Is there a difference between foo(void) and foo() in C++ or C?

Question

Consider these two function definitions:

void foo() { }

void foo(void) { }

Is there any difference between these two? If not, why is the void argument there? Aesthetic reasons?

Forgiving

For C the Q/A is here – Saloma

Please note that most answers to this question are now outdated as per the C23 standard. – Copenhaver

Answer 1

Historical note: this answer applies to C17 and older editions. C23 and later editions treat void foo() differently.

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In C:

• void foo() means "a function foo taking an unspecified number of arguments of unspecified type"
• void foo(void) means "a function foo taking no arguments"

In C++:

• void foo() means "a function foo taking no arguments"
• void foo(void) means "a function foo taking no arguments"

By writing foo(void), therefore, we achieve the same interpretation across both languages and make our headers multilingual (though we usually need to do some more things to the headers to make them truly cross-language; namely, wrap them in an extern "C" if we're compiling C++).

Textile

But if C++ had required the void, then it could have avoided the "most vexing parse" problem. – Moonset

True, but there are so many other crappy parses in C++ there's no real point in kvetching about any one of them. – Textile

On a recent question, @James Kanze posted an interesting tidbit. Repost here to avoid losing it: the first versions of C did not allow to specify the number of parameters a function might take, thus void foo() was the only syntax to declare a function. When signatures where introduced, the C committee had to disambiguate the no-parameter from the old syntax, and introduced the void foo(void) syntax. C++ took it for the sake of compatibility. – Bothnia

Can you give me an example of C C90 and later where using void foo() instead of void foo(void) will produce a functional difference? I.e. I have been using the version without the void for many years and havent seen any problem, am I missing something? – Utilitarian

Ok, I understand a difference now, but what I will gain in C if I write void foo( void ) instead void foo() ? What is a practical reason to write void explicitly? – Needlepoint

You'll receive warnings, if not errors, if you ever mistakenly try to pass args to a no-args function. – Textile

I now understand why void might be placed in a non parameter accepting function. So would it be best practice to do so in all C++ functions that have no parameters? – Beedon

@AdrianMcCarthy: MVP is hardly a "problem". – Dais

@AdrianMcCarthy I don't think MVP would've been solved. What about std::vector<int> v(std::vector<int>()); or any combination of 2 types, constructing one directly in the constructor of the other. – Permittivity

@Utilitarian void foo() { if ( rand() ) foo(5); } compiles and runs (causing undefined behaviour unless you're very lucky), whereas void foo(void) with the same body would cause a compilation error. – Sparklesparkler

@MatthieuM. that is an interesting piece of information. I would love to have a link to read the question/text that you mention. Yeah, I know, 7 years later haha – Fork

extern "C" void foo(void) is NOT required, since functions are extern by default and by the very nature of it :) – Correct

If you want your C++ compiler to apply C name mangling rules to C declarations--and you do, if you want the linker to know what you're talking about--then you need extern "C" around the C names. – Textile

@Sparklesparkler You can't "cause" undefined behaviour. Either the program has well-defined behaviour, or it does not. It's a property, not an event. However, whether you see "strange things happening" as a result of executing a program whose behaviour is undefined is down to chance; perhaps that's what you meant? – Rambo

@AsteroidsWithWings It can be both. Another similar example to the above is if (getchar() == 'x') 1/0; , the behaviour is only undefined if execution reaches this line, and certain input is given. – Sparklesparkler

@Sparklesparkler It's still a property of the program; it seemed you were conflating "undefined behaviour" with "possible strange symptoms of undefined behaviour", since you posited the idea that you could somehow avoid it by being "very lucky" (which isn't true in the given example unless the source code changes!) – Rambo

@AsteroidsWithWings The "lucky" element depends on the return value of rand(); if it happens to return 0 then there is no undefined behaviour: having foo(5) in code that is never executed does not imply the program has UB. I'm not conflating UB with symptoms of UB. Maybe the getchar() example is better than the rand() example to demonstrate this point in order to avoid arguments about guarantees of the sequence of numbers generated by rand() – Sparklesparkler

@M.M. Oh, fair :P – Rambo

Answer 2

I realize your question pertains to C++, but when it comes to C the answer can be found in K&R, pages 72-73:

Furthermore, if a function declaration does not include arguments, as in

double atof();

that too is taken to mean that nothing is to be assumed about the arguments of atof; all parameter checking is turned off. This special meaning of the empty argument list is intended to permit older C programs to compile with new compilers. But it's a bad idea to use it with new programs. If the function takes arguments, declare them; if it takes no arguments, use void.

Uncaredfor

But the question is about definitions, in that case the relevant C rule is An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters. – Remainder

Answer 3

C++11 N3337 standard draft

There is no difference.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf

Annex C "Compatibility" C.1.7 Clause 8: declarators says:

8.3.5 Change: In C ++ , a function declared with an empty parameter list takes no arguments. In C, an empty parameter list means that the number and type of the function arguments are unknown.

Example:

int f();
// means int f(void) in C ++
// int f( unknown ) in C

Rationale: This is to avoid erroneous function calls (i.e., function calls with the wrong number or type of arguments).

Effect on original feature: Change to semantics of well-defined feature. This feature was marked as “obsolescent” in C.

8.5.3 functions says:

4. The parameter-declaration-clause determines the arguments that can be specified, and their processing, when the function is called. [...] If the parameter-declaration-clause is empty, the function takes no arguments. The parameter list (void) is equivalent to the empty parameter list.

C99

As mentioned by C++11, int f() specifies nothing about the arguments, and is obsolescent.

It can either lead to working code or UB.

I have interpreted the C99 standard in detail at: https://mcmap.net/q/14687/-is-it-better-to-use-c-void-arguments-quot-void-foo-void-quot-or-not-quot-void-foo-quot-duplicate

Exorcise

The syntax highlighting is off near "means int". Can you fix it? For instance, turned off inside comments or "void" more like here. – Chilcote

Answer 4

As of C23, C has adopted C++'s semantics for function declarations with empty parameter lists.

In C++ (all editions) and C (C23 and later):

• void foo() means "a function foo taking no arguments".
• void foo(void) means "a function foo taking no arguments".

Non-prototype function declarations (which is what void foo() used to be) have been removed from the C language.

This change was introduced by the proposal N2841: No function declarators without prototypes, which has the summary:

This removes the obsolescent support for function declarators without prototypes. The old syntax for function declarators without prototypes is instead given the C++ semantics.

Quoting the current C23 standard draft (N3096, Annex M.2 Fifth Edition):

Major changes in this fifth edition (__STDC_VERSION__ 202311L) include
...

• mandated function declarations whose parameter list is empty be treated the same as a parameter list which only contain a single void;

In both C and C++, void foo() and void foo(void) are now exactly equivalent, and mean "a function taking no arguments."

Kilar

Notably, non-prototype declarations/definitions were flagged as obsolescent in the C99 version of the standard (C99 6.11.6 and 6.11.7). So we had transit period of 24 years during which we should have replaced all () with (void). But as of C23, () is once again fine to use. And those who've lived underneath a rock for 25 years need not worry either. – Copenhaver

They could change also 5.1.2.2.1 writing both int main(void) { /* ... */ } and int main() { /* ... */ } or make a note about it, as per the other case. – Galvano

Answer 5

In C, you use a void in an empty function reference so that the compiler has a prototype, and that prototype has "no arguments". In C++, you don't have to tell the compiler that you have a prototype because you can't leave out the prototype.

Villosity

"prototype" means the argument list declaration and return type. I say this because "prototype" confused me as to what you meant at first. – Heterochromous