What does `template<>` mean in front of a variable definition in C++98

Question

I'm recently working on some legacy codes written in C++98.

I am trying to get them compiled by C++17 compiler. A whole lot of warnings popped out as I was doing that. However, almost all the warnings were easily resolved. Except for this one:

struct Counter { Counter(int v) : m_val(v) {} int m_val; }; struct AClass { static Counter SEQ; }; template<> Counter AClass::SEQ = 0;

The C++17 compiler gave a warning "too many template headers for...(should be 0)" for the line template<> Counter AClass::SEQ = 0;. (https://godbolt.org/z/G845K3cvv)

But for C++98 compiler it was totally OK. (https://godbolt.org/z/xErje7T8o)

Now, in order to resolve the warning, I need a complete understanding as to what does the statement mean in the C++98 world.

My gut feeling is that it is a full template specialization. And the template argument is empty. So it is equivalent to Counter AClass::SEQ = 0; and template<> means nothing. (https://stackoverflow.com/a/4872816/1085251)

Can anyone tell me exactly what it means?

I don't think it's a Standard change. Might be a defect fix or or a bug in older compilers. Or since it's a warning and not a hard error, it might be something that's now reported and previously wasn't. — user4581301
– user4581301, Commented Sep 8, 2022 at 7:02
You get a warning with gcc 5.1 onwards. See demo. Also the program is ill-formed. — Richard
– Richard, Commented Sep 8, 2022 at 7:03
It's a template specialisation. I'd argue that the C++98 compiler was incorrect in not diagnosing an error since none of the types/objects in your sample are templates that need to be specialised. (I'd also guess the template<> is either a typo, or an artefact resulting from editing a templated type to produced non-templated type). You may wish to look at stackoverflow.com/questions/4872809/… — Peter
– Peter, Commented Sep 8, 2022 at 7:04
This is the emulation of inline static Counter SEQ = 0; in the class definition. — 3CEZVQ
– 3CEZVQ, Commented Sep 8, 2022 at 7:04

Richard · Accepted Answer · 2022-09-08 07:09:08Z

It seems like gcc 4.7.4 is unable to give a diagnostic. Note that from gcc 5.1 onwards, we get a diagnostic from gcc. Demo.

The given program is ill-formed in both C++17 as well as C++98 as you're trying to provide an explicit specialization when there is nothing to specialize(as there is no templated entity anywhere in the program and AClass is also not a class template).

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