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." 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Domagoj Saric wrote:
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() ); }
Using a single type allows unrelated classes to compare equal with operator==.
"Peter Dimov" <pdimov@pdimov.com> wrote in message news:A062A9144EEA45339956BFD0D4FDC906@pdimov2...
Using a single type allows unrelated classes to compare equal with operator==.
Do'h :) Even if I thought of that I would have thought that the 'only-a-single-conversion-may-take-place' rule would apply...Thanks for the correction ;) ...but still the proposal can work, only in the usual template-uglified form: class my_class { public: bool some_is_valid_function() const; operator boost::safe_bool<my_class>::type() const { return boost::safe_bool<my_class>::make( some_is_valid_function() ); } //or by using a macro BOOST_SAFE_BOOL_FROM_FUNCTION( my_class, some_is_valid_function() ); //or by using a data member unsigned int size_; operator boost::safe_bool<my_class>::type() const { return boost::safe_bool<my_class>::make( size_ ); } BOOST_SAFE_BOOL_FROM_DATA( my_class, size_ ); } ...still less hassle than status quo... ...or maybe even more so if we take advantage of the fact that now we can use the CRTP idiom (or part of it): class my_class : boost::safe_bool<my_class> { public: bool some_is_valid_function() const; operator unspecified_bool_type() const { return make( 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." 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On 1 March 2010 15:36, Domagoj Saric <domagoj.saric@littleendian.com> wrote:
//or by using a macro BOOST_SAFE_BOOL_FROM_FUNCTION( some_is_valid_function() )
Is there a reason it can't just always look at operator!? I've been using a macro like this, since I don't need the workarounds: #define OPERATOR_SAFE_BOOL(for_type) \ typedef bool (for_type::*unspecified_bool_type)() const; \ operator unspecified_bool_type() const { \ return !*this ? 0 : &for_type::operator!; \ } so then it's just class my_class { public: bool operator!() const; OPERATOR_SAFE_BOOL(my_class) };
"Scott McMurray" <me22.ca+boost@gmail.com> wrote in message news:fa28b9251003011306k77a61047l4e0236ebbd6d8dc3@mail.gmail.com...
Is there a reason it can't just always look at operator!?
I've been using a macro like this, since I don't need the workarounds:
#define OPERATOR_SAFE_BOOL(for_type) \ typedef bool (for_type::*unspecified_bool_type)() const; \ operator unspecified_bool_type() const { \ return !*this ? 0 : &for_type::operator!; \ }
This forces classes to have operator! and it still suffers from efficiency issues outlined in the first post... -- "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
Domagoj Saric wrote:
"Scott McMurray" <me22.ca+boost@gmail.com> wrote in message news:fa28b9251003011306k77a61047l4e0236ebbd6d8dc3@mail.gmail.com...
Is there a reason it can't just always look at operator!?
I've been using a macro like this, since I don't need the workarounds:
#define OPERATOR_SAFE_BOOL(for_type) \ typedef bool (for_type::*unspecified_bool_type)() const; \ operator unspecified_bool_type() const { \ return !*this ? 0 : &for_type::operator!; \ }
This forces classes to have operator! and it still suffers from efficiency issues outlined in the first post...
I don't think the requirement for operator! is so bad (should certainly satisfy the majority of use cases, right?), and perhaps efficiency can be addressed by defining the "unspecified_bool_type" conversion in debug builds and a straight bool conversion in release builds for "problem" compilers...? Or perhaps some other preprocessor macro can direct whether to go with the type-safe version (default) or the efficient version (which may in fact be the same for some compilers)...? - Jeff
"Jeffrey Hellrung" <jhellrung@ucla.edu> wrote in message news:4B8F777C.6000202@ucla.edu...
I don't think the requirement for operator! is so bad (should certainly satisfy the majority of use cases, right?),
FWIW I don't remember writing one, ever... ;)
and perhaps efficiency can be addressed by defining the "unspecified_bool_type" conversion in debug builds and a straight bool conversion in release builds for "problem" compilers...? Or perhaps some other preprocessor macro can direct whether to go with the type-safe version (default) or the efficient version (which may in fact be the same for some compilers)...?
But there is no need for any of that...shared_ptr already has an efficient implementation... I just wrapped this implementation along with one of my one (that is perhaps slightly less 'clean' but easier on the optimizer) into a 'reusable' solution... -- "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
Domagoj Saric wrote:
But there is no need for any of that...shared_ptr already has an efficient implementation... I just wrapped this implementation along with one of my one (that is perhaps slightly less 'clean' but easier on the optimizer) into a 'reusable' solution...
User testimonial: in my own boost absed library when i have a pointer like class and i want the safe bool conversion, I make it so my members are named in such a way tha including the shared_ptr safe_bool header is all I need. Never had any problem
"Joel Falcou" <joel.falcou@lri.fr> wrote in message news:4B8FAD36.1020500@lri.fr...
User testimonial: in my own boost absed library when i have a pointer like class and i want the safe bool conversion, I make it so my members are named in such a way tha including the shared_ptr safe_bool header is all I need. Never had any problem
Isn't that actually yet another 'pointer' that a clean, public, reusable utility component is needed for this 'age old' idiom? -- "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
On Fri, Mar 5, 2010 at 9:18 AM, Domagoj Saric < domagoj.saric@littleendian.com> wrote:
User testimonial: in my own boost absed library when i have a pointer
"Joel Falcou" <joel.falcou@lri.fr> wrote in message news:4B8FAD36.1020500@lri.fr... like
class and i want the safe bool conversion, I make it so my members are named in such a way tha including the shared_ptr safe_bool header is all I need. Never had any problem
Isn't that actually yet another 'pointer' that a clean, public, reusable utility component is needed for this 'age old' idiom?
Yes, it is more supporting data that we need a reusable bool conversion mechanism. I am finding that I need this in Boost.Range (since the implementation in here is sub-optimal). I am also finding in my other application development that a general utility for providing such a mechanism would be extremely useful. I intend to develop such a reusable Boost-ified macro and get it into the core of Boost. Any objections, or comments? Regards, Neil Groves
participants (7)
-
Domagoj Saric -
Domagoj Saric -
Jeffrey Hellrung -
Joel Falcou -
Neil Groves -
Peter Dimov -
Scott McMurray