Discussion on: C++ Template Specialization - Syntax Note

[Roger Zanoni]

Hello!

It works if you define it it outside class scope: godbolt.org/z/mL7JsO

I tried to find a reason in the standard for it to work like this but didn't find anything

[Ben Lovy]

Huh, interesting. Thanks!

[Jason C. McDonald]

Thanks for the insight, Roger. It's odd that this form of specialization, just limiting the valid argument types for a generic function, is not documented very well...or, really, at all as far as I can tell.

[Roger Zanoni]

Hello Jason, I forgot to post two bits of documentation I saved in my first reply:

14.7.3/2:
An explicit specialization shall be declared in a namespace enclosing the specialized template. An explicit specialization whose declarator-id is not qualified shall be declared in the nearest enclosing namespace of the template, or, if the namespace is inline (7.3.1), any namespace from its enclosing namespace set. Such a declaration may also be a definition. If the declaration is not a definition, the specialization may be defined later (7.3.1.2).

7.1.1/1:
At most one storage-class-specifier shall appear in a given decl-specifier-seq, except that thread_local
may appear with static or extern. If thread_local appears in any declaration of a variable it shall be
present in all declarations of that entity. If a storage-class-specifier appears in a decl-specifier-seq, there
can be no typedef specifier in the same decl-specifier-seq and the init-declarator-list of the declaration shall
not be empty (except for an anonymous union declared in a named namespace or in the global namespace,
which shall be declared static (9.5)). The storage-class-specifier applies to the name declared by each
init-declarator in the list and not to any names declared by other specifiers. A storage-class-specifier shall
not be specified in an explicit specialization (14.7.3) or an explicit instantiation (14.7.2) directive.

This code

class idksomefunctions {
public:
    idksomefunctions() = delete; // specify there should be no constructor

    template <typename T>
    static T myIdentityFunction(T val)
    {
        return val;
    }

    template static int myIdentityFunction(int val);
};

looks ok according to the first bit, but if I got it correctly, it fails because of this rule from the second one "A storage-class-specifier shall not be specified in an explicit specialization"

[Jason C. McDonald]

Delicious spec quotes. :)

Makes total sense. I've always done this sort of specialization in the .cpp file, but the transition to header-only on this library has forced me to consider some of the deeper technical implications.