Wednesday, July 09, 2008

return void

I thought it would be interesting to discuss a subtle C/C++ interview question I learned recently. Question is deceptively simple: "Can you write a return statement in a function that returns void?" The answer is "Yes! You can return void!" The following program seems to be a valid C/C++ program.

static void foo (void) { }
static void bar (void) {
return foo(); // Note this return statement.
}
int main (void) {
bar();
return 0;
}

I tested it using gcc4 and VS9. With -ansi and -pedantic compiler options for gcc, it throws just a warning pointing at line #5.

return_void.c:5: warning: return with a value, in function returning void

Although use of such a feature is not clear in a C program, it is particularly useful while using templates. Consider,

template <class T>
T FOO (void) {
return T(); // Default construction
}

template <class T>
T BAR (void) {
return FOO<T>(); // Syntactic consistency. Same for int, void and everything else.
}

int main (void) {
BAR<void>();
}

It suddenly makes sense when templates are in picture. Take home point: Syntactic consistency is of paramount importance for supporting generic programming and writing generic libraries.

12 comments:

Tarun Ramakrishna Elankath said...

This is a *lovely* blog with a great set of articles and pointers to modern C++ programming.

I come from a Java background and have only begun to recently write C++ API's and I realize that the modern C++ way of doing things (generic programming) is just *so* totally different from OOP.

It's no wonder that people find it obtuse. :)

Sumant said...

Thanks, I'm glad that you liked it.

Dascandy said...

Since when is this allowed? That could save me hundreds of lines of generic code, written for compilers hating me returning some other functions' void as my own...

In particular, do you have a iso14882 reference for this?

Sumant said...

@Dascandy - Section 6.6.3 [stmt.return] of ISO-14882 and the C++0x public draft says: "A return statement with an expression of type “cv void” can be used only in functions with a return type of cv void;"

Dascandy said...

I recall where it went wrong. I needed to be able to store the to-be-return-value somewhere and retrieve it too. Since you can't store a void (void *x = new void(); doesn't work) and you can't retrieve a void from it again (return *(void *)x; also doesn't work) you can't really use this generically.

And you can't even overload the function template that hides this, so I had to make another N overloads to hide that again.

Too bad they didn't pull this through all the way.

Zahid Ghadialy said...

Interesting post Sumant. One thing that always intrigues me is that why do interviews have such questions. I know of people who can clear these kind of interviews but they are not really good programmers and struggle with day to day coding.

Anyway, I will blog on this topic here:
http://advancedcppwithexamples.blogspot.com/

Viktor said...

Altough its an interesting problem, unless the job is finding bugs in a c++ compiler, it's a very strange interview question.

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Anonymous said...

I don't remember where i read it, but the possibility to return void was meant to ease writing programs that generate C code. i think I read it in "the design and evolution of c++", but am not sure.

Icila said...

I'm writing in C and I receive the " 'return' with a value, in function returning void" warning like you did in a program of my own, but my compiler refuses to compile with this warning. is there a way around it?

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