[boost] Header-only Boost.System

[Ashish Singh via Boost]

Hi All,

In my application, I'm trying to use Boost ASIO via boost 1.66 and defining the following two macros in the Makefile. Basically trying to use the header-only approach.

CFLAGS += \
-DBOOST_ERROR_CODE_HEADER_ONLY \
-DBOOST_SYSTEM_NO_DEPRECATED
But then I came across this thread: https://lists.boost.org/Archives/boost/2017/10/239106.php
"In header only mode, multiple instances may appear and thus no
longer be proper singletons. Stuff breaks in this situation, badly."
Is the above applicable to 1.66 too?How can I safely use the header-only approach in boost 1.66?

Thanks,Ashish

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[Peter Dimov via Boost]

Ashish Singh wrote:
> Hi All,
>
> In my application, I'm trying to use Boost ASIO via boost 1.66 and defining the
> following two macros in the Makefile. Basically trying to use the header-only
> approach.
>
> CFLAGS += \
> -DBOOST_ERROR_CODE_HEADER_ONLY \
> -DBOOST_SYSTEM_NO_DEPRECATED
> But then I came across this thread:
> https://lists.boost.org/Archives/boost/2017/10/239106.php
> "In header only mode, multiple instances may appear and thus no longer be
> proper singletons. Stuff breaks in this situation, badly."
> Is the above applicable to 1.66 too?How can I safely use the header-only
> approach in boost 1.66?

Do you have to use Boost 1.66? System is header-only in Boost 1.69 and later.

[Peter Dimov via Boost]

Ashish Singh wrote:
> >Do you have to use Boost 1.66? System is header-only in Boost 1.69 and
> > later.
>

> For the time being, yes. The update is not planned yet.
>
>
> Is there a workaround that can be used in Boost 1.66 to achieve that?
> Or are there any consequences with the following approach that I'm thinking
> to use? :
>
>
> > -DBOOST_ERROR_CODE_HEADER_ONLY \
> > -DBOOST_SYSTEM_NO_DEPRECATED

Defining BOOST_ERROR_CODE_HEADER_ONLY is the correct way to switch
Boost.System 1.66 to header-only mode. Whether this will create problems
with duplicate category instances depends on whether and how your program
uses dynamic libraries that also use Boost.System and if so, whether these
libraries under POSIX use hidden visibility.

[Peter Dimov via Boost]

Ashish Singh wrote:
> I see.

>
> > Whether this will create problems
>
> > with duplicate category instances depends on whether and how your
> > program uses dynamic libraries that also use Boost.System and if so,
> > whether these libraries under POSIX use hidden visibility.
>

> So, does that also mean that the problems with duplicate category instances
> caused by a scenario like above, will NOT occur if header-only approach is
> used in Boost 1.69 and later?

Yes, that's correct. Boost 1.69 and later do not suffer from these issues.

[Peter Dimov via Boost]

Ashish Singh wrote:
> Hi Peter,
>
> Forgot to ask:
>
> For Boost 1.69 and later, does the "way" to use the header-only approach
> remain the same as below?
>
>
> CFLAGS += \
> -DBOOST_ERROR_CODE_HEADER_ONLY \
> -DBOOST_SYSTEM_NO_DEPRECATED

1.69 is always header-only, so the BOOST_ERROR_CODE_HEADER_ONLY
macro has no effect. It doesn't hurt to define it though.

[Ashish Singh via Boost]

>Do you have to use Boost 1.66? System is header-only in Boost 1.69 and later.

For the time being, yes. The update is not planned yet.

Is there a workaround that can be used in Boost 1.66 to achieve that?Or are there any consequences with the following approach that I'm thinking to use? :
> -DBOOST_ERROR_CODE_HEADER_ONLY \
> -DBOOST_SYSTEM_NO_DEPRECATED
Thanks,Ashish

[Ashish Singh via Boost]

I see.

> Whether this will create problems
> with duplicate category instances depends on whether and how your program
> uses dynamic libraries that also use Boost.System and if so, whether these
> libraries under POSIX use hidden visibility.

So, does that also mean that the problems with duplicate category instances caused by a scenario like above, will NOT occur if header-only approach is used in Boost 1.69 and later?

Thanks,Ashish

[Ashish Singh via Boost]

Thank you so much Peter, for your time and information!
My app does use couple of libraries which also use boost by headers.
Seems like updating the boost version to >= 1.69 can't be avoided anymore.
Thanks,Ashish

[Ashish Singh via Boost]

Hi Peter,
Forgot to ask:
For Boost 1.69 and later, does the "way" to use the header-only approach remain the same as below?

CFLAGS += \
-DBOOST_ERROR_CODE_HEADER_ONLY \
-DBOOST_SYSTEM_NO_DEPRECATED

Thanks,Ashish