Is there a difference between foo(void) and foo() in C++ or C?
C++CArgumentsC++ Problem Overview
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?
C++ Solutions
Solution 1 - C++
In C:
void foo()
means "a functionfoo
taking an unspecified number of arguments of unspecified type"void foo(void)
means "a functionfoo
taking no arguments"
In C++:
void foo()
means "a functionfoo
taking no arguments"void foo(void)
means "a functionfoo
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++).
Solution 2 - C++
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.
Solution 3 - C++
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:
>
> c > 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://stackoverflow.com/a/36292431/895245
Solution 4 - C++
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.