Re: [std-proposals] pure virtual functions

[Douglas Boffey]

On Mon, Apr 20, 2015 at 5:52 PM, Columbo <r....@gmx.net> wrote:

Is there an objective reason that

    virtual void f() = 0 {};
    virtual void g() = 0 = default;

 

I don’t understand what you would expect these definitions to do.  To me, the ‘= 0’ indicating the functions are pure virtual and thus have no body, conflict with the {} or =default parts, which state otherwise.  Please could you elaborate.

 

should not be allowed? Is it possible to alter the definition of function-definition to allow pure-specifiers? It seems to me that parsers are fine with it,
and it's a hassle to define pure virtual destructors outside a class or class template. The changes necessary seem rather straightforward:
Change the grammar and make one or two semantical paragraphs (i.e. forbidding the use of pure-specifiers for anything but in-class virtual member function definitions).

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[Columbo]

(This is a double post. This thread did not appear until after I tried to post it a second time.)

[Nevin Liber]

On 23 April 2015 at 09:43, Douglas Boffey <douglas...@gmail.com> wrote:

On Mon, Apr 20, 2015 at 5:52 PM, Columbo <r....@gmx.net> wrote:

Is there an objective reason that

    virtual void f() = 0 {};
    virtual void g() = 0 = default;

 

I don’t understand what you would expect these definitions to do.  To me, the ‘= 0’ indicating the functions are pure virtual and thus have no body, conflict with the {} or =default parts, which state otherwise.  Please could you elaborate.

= 0 means that the base class is abstract and that the function must be overridden by the time we have a concrete derived class.

{} or = default gives an implementation to that function, which, for instance, may be called directly from a derived class.

They are not in conflict, and we can do that now.

Example:

struct Base {

    virtual void Alpha() = 0;

};

void Base::Alpha() { std::cout << "Base\n"; }

struct Derived : Base {

    void Alpha() override { std::cout << "Derived\n"; Base::Alpha(); }

};

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 Nevin ":-)" Liber  <mailto:ne...@eviloverlord.com>  (847) 691-1404