attribute_value.h

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#ifndef _BBM_ATTRIBUTE_VALUE_H_
#define _BBM_ATTRIBUTE_VALUE_H_
 
#include "concepts/attribute.h"
 
/************************************************************************/
/*! \file attribute_value.h
 
  \brief Helper methods for extracting the value of an attribute (according to
  concepts::attribute).
 
*************************************************************************/
 
namespace bbm {
 
  /**********************************************************************/
  /*! \brief return the value of an attribute, or if not an attribute the object
    *********************************************************************/
  template<typename T>
    decltype(auto) value(T&& t)
  {
    if constexpr (concepts::attribute<std::decay_t<T>>) return t.value();
    else return t;
  }
 
  
  /*** Implementation detail ***/
  namespace detail {
    template<typename T> struct attribute_value { using type = T; };
    template<typename T> requires bbm::concepts::attribute<T>
      struct attribute_value<T>
    {
      using type = typename std::decay_t<T>::type;
    };
  } // end detail namespace
 
  /**********************************************************************/
  /*! \brief return the type of value(t)
   **********************************************************************/
  template<typename T>
    using attribute_value_t = bbm::detail::attribute_value<std::decay_t<T>>::type;
 
} // end bbm namespace
 
#endif /* _BBM_ATTRIBUTE_VALUE_H_ */