illegal void argument

Discussion in 'C++' started by Öö Tiib, Jan 29, 2014.

  1. Öö Tiib

    Öö Tiib Guest

    In C++ we can have void return type and we can "return" it:

    void foo();

    void bar()
    return (void)42; // ok #1
    return foo(); // ok #2

    Fun way to confuse novices indeed. However, it appears that we can not (for
    whatever unknown reason) pass void arguments:

    void bad()
    foo((void)42); // illegal

    Even if I think I will be extra clever and add overload of 'foo' that supposedly
    accepts anything ...

    void foo(...);

    .... then I get different failures or successes on different mac/ubuntu clang/gcc
    versions. Seems that compilers are confused.

    Is there reason why we have such inconsistency?
    Öö Tiib, Jan 29, 2014
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  2. Inconsistency? Returning from a 'void' function by calling another void
    function is syntactic sugar. How would you reconcile passing a void
    argument with overloading? What about conversions? I am too lazy to
    check (with a compiler or the Standard), maybe you know, what would
    happen if you do

    int someth();

    void foo() {
    return someth();

    ? Would it be OK? IOW, is it the same as

    void foo() {

    ? If it's OK, then any expression value *can* be converted to 'void'
    when used in a 'return' from a 'void' function. Would you allow the
    same for any expression converted as a single argument? Or does it have
    to be explicit?

    And the clincher for me is the question "what problem does it solve?"
    The ability to return an expression from a void function was added to
    help resolve some template generation problems. And in this case it
    seems that the sole purpose is obfuscation...

    Victor Bazarov, Jan 29, 2014
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  3. `(void)` as a formal argument list doesn't indicate a formal argument of
    type `void`. It's just a special syntax from C, where it specifies that
    the function really doesn't take *any* arguments, at all.

    In C++ the special syntax is unnecessary, since in C++ `()` also says
    that, but the special syntax is supported for C compatibility.

    Also baffling: why you can create a "void value" via `void()`. I guess
    for uniform treatment in template code. But still it's kind of
    inconsistent with the basic idea of `void` as an incomplete type.

    Cheers & hth.,

    - Alf
    Alf P. Steinbach, Jan 29, 2014
  4. I do not agree with you. In template programming it is essential to
    accept this syntax, e.g. for function adapters.
    This point is a different story. First, it have to be defined what
    should happen when void is passed in the parameter list. This has not
    yet been defined.

    Exactly. And maybe in conjunction with variadic templates at some time
    passing void to a function may become well defined.

    Marcel Müller, Jan 29, 2014
  5. Just my reference to the OP's comment, "Fun way to confuse novices".

    Victor Bazarov, Jan 29, 2014
  6. Öö Tiib

    Öö Tiib Guest

    Yes, if to use explicit void cast in return then it would work like that.
    The compiler may not implicitly cast to void.
    Yes it has to be explicit. The void cast is perhaps illusionary.
    My comment about "fun way to confuse novices" was may be too
    harsh but that is how it feels because there is *illusionary* usage of
    "values" of void type but such usage is only available for rather
    *narrow* set of cases (like that 'return' statement).

    Why ability to return is more essential for template programming than
    ability to "pass" it to other callable? Other problem is that passing void
    arguments does not seem to be even forbidden ... it seems to be
    Öö Tiib, Jan 30, 2014
  7. A function wrapper needs the return value type defined (or deduced) and
    if/when the call is forwarded to the wrapped object, the return value
    needs to be retained and returned to the caller [of the wrapper]. I am
    guessing (since I've not followed that particular development in the
    language) it to be the reason for introducing the ability to have a void
    expression in a return statement. If I were to invent a situation in
    which "passing" would be generically necessary, I can only think (right
    now) of a "broker" emulator, some mechanism that calls one side and then
    the other, while passing the return value of one to the other as its
    argument, without necessarily knowing (or caring) what that value is or
    where it exists (in the case of 'void'). Apparently the library
    designers didn't think it common enough a problem to introduce such a
    mechanism into the library. IOW, if you need one, you'd roll your own
    instead of instantiating some standard template...

    Victor Bazarov, Jan 30, 2014
  8. Öö Tiib

    Öö Tiib Guest

    I noticed this whole trouble when working with variadic templates.
    We can have rather variable list of parameters with rather variable types.
    Trouble is only with 'void' that represents several things: lack of object,
    lack of object's value, lack of object's type. On cases when 'void' value
    is legal it differs what it is by context; on several cases it is illegal and
    as actual argument it is undefined.
    Öö Tiib, Jan 31, 2014
  9. Can you please post with a short example where a variadic template's
    inability to pass 'void' as argument makes it hard for you to use it to
    accomplish your task?

    Victor Bazarov, Jan 31, 2014
  10. Öö Tiib

    Öö Tiib Guest

    I can accomplish anything with C++. It is just trouble when object has
    alleged member function (lets say for random example 'getOrigin()')
    but that is with 'void' return type. Needs to be extra checked or
    enable_iffed, no way to resolve it with some overload that accepts void
    arguments (because there are no such things); even the one with ellipsis
    parameter crashes some compilers.
    Öö Tiib, Jan 31, 2014
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