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joaquintides — Tue, 02 Jan 2007 17:45:01 GMT

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Hello Braden,
Having guard_impl accept its args by reference has some
drawbacks of its own; accepting T& would preclude us to
pass temporaries:
scope_guard guard = make_guard(do_something_with_an_int,10);
// error: cannot match to T& to int
Overloading guard_impl to accept both T& and const T& fixes
this, but a subtle problem is introduced then:
scope_guard guard = make_guard(do_something_with_an_int,10);
A reference to the temporary 10 is stored, but this
temporary is no longer valid by the time the scope guard
commits! So it was decided that taking args by value is
safer. When you need to really pass a reference, you can do
it by using Boost.Ref (http://boost.org/doc/html/ref.html):
#include <boost/ref.hpp>
T * obj = 0;
scope_guard guard = make_guard(delete_T, boost::ref(obj));
obj = create_T();
...
Hope this helps. One final warning: the implementation of
scope_guard inside Boost.MultiIndex is stored in the detail
directory, which means it's not provided for public
consumption and you cannot really rely on it being available
in a future release of Boost. I don't plan to remove it, but
to be really safe you should use your homegrown version (it
suffices to use a modified version of the implementaton in
B.MI appropriately placed in a namespace of your own) until
an official scope guard lib makes it into Boost.
Hope this helps,
JoaquÃn M LÃ³pez MuÃ±oz
TelefÃ³nica, InvestigaciÃ³n y Desarrollo

nobody — Wed, 03 Jan 2007 16:52:21 GMT

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You're right, of course. For some reason I thought boost::ref wouldn't work here; but indeed it does.
With regard to using stuff in a "detail" namespace, it's my understanding that boost doesn't make any guarantees about the consistency of its API between releases. Unless/until that changes, I think the practical value of the hint "detail" provides is only to segregate the intentional interface from the incidental one.

Boost C++ Libraries: Ticket #801: guard_impl objects should store references

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