[boost] safe bool operator

1 Mar 2010

      While examining source and binary code for the various
safe_bool/unspecified_bool_type implementations two issues can be noticed:
 - source code verbosity/repetition
 - binary code inefficiency

The first issue is made worse by the fact that even this simple idiom is broken
on some compilers ('pointed out' by workarounds in
smart_ptr/detail/operator_bool.hpp) so workarounds must also be
copied/'rediscovered'...

The second issue (related to the usual implementation with member function
pointers) has two sides, one is that taking the address of a function will force
it to be included in the resulting binary even if it is never used or is always
inlined and the second is that some compilers produce non-optimal code. For
example for the following source code:

boost::optional<int> test1;
boost::optional<int> test2;

__declspec( noinline )
void func()
{
    if ( test1 )
        test2 = *test1;
}

MSVC++ 8.0 and 9.0 will produce the following binary code
...
0F923A26  movzx       eax,byte ptr [test1 (0F968210h)]
0F923A2D  neg         eax
0F923A2F  sbb         eax,eax
0F923A31  test        eax,offset
boost::optional_detail::optional_base<int>::is_initialized (0F923B9Eh)
0F923A36  je          ...
...

but if the if line is changed to if ( test1.is_initialized() ) the following
code will be produced:
...
0F923A26  cmp         byte ptr [test1 (0F968210h)],0
0F923A2D  je          ...
...

At first I tried a solution that takes advantage of the fact that MSVC++ has
plain-pointer-sized/register-sized member function pointers for plain single
inheritance non virtual member functions and worked properly but than I
found old discussions about the same subject:
http://lists.boost.org/Archives/boost/2003/09/52846.php
http://lists.boost.org/Archives/boost/2002/10/38624.php
where the solution for MSVC++ was to use a pointer member data which gives the
same results as my initial solution...

Anyways I've attached a .hpp file with a draft implementation proposal (missing
the compiler workarounds from smart_ptr/smart_ptr/detail/operator_bool.hpp for
example) that uses both approaches...

It enables a one liner implementation for a safe_bool operator:

class my_class
{
public:
    bool some_is_valid_function() const;
    operator boost::unspecified_bool_type() const { return
          boost::make_safe_bool( some_is_valid_function() ); }

    //or by using a macro
    BOOST_SAFE_BOOL_FROM_FUNCTION( some_is_valid_function() )
}

--
"What Huxley teaches is that in the age of advanced technology, spiritual
devastation is more likely to come from an enemy with a smiling face than from
one whose countenance exudes suspicion and hate."
Neil Postman

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end

[boost] safe bool operator

Domagoj Saric