Pure virtual functions in C++11

Question

In C++98, the null pointer was represented by the literal 0 (or in fact any constant expression whose value was zero). In C++11, we prefer nullptr instead. But this doesn't work for pure virtual functions:

struct X
{
    virtual void foo() = nullptr;
};

Why does this not work? Would it not make total sense? Is this simply an oversight? Will it be fixed?

Answer 1

Because the syntax says 0, not expression or some other non-terminal matching nullptr.

For all the time only 0 has worked. Even 0L would be ill-formed because it does not match the syntax.

Edit

Clang allows = 0x0, = 0b0 and = 00 (31.12.2013). That is incorrect and should be fixed in the compiler, of course.

Answer 2

The = 0 notation for virtual functions wasn't literally "assign null" but rather a special notation which is actually deceptive: a pure virtual function can also be implemented.

With various context keywords it would make more sense to allow abstract rather than = nullptr and have abstract be a context keyword.

Answer 3

This is just how the grammar is defined, if we look at the draft C++ standard section 9.2 Class members the relevant grammar is as follows:

[...]
member-declarator:
 declarator virt-specifier-seqopt pure-specifieropt
[...]
pure-specifier:
  = 0
  ^^^

The grammar specifically indicates that a pure-specifier is = 0 and not an integer literal or expression, that does not seem to leave any wiggle room. If I attempt things like:

virtual void foo() = 0L;

or:

virtual void foo() = NULL ;

gcc tells me:

error: invalid pure specifier (only '= 0' is allowed) before ';' token

and clang says:

error: initializer on function does not look like a pure-specifier

Although the following does work in both:

#define bar 0
//...
virtual void foo() = bar;

It also seems like clang allows octal literal, hexadecmical literal and binary literal zero which is incorrect behavior.

Update

Apparently Visual Studio accepts NULL and any zero integer literal including 0L, 0x0, 00 etc... Although it does not accept nullptr.

Answer 4

= 0 has a fixed meaning there. It's not really an integer zero there. Therefore, you can't simply replace it like that.

Answer 5

The whole point of nullptr (or most of the point anyway) is that can only be assigned to (or used to initialize) pointers.

In this case, you're not initializing or assigning to a pointer, so it doesn't even make sense that you would be able to use it in this situation.

Answer 6

The = 0 syntax wasn't used to initialize a pointer, it was simply to indicate syntactically that the provided virtual was pure.

Hence the = 0 syntax for declaring pure virtuals is unchanged.

Answer 7

That doesn't mean it is a pointer, or that it has to be equal to nullptr.

= 0 is sufficient and means that the virtual function is to be pure.

Answer 8

The C++11 grammar only allows 0 here (and it doesn't mean a pointer). As nullptr is not 0, it fails. NULL only works when NULL is defined to be 0 (sometimes it's the case, but not always). Just use 0 here, or use following define (if you really want to use a null, when it's not a pointer).

#define VIRTUAL_NULL 0
struct X
{
    virtual void foo() = VIRTUAL_NULL;
};