[boost] Header-only singletons (was Re: [config] RFC PR 82)

Subject: [boost] Header-only singletons (was Re: [config] RFC PR 82)
From: Vinnie Falco (vinnie.falco_at_[hidden])
Date: 2016-09-09 13:18:11

On Mon, Aug 1, 2016 at 4:13 PM, Vinícius dos Santos Oliveira
>...I want my library header only and Boost.System requires the category to
> be a single static object and it'll even use address comparison to test if the
> categories are the same:
> ...
> Is it guaranteed the above code will return the same object even if this
> code was included in different libraries used by the same program?

Sorry for the late reply. This header-only idiom works:

    inline
    boost::system::error_category const&
    my_category()
    {
        struct my_error_category : boost::system::error_category { ... };
        static my_error_category cat;
        return cat;
    }

This idiom also works:

    template<class = void>
    boost::system::error_category const&
    my_category()
    {
        struct my_error_category : boost::system::error_category { ... };
        static my_error_category cat;
        return cat;
    }

Justification:

>From the C++ standard (see https://isocpp.org/std/the-standard)

7.1.6 describes inline functions. It says they must exist in every TU
that they are used in, and must be the same in all TUs (no diagnostic
required).

7.1.6 p6 also states that the address of an (extern) inline function
shall be the same in all TUs.

Elsewhere, the standard specifies that inline functions are extern by default.

Empirical evidence also supports these claims, both of the
aforementioned idioms have been used extensively in my projects under
gcc, clang, and msvc.

This likely upcoming feature (maybe its already in?) further cements
support for global variables in header-only libraries:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4424.pdf

However, even in the absence of additional language features, the
ability to have global variables in header-only libraries exists
today, with nothing more than C++11 support.

Hope this helps!