Question

Does the boost library provide an implementation of a safe bool idiom, so that I could derive my class from it?

If yes – where is it?

If no – what are my alternatives beyond implementing it myself?

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I found the following similar question: ” Is there a safe bool idiom helper in boost? ” and the accepted answer suggests using bool_testable<> in Boost.Operators.

Unfortunately, when I checked the boost manual I couldn’t find it there. Code using it fails to compile too.

I also stumbled on another SO question ” Was boost::bool_testable<> relocated or removed? ” and the comment there suggests that the bool_testable actually never made to any release version of the boost.

There is also an interesting article by Bjorn Karlsson on the subject which contains a code which could be copy-pasted into my project. I am hoping however, that there is a commonly accepted and maintained utility library (e.g. boost) that implements that already.

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For compatibility reasons, I do not want to rely on C++11.

Answer 1

I do not know of a commonly accepted utility library that provides the safe-bool idiom. There have been a few attempts within Boost, and they often result in debates about how to provide a safe-bool implementation (naming conventions, macros, inline includes, inheritance). As a result, there are at least three implementations existing within Boost, with only one of the implementations, Boost.Spirit.Classic’s safe_bool, designed for external use.

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Details and concepts for each implementation:

• Boost.Range’s safe_bool
  • Contained within the detail directory, so not explicitly designed for external use.
  • Implemented by using a template helper type and static member functions.
  • The safe-bool enabled class is expected to:
    • Provide an operator boost::range_detail::safe_bool< MemberPtr >::unspecified_bool_type() const member function that delegates to the static safe_bool::to_unspecified_bool() function.
• Boost.SmartPtr’s operator_bool:
  • Contained within the detail directory, so not explicitly designed for external use.
  • The header file is intended to be included directly within a class definition. See shared_ptr.hpp for an example.
  • Requires including boost/detail/workaround.hpp before including smart_ptr/detail/operator.hpp.
  • The surrounding safe-bool enabled class is expected to:
    • Provide a this_type type.
    • Provide a T type.
    • Provide a T* px member variable.
• Boost.Spirit.Classic’s safe_bool
  • Designed for external use.
  • Uses the CRTP pattern.
  • Designed to support base class chaining, allowing boost::spirit::class::safe_bool to be used without mandating multiple inheritance on the derived class.
  • The safe-bool enabled class is expected to:
    • Publicly derive from boost::spirit::classic::safe_bool< Derived >. If Derived already inherits from Base, then use boost::spirit::classic::safe_bool< Derived, Base >.
    • Provide a bool operator_bool() const member function.

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This example uses Boost 1.50. Each class should evaluate to true in boolean context if the integer passed to the constructor is greater than 0:

// Safe-bool idiom with Boost.Range.
#include <boost/range/detail/safe_bool.hpp>
class range_bool
{
public:
  range_bool( int x ) : x_( x ) {}
private:
  // None of these are required, but makes the implementation cleaner.
  typedef boost::range_detail::safe_bool< int range_bool::* > safe_bool_t;
  typedef safe_bool_t::unspecified_bool_type unspecified_bool_type;
  int dummy;
public:
  operator unspecified_bool_type() const
  {
    return safe_bool_t::to_unspecified_bool( x_ > 0, &range_bool::dummy );
  }
private:
  int x_;
};

// Safe-bool idiom with Boost.SmartPtr.
#include <boost/detail/workaround.hpp>
class smart_ptr_bool
{
public:
  smart_ptr_bool( int x ) { px = ( x > 0 ) ? &dummy : 0 ; }
private:
  typedef smart_ptr_bool this_type; // -.
  typedef int T;                    //   :- Required concepts when using
  T* px;                            // -'   smart_ptr's operator_bool.
private:
  T dummy; // Simple helper.
public:
  #include <boost/smart_ptr/detail/operator_bool.hpp>
};

// Safe-bool idiom with Boost.Spirit.
#include <boost/spirit/include/classic_safe_bool.hpp>
class spirit_bool: public boost::spirit::classic::safe_bool< spirit_bool >
{
public:
  spirit_bool( int x ) : x_( x ) {} 
public:
  // bool operator_bool() is required by the spirit's safe_bool CRTP.
  bool operator_bool() const { return x_ > 0; }
private:
  int x_;
};

#include <iostream>

int main()
{
  std::cout << "range_bool( -1 ):     " << range_bool( -1 )     << std::endl
            << "range_bool(  1 ):     " << range_bool(  1 )     << std::endl
            << "smart_ptr_bool( -1 ): " << smart_ptr_bool( -1 ) << std::endl
            << "smart_ptr_bool(  1 ): " << smart_ptr_bool(  1 ) << std::endl
            << "spirit_bool( -1 ):    " << spirit_bool( -1 )    << std::endl
            << "spirit_bool(  1 ):    " << spirit_bool(  1 )    << std::endl;
  return 0;
}

Resulting output:

range_bool( -1 ):     0
range_bool(  1 ):     1
smart_ptr_bool( -1 ): 0
smart_ptr_bool(  1 ): 1
spirit_bool( -1 ):    0
spirit_bool(  1 ):    1

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I do not know of any alternatives. When I have ran across safe-bool idioms, most of the implementations have been a copy-and-paste variants of the implementation provided in Bjorn Karlsson’s article.