Why can't an incomplete type be casted to void

2020-04-09 16:45发布

问题:

Why does the following code give the following error?

Why does the type need to be complete in order to be casted to void?

struct Incomplete;
class Class
{
    virtual void foo(Incomplete &incomplete)
    {
        (void) incomplete;
        throw std::logic_error("not implemented");
    }
};

Error:

error C2027: use of undefined type 'Incomplete'  
    see declaration of 'Incomplete'

回答1:

It's a change between C and C++, where Microsoft previously implemented the C rules. As noted in remyabel's answer, that has since been fixed.

In C, a cast to void, or simply using an expression as a statement by itself (as in incomplete;), still involves the lvalue-to-rvalue conversion. C calls it slightly differently, but it's the same conversion.

In C++, a cast to void, or simply using an expression as a statement by itself doesn't involve the lvalue-to-rvalue conversion. This is needed because C++ makes assignment operators return lvalues, so if the lvalue-to-rvalue conversion were applied, then

volatile int i;
i = 1;

would not merely store, it would also immediately load afterwards.

The lvalue-to-rvalue conversion requires a complete type, even if the value is then discarded, since otherwise, it's impossible to know how many bytes should be read.



回答2:

I don't see anything prohibiting this, quoting N4140:

§5.4/4 The conversions performed by

[...]

— a static_cast (5.2.9),

[...]

can be performed using the cast notation of explicit type conversion.

§5.2.9/5 Otherwise, the static_cast shall perform one of the conversions listed below. No other conversion shall be performed explicitly using a static_cast.

§5.2.9/6 Any expression can be explicitly converted to type cv void, in which case it becomes a discarded-value expression (Clause 5). [...]

It's most likely a bug as tested on Rextester, an online VS2013 compiler, but compiles in rise4fun, Microsoft's online compiler which is bleeding edge.