
> get a warning message from the C++ compiler (without the extra step
> of an option to the compiler).
>
> > void f();
> >
> > bool g() { f(); }
> >
> > Is this an error?  What if f() never returns?
> >
> > In C++11 we could mark f() with [[noreturn]] (like GCC's
> > __attribute((noreturn))) but just because a function isn't marked
> > noreturn doesn't mean it does return.
>
> True, but that does seem to be rather poor design (I know you're
> giving me an example).  Probably, the language supports this because
> of some need which I've not run into and am unaware of its merits.
>
> Thanks for the info.
>
> Andy

Most embedded systems have something like this:

int main(void) {
	initialise_everything();
	while (true) {
		run_everything();
	}
}

Putting a "return 0;" at the end would be unreachable code, which you 
don't want.

Anyway, the point is that for various reasons (technical, historical, 
simplicity of implementation, or complexity of checking) there are 
plenty of things that are legal C but have undefined behaviour. 
Returning from a non-void function without specifying the return value 
is one such case.  And like many similar cases, gcc can warn about it in 
almost all circumstances - /if/ you enable the warning flags.

So learn to use -Wall, -Wextra, and many of the other flags.  They are a 
very easy way to keep your code safe and clean (well, /safer/ and 
/cleaner/), and they help catch many small bugs or typos.