Skip to main content

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) {
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) {

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.


Tarun 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.
Peter Bindels 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;"
Peter Bindels 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:
Gloinart said…
Altough its an interesting problem, unless the job is finding bugs in a c++ compiler, it's a very strange interview question.
Cool Stuff, can you check out my C++ Code Samples too?
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?
Unknown said…
ya this is good blog helping me to understand some truths of C++:)

Popular Content

Unit Testing C++ Templates and Mock Injection Using Traits

Unit testing your template code comes up from time to time. (You test your templates, right?) Some templates are easy to test. No others. Sometimes it's not clear how to about injecting mock code into the template code that's under test. I've seen several reasons why code injection becomes challenging. Here I've outlined some examples below with roughly increasing code injection difficulty. Template accepts a type argument and an object of the same type by reference in constructor Template accepts a type argument. Makes a copy of the constructor argument or simply does not take one Template accepts a type argument and instantiates multiple interrelated templates without virtual functions Lets start with the easy ones. Template accepts a type argument and an object of the same type by reference in constructor This one appears straight-forward because the unit test simply instantiates the template under test with a mock type. Some assertion might be tested in

Covariance and Contravariance in C++ Standard Library

Covariance and Contravariance are concepts that come up often as you go deeper into generic programming. While designing a language that supports parametric polymorphism (e.g., templates in C++, generics in Java, C#), the language designer has a choice between Invariance, Covariance, and Contravariance when dealing with generic types. C++'s choice is "invariance". Let's look at an example. struct Vehicle {}; struct Car : Vehicle {}; std::vector<Vehicle *> vehicles; std::vector<Car *> cars; vehicles = cars; // Does not compile The above program does not compile because C++ templates are invariant. Of course, each time a C++ template is instantiated, the compiler creates a brand new type that uniquely represents that instantiation. Any other type to the same template creates another unique type that has nothing to do with the earlier one. Any two unrelated user-defined types in C++ can't be assigned to each-other by default. You have to provide a

Multi-dimensional arrays in C++11

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together. It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array? int native[3][4]; std::array<std::array<int, 3>, 4> arr; No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is: std::array<std::array<int, 4>, 3> arr; That's quite annoying for