Construction of a void Type?

问题:

I was given a piece of code that uses void() as an argument. The code doesn't compile... obviously?

Can we instantiate anything of type void? I believed the answer was no, with the exception of a void*. For example:

1. Writing the function void askVoid(void param) {} errors:

A parameter may not have void type

2. Writing the function void askNaught() {} and calling it with askNaught(void())` errors:

error C2660: takeNaught: function does not take 1 arguments

3. Writing the templatized function template <typename T> void takeGeneric(T param) {} and calling it with takeGeneric(void()) errors:

error C2893: Failed to specialize function template void takeGeneric(T)

4. Declaring void voidType errors:

Incomplete type is not allowed

5. Declaring auto autoVoid = void() errors:

Cannot deduce auto type

6. Declaring void* voidPtr works fine, but remove_pointer_t<decltype(voidPtr)> decltypeVoid errors:

error C2182: decltypeVoid: illegal use of type void

That's it, right? There is no place for void() in C++ is there? This is just bad code I've been given, right?

回答1:

The expression void() is a prvalue of type void and can be used anywhere such an expression may be used, which [basic.fundamental]/9 helpfully provides a list:

• As an expression-statement: void();
• As the second or third operand of a conditional operator: true ? throw 1 : void()
• As an operand of the comma operator: ++it1, void(), ++it2
• As the operand of decltype or noexcept: using my_void = decltype(void()); static_assert(noexcept(void()), "WAT");
• In a return statement of a function returning (possibly cv-qualified) void: const void f() { return void(); }
• As an operand of an explicit conversion to (possibly cv-qualified) void: static_cast<const void>(void())

An expression of type void can also be used as the operand of typeid, but void() in particular would be parsed as a type, not an expression, in this context.

回答2:

C++ (and I say C++, not C) allows (§6.6.3 comma 2) functions with void return type to return a void expression, that is:

void foo() { return void(); }

But notice it is not constructing a temporary void!

回答3:

You can take a void() as function parameter:

void test(void()) { ... }

Which expands to:

void test(void (*)())

Which is a function pointer to a method which returns void and takes no arguments.

Full example:

void abc() {}
void test(void()) { }

int main() {
    test(abc);
}

回答4:

You can use void() as a callable type, as an example std::function<void()> f; is a valid statement.

Moreover, as from 8.3.5/4:

A parameter list consisting of a single unnamed parameter of non-dependent type void is equivalent to an empty parameter list.

That means that this is valid:

template<typename T>
struct F;

template<typename R, typename... A>
struct F<R(A...)> { };

int main () {
   F<void(void)> s;
}

Here you are not instantiating anything of type void, but you are using it (let me say) as a parameter list of a callable type.

Not sure if this replies to your question, I've not clear what the question actually is.

回答5:

There is no place for void() in C++ is there?

As an expression, void() is valid in C++.

From the standard, $5.2.3/2 Explicit type conversion (functional notation) [expr.type.conv]:

The expression T(), where T is a simple-type-specifier or typename-specifier for a non-array complete object type or the (possibly cv-qualified) void type, creates a prvalue of the specified type, whose value is that produced by value-initializing (8.5) an object of type T; no initialization is done for the void() case.

From cppreference.com:

new_type ( )

If new_type is an object type, the object is value-initialized; otherwise, no initialization is done. If new_type is (possibly cv-qualified) void, the expression is a void prvalue.