Perfectly emulate nullptr

Question

I got tired of waiting for compiler support of nullptr (gcc 4.6 does but it's so new few distributions support it).

So as a stop gap until nullptr is fully supported I decided to emulate it. There are two examples of emulation: one from here, and one from wikibooks.

Of note, neither implementation mentions an operator ==. However, without one, the following code will not compile.

int* ptr = nullptr;
assert( ptr == nullptr ); // error here: missing operator ==

Is this operator == error a compiler bug?
Is operator == (and !=, <, <=, etc) needed to more perfectly emulate nullptr?
What else is different between an emulated nullptr and the real deal?

Why would you do this instead of `assert(ptr);` in the first place? — ildjarn, Jun 07 '11 at 17:23
@Neil Butterworth: That's not true. `nullptr` has to be part of the Standard because it only has value when everybody uses the same null pointer type- `std::nullptr_t`. If everyone implemented their own null pointer, how would you write a function that would accept a null pointer? You don't know what the null pointer type is. — Puppy, Jun 07 '11 at 17:28
@ildjarn: I used an assertion to clean up the code. My actual example was `std::remove( v.begin(), v.end(), nullptr );` which uses the equality operator. And the reason I wont just use `std::remove_if` is I don't want to. I want it to just work like the real `nullptr`. — deft_code, Jun 07 '11 at 17:41
Fair enough, I didn't think about implications within the standard library. — ildjarn, Jun 07 '11 at 23:20

score 3 · Accepted Answer · answered Jun 07 '11 at 17:34

3

You compiled it with C++0x compiler that failed for unknown reason. It compiles fine in C++03.

answered Jun 07 '11 at 17:34

Yakov Galka

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So, back to my original question. Is this error an effect of new c++0x language rules? Or is it a bug that was introduced into gcc when the other c++0x features were added? – deft_code Jun 07 '11 at 17:44
3

@deft_code: 1) your code is invalid C++0x because it uses the keyword nullptr. 2) I'm 90% sure it's a GCC bug 3) there already was a [similar bug](http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33990). – Yakov Galka Jun 07 '11 at 17:45
Great find with that bug. My [exact error case](http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33990#c4) is listed in the comments. In response to point 1 I've gotta say _no duh!_ It's obviously invalid in a fully compliant c++0x compiler. gcc 4.5 isn't fully compliant yet, and hence my need to emulate a keyword. – deft_code Jun 07 '11 at 17:57

score 1 · Answer 2 · answered Jun 07 '11 at 17:27

Yes, you should implement such a thing. I am, however, surprised that the implicit conversion operators aren't kicking in and allowing you to compare without providing an explicit operator.

template<typename T> bool operator==(T* ptr, nullptr_t null) {
    return ptr == 0;
}
template<typename C, typename R> bool operator==(R C::* ptr, nullptr_t null) {
    return ptr == 0;
}
// And the reverse

score 1 · Answer 3 · answered Jun 07 '11 at 17:28

It's actually mentioned in the official proposal from your first example reference:

Experiments with several popular existing compilers show that it generates poor and/or misleading compiler diagnostics for several of the common use cases described in section 2. (Examples include: “no conversion from „const ‟ to „int‟”; “no suitable conversion function from „const class ‟ to „int‟ exists”; “a template argument may not reference an unnamed type”; “no operator „==‟ matches these operands, operand types are: int == const class ”.) We believe that compilers will still need to add special knowledge of nullptr in order to provide quality diagnos- tics for common use cases.

So you should fill this gap yourself if the compiler doesn't yet.

That poor compiler diagnostics discussion is in reference to an unnamed `nullptr` type. However, the standards committee decided that `nullptr` would have the type `nullptr_t`. All of those cryptic error messages get a lot better when the `nullptr` class is named `nullptr_t`. — deft_code, Jun 07 '11 at 18:06

Perfectly emulate nullptr

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