Difference between foo(void) and foo()

Source: http://stackoverflow.com/

Problem:
Consider these two function definitions 
void foo(){ ... }
void foo(void){ ..... }
What is the difference between these two functions?

Hint: The answer depends whether this is C code or C++ code.



Solution:

Highlight the part between the * symbols for the answer.
* In C++ there is no difference: both declare a function taking no arguments.

In C they differ. void foo(void) declares a function taking no arguments - calling foo(42) is a compile error. void foo() declares a function with an unspecified (unprototyped) parameter list: the compiler accepts calls with any arguments, e.g. foo(66) compiles fine and the argument is simply ignored. So in C, foo() means "parameters not specified", not "no parameters". (Note: in a *definition*, void foo(){...} defines a no-argument function, but calls to it are still unchecked without a prototype; writing (void) is the correct habit in C.)

Solution by DETERMINANT from the comments, with the illustrative C/C++ examples from an Anonymous comment. *

Comments

  1. DETERMINANT11:33 PM, January 30, 2012

    The difference is this,
    foo(void) is the same in C/C++ a function with no arguments.
    foo() in c++ also means the same as above.
    foo() in c means that we there is not specified about the arguments of function foo() and about the type.

    ReplyDelete
  2. Unknown10:46 PM, February 02, 2012

    Taken from the book :The Complete Reference C++ - Page 626 - Edition #4

    ---
    int f();
    In C this means that the function f may or Might not have any parameters !!
    --
    ---
    In C++ "Super Set Of C"
    int f() is same as int f(void)
    because void parameter list is optional in C++
    But programmers put void inside () making it clear that no parameters are accepted by function !!




    Example ::

    C Code :
    #include
    void f()
    {
    puts("hello there");
    }
    int main()
    {
    f(66);
    }
    ----


    The above code will compile without error and print "hello there"



    C++ example:

    #include
    using namespace std;
    void f( )
    {
    cout<<"from f"<<endl;
    }
    int main()
    {
    f(3);
    return 0;
    }


    The above code will say that(compiler)
    too many arguments supplied !!
    ---

    ReplyDelete
  3. Pratik Poddar2:09 PM, September 09, 2026

    Correct. In C++, foo() and foo(void) are identical: no parameters. In C, foo() declares a function with UNSPECIFIED parameters (it can be called with any arguments, no checking), while foo(void) explicitly takes none. That is why C style guides insist on (void). (replied using AI)

    ReplyDelete
  4. Pratik Poddar2:10 PM, September 09, 2026

    Exactly right, and the example makes it concrete: in C, void f() { ... } followed by f(66) compiles silently, which is precisely the trap foo(void) protects against. Thanks for posting the reference excerpt. (replied using AI)

    ReplyDelete

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