Can reserved function names be overloaded?

Question

This question is a follow-up question of this one.

Consider the following program:

#include <cmath>

// meaningless, only for language-lawyer purpose
void abs(void*) {}

int main(){
    abs(nullptr);
}

Does this program result in undefined behavior?

---

The related part in the standard is [extern.names]/4:

Each function signature from the C standard library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage, or as a name of namespace scope in the global namespace.

I'm not sure whether overloading is permitted.

Muns

I don't know whether the behaviour is undefined, but it is certainly unexpected. – Dropsonde

That's a weird paragraph. It starts with signatures, which is quite clear, and then segues to names. – Burnett

@Ron int abs(int); is a function signature from the C Standard library declared with external linkage. The non-bold text says this signature is reserved; but then the bold part goes on to say that the signature is reserved as a name, which makes no sense – Despain

Some "functions" are sometimes implemented as marcos - htons for example. While not a "reserved" function that I know of, it highlights how poor some implementations can be. – Halfbreed

@UKMonkey: C++ specifically says the C names which can be implemented as functions or macros have to be functions in C++. However, htons is not a Standard C function, which is why it's not covered by that rule. – Kenakenaf

Why would you ever want to do something like this? – Allbee

@Allbee there are a number of questions about overriding std::to_string() for custom objects I imagine this has similar motivations – Halfbreed

@Halfbreed Nope. This question is not about overloading any methods in std. So your example doesn't apply. – Allbee

@Allbee Yup If you seriously can't think of a function that would be of use to overload in global namespace, and you can't understand the word "similar" Then I don't see how you can say doesn't apply. Maybe you mean "I can't think of any similar examples - could you give me one"? – Halfbreed

@Halfbreed No, I cannot think about such a thing. A user should add nothing to the global namespace except int main(). All other stuff can happily live in an anonymous namespace. – Allbee

@Allbee Well don't ask questions if you don't want to know the answer then. – Halfbreed

Answer 1

There are two parts to this statement, as it talks about names (from the C standard) that are reserved (for C++ implementations). In particular,

Part 1: Each function signature from the C standard library declared with external linkage

This includes the C library function abs

Part 2: is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage, or as a name of namespace scope in the global namespace.

So the name ::abs is reserved for the C++ implementation. You can't use it. Overloading is irrelevant.

Kenakenaf

If it means the name ::abs is reserved, then all the un-bold text in the original quote would be redundant. Not saying you're wrong but there still seems to be some mystery about this paragraph. – Despain

@M.M: Names with "C" linkage don't live precisely in the global namespace, IIRC. – Kenakenaf

abs is not a function signature, so cannot be referred to in Part 1. – Allbee

@Despain The un-bold text brings the constraint into non-global namespace. This is a related discussion. – Muns

For some reason everyone skips p3: "Each name from the C standard library declared with external linkage is reserved to the implementation for use as a name with extern "C" linkage, both in namespace std and in the global namespace." – Bonn

@Bonn That seems very relevant. But I don't understand it really. Does this imply that using namespace foo; if foo contains, say, abs(funny) is illegal in the global namespace? – Allbee

@Bonn Why is that relevant? I don't declare abs with C linkage. – Muns

@Walter: Why would that be the case? using namespace Foo only makes Foo::abs visible, it doesn't move Foo::abs to the global namespace. – Kenakenaf

The name is reserved for use by the implementation in (among other things) the global namespace. The use to which the implementation puts it is not relevant. – Bonn

Answer 2

tl;dr - yes, you can

http://www.eel.is/c++draft/reserved.names#extern.names

Pulling in the rest of the context:

20.5.4.3.2: If a program declares or defines a name in a context where it is reserved, other than as explicitly allowed by this Clause, its behavior is undefined.

And then also

20.5.4.3.3.2 Each global function signature declared with external linkage in a header is reserved to the implementation to designate that function signature with external linkage.

20.5.4.3.3.4 Each function signature from the C standard library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage,182 or as a name of namespace scope in the global namespace.

These suggest you can, as it is only the signature that is reserved.

Bonus for namespace ::std

http://www.eel.is/c++draft/library#namespace.std

20.5.4.2.1.1 Unless otherwise specified, the behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std.

http://en.cppreference.com/w/cpp/language/extending_std has summarised the exceptions

Halfbreed

There was no question of adding to namespace std, but to adding to the global namespace. – Allbee

@Allbee if the standard states explicitly that std can not be extended, but then doesn't say anything about the global namespace; and explicitly states that it is the function signatures that are reserved - then adding new ones in the global namespace isn't rejected by the standard... – Halfbreed

Do we need the part with std in this argumentation? – Allbee

@Allbee because the first parts suggest that you can overload any function in std too - which you can't, and I don't want this answer to suggest you can. – Halfbreed

This is a red herring. The question clearly related to the global namespace (only). – Allbee

But the specification says "or as a name". Note it says "as a function signature" in the first half part. – Muns

@Muns "or" - used to link alternatives. That means one OR the other. If they're a function - they're reserved as a function. If they're a name, they're reserved as a name. – Halfbreed

But the subject is "each function signature". Well, I agree with you that this paragraph is weird, as StorryTeller pointed out in the comment. – Muns

@Muns To be honest, not doing so is by far the safest thing; the c++ standard has been changing so fast in recent years the whole of the global namespace could end up reserved soon, but that wasn't your question ;) – Halfbreed