Boost.MultiIndex Key extraction reference"boost/multi_index/key_extractors.hpp" synopsis
"boost/multi_index/identity.hpp" synopsis
"boost/multi_index/member.hpp" synopsis
"boost/multi_index/mem_fun.hpp" synopsis
const_mem_funcv_mem_fun, cref_mem_fun and cvref_mem_funmem_funvolatile_mem_fun, ref_mem_fun and vref_mem_funconst_mem_fun_explicit and mem_fun_explicitBOOST_MULTI_INDEX_CONST_MEM_FUN and BOOST_MULTI_INDEX_MEM_FUN"boost/multi_index/global_fun.hpp" synopsis
"boost/multi_index/composite_key.hpp" synopsis
"boost/multi_index/key.hpp" synopsis
Key extraction classes are used by
key-based indices to
obtain the indexing keys from the elements of a multi_index_container.
A CopyConstructible and CopyAssignable
class KeyFromValue is said to be a key extractor from a
type Type if
KeyFromValue::result_type is defined,k1(ca) is defined and returns a value convertible
to const KeyFromValue::result_type&,k2 is a copy of k1, k1(ca) is the
same value as k2(ca),k1, k2 of type const KeyFromValue,
and ca of type const Type&.
Additionally, KeyFromValue is a read/write key extractor
if the following extra conditions are met:
k1(a) is defined and returns a value convertible
to KeyFromValue::result_type&,const_cast<const KeyFromValue::result_type&>(k1(a))
is the same value as
k1(const_cast<const Type&>(a)),k1 of type const KeyFromValue and
a of type Type&.
Boost.MultiIndex provides twelve general-purpose key extractors:
identity,member,const_mem_fun
(and three variants),
mem_fun
(and three variants),
global_fun andcomposite_key,key
allowing for a very terse specification of the previous extractors
(except identity).
The key extractors provided by Boost.MultiIndex are templatized according
to the type Type and serve to extract keys not only from objects
of type Type, but also from reference wrappers provided by
Boost.Ref and from chained pointers
to Type (or to reference wrappers of Type): a chained pointer
is any type P such that, for an object p of type
const P
*p yields an object of type Type& or
boost::reference_wrapper<Type>, OR*p yields a chained pointer to Type,Type& or
boost::reference_wrapper<Type>.
"boost/multi_index/key_extractors.hpp" synopsis
#include <boost/multi_index/identity.hpp> #include <boost/multi_index/member.hpp> #include <boost/multi_index/mem_fun.hpp> #include <boost/multi_index/global_fun.hpp> #include <boost/multi_index/composite_key.hpp>
This header includes all the key extractors provided by Boost.MultiIndex.
"boost/multi_index/identity.hpp" synopsisnamespace boost{ namespace multi_index{ template<typename T> struct identity; } // namespace boost::multi_index } // namespace boost
identity
identity is a Key Extractor
that acts as a do-nothing identity functor.
template<typename Type> struct identity { typedef Type result_type; // only provided if const ChainedPtr& is not convertible to const Type& template<typename ChainedPtr> Type& operator()(const ChainedPtr& x)const; const Type& operator()(const Type& x)const; Type& operator()(Type& x)const; // only provided if Type is non-const // only provided if Type is non-const const Type& operator()(const reference_wrapper<const Type>& x)const; // only provided if Type is const Type& operator()( const reference_wrapper<typename remove_const<Type>::type>& x)const; Type& operator()(const reference_wrapper<Type>& x)const; };
identity<Type> is a model of:
Key Extractor
from Type,Key Extractor
from reference_wrapper<const Type>,Key Extractor
from reference_wrapper<Type>,Key Extractor
from any chained pointer to
const Type,Key Extractor
from any chained pointer
to Type.identity memberstemplate<typename ChainedPtr> Type& operator()(const ChainedPtr& x)const;
Requires:ChainedPtris a chained pointer type toType.
Returns: a reference to the object chained-pointed to byx.
const Type& operator()(const Type& x)const;
Returns: x.
Type& operator()(Type& x)const;
Returns: x.
const Type& operator()(const reference_wrapper<const Type>& x)const;
Returns: x.get().
Type& operator()(const reference_wrapper<typename remove_const<Type>::type>& x)const;
Returns: x.get().
Type& operator()(const reference_wrapper<Type>& x)const;
Returns: x.get().
"boost/multi_index/member.hpp" synopsisnamespace boost{ namespace multi_index{ template<class Class,typename Type,Type Class::*PtrToMember> struct member; template<class Class,typename Type,std::size_t OffsetOfMember> struct member_offset; // deprecated #define BOOST_MULTI_INDEX_MEMBER(Class,Type,MemberName) implementation defined } // namespace boost::multi_index } // namespace boost
member
member is a Key Extractor
aimed at accessing a given member of a class.
template<class Class,typename Type,Type Class::*PtrToMember> struct member { typedef Type result_type; // only provided if const ChainedPtr& is not convertible to const Class& template<typename ChainedPtr> Type& operator()(const ChainedPtr& x)const; const Type& operator()(const Class& x)const; Type& operator()(Class& x)const; // only provided if Type is non-const const Type& operator()(const reference_wrapper<const Class>& x)const; Type& operator()(const reference_wrapper<Class>& x)const; };
The PtrToMember template argument specifies the particular
Type Class::* pointer to the member to be extracted.
member<Class,Type,PtrToMember> is a model of:
Key Extractor
from Class,Key Extractor
from reference_wrapper<const Class>,Key Extractor
from reference_wrapper<Class>,Key Extractor
from any chained pointer
to const Class,Key Extractor
from any chained pointer
to Class.member memberstemplate<typename ChainedPtr> Type& operator()(const ChainedPtr& x)const;
Requires:ChainedPtris a chained pointer type toType.
Returns: a reference to the object chained-pointed to byx.
const Type& operator()(const Class& x)const;
Returns: x.*PtrToMember.
Type& operator()(Class& x)const;
Returns: x.*PtrToMember.
const Type& operator()(const reference_wrapper<const Class>& x)const;
Returns: x.get().*PtrToMember.
Type& operator()(const reference_wrapper<Class>& x)const;
Returns: x.get().*PtrToMember.
member_offset
member_offset was designed to overcome limitations of some legacy
compilers and its use is currently deprecated. Refer to a
former version
of Boost.MultiIndex for further information.
BOOST_MULTI_INDEX_MEMBERBOOST_MULTI_INDEX_MEMBER(Class,Type,MemberName)
This macro was designed as a portability mechanism for legacy compilers where member
could not be supported.
As such it is no longer needed in modern environments, though some users might still prefer it
to plain member because it provides a slightly more concise syntax.
"boost/multi_index/mem_fun.hpp" synopsisnamespace boost{ namespace multi_index{ template<class Class,typename Type,Type (Class::*PtrToMemberFunction)()const> struct const_mem_fun; template< class Class,typename Type,Type (Class::*PtrToMemberFunction)()const volatile > struct cv_mem_fun; template<class Class,typename Type,Type (Class::*PtrToMemberFunction)()const&> struct cref_mem_fun; template< class Class,typename Type,Type (Class::*PtrToMemberFunction)()const volatile& > struct cvref_mem_fun; template<class Class,typename Type,Type (Class::*PtrToMemberFunction)()> struct mem_fun; template<class Class,typename Type,Type (Class::*PtrToMemberFunction)()volatile> struct volatile_mem_fun; template<class Class,typename Type,Type (Class::*PtrToMemberFunction)()&> struct ref_mem_fun; template< class Class,typename Type,Type (Class::*PtrToMemberFunction)()volatile& > struct vref_mem_fun; template< class Class,typename Type, typename PtrToMemberFunctionType,PtrToMemberFunctionType PtrToMemberFunction > struct const_mem_fun_explicit; // deprecated template< class Class,typename Type, typename PtrToMemberFunctionType,PtrToMemberFunctionType PtrToMemberFunction > struct mem_fun_explicit; // deprecated #define BOOST_MULTI_INDEX_CONST_MEM_FUN(Class,Type,MemberFunName) \ implementation defined #define BOOST_MULTI_INDEX_MEM_FUN(Class,Type,MemberFunName) \ implementation defined } // namespace boost::multi_index } // namespace boost
const_mem_fun
const_mem_fun is a Key Extractor
returning as key the result of invoking a given constant member function of a class.
template<class Class,typename Type,Type (Class::*PtrToMemberFunction)()const> struct const_mem_fun { typedef typename remove_reference<Type>::type result_type; // only provided if const ChainedPtr& is not convertible to const Class& template<typename ChainedPtr> Type operator()(const ChainedPtr& x)const; Type operator()(const Class& x)const; Type operator()(const reference_wrapper<const Class>& x)const; Type operator()(const reference_wrapper<Class>& x)const; };
The PtrToMemberFunction template argument specifies the particular
Type (Class::*PtrToMemberFunction)()const pointer to the constant
member function used in the extraction.
const_mem_fun<Class,Type,PtrToMemberFunction> is a model of:
Key Extractor
from Class,Key Extractor
from reference_wrapper<const Class>,Key Extractor
from reference_wrapper<Class>,Key Extractor
from any chained pointer
to const Class,Key Extractor
from any chained pointer
to Class.const_mem_fun memberstemplate<typename ChainedPtr> Type operator()(const ChainedPtr& x)const;
Requires:ChainedPtris a chained pointer type toType.
Returns:(y.*PtrToMemberFunction)(), whereyis the object chained-pointed to byx.
Type operator()(const Class& x)const;
Returns: (x.*PtrToMemberFunction)().
Type operator()(const reference_wrapper<const Class>& x)const;
Returns: (x.get().*PtrToMemberFunction)().
Type operator()(const reference_wrapper<Class>& x)const;
Returns: (x.get().*PtrToMemberFunction)().
cv_mem_fun, cref_mem_fun and cvref_mem_fun
These Key Extractors are variants of
const_mem_fun used in the case that the
passed member function is:
const volatile (cv_mem_fun),const &-qualified (cref_mem_fun),const volatile &-qualified (cvref_mem_fun).const_mem_fun.
mem_fun
mem_fun is a Key Extractor
returning as key the result of invoking a given member function of a class.
template<class Class,typename Type,Type (Class::*PtrToMemberFunction)()> struct mem_fun { typedef typename remove_reference<Type>::type result_type; // only provided if ChainedPtr& is not convertible to Class& template<typename ChainedPtr> Type operator()(const ChainedPtr& x)const; Type operator()(Class& x)const; Type operator()(const reference_wrapper<Class>& x)const; };
The PtrToMemberFunction template argument specifies the particular
Type (Class::*PtrToMemberFunction)() pointer to the member
function used in the extraction.
mem_fun<Class,Type,PtrToMemberFunction> is a model of:
Key Extractor
from reference_wrapper<Class>,Key Extractor
from any chained pointer
to Class.mem_fun memberstemplate<typename ChainedPtr> Type operator()(const ChainedPtr& x)const;
Requires:ChainedPtris a chained pointer type toType.
Returns:(y.*PtrToMemberFunction)(), whereyis the object chained-pointed to byx.
Type operator()(Class& x)const;
Returns: (x.*PtrToMemberFunction)().
Type operator()(const reference_wrapper<Class>& x)const;
Returns: (x.get().*PtrToMemberFunction)().
volatile_mem_fun, ref_mem_fun and vref_mem_fun
These Key Extractors are variants of
mem_fun used in the case that the
passed member function is:
volatile (volatile_mem_fun),&-qualified (ref_mem_fun),volatile &-qualified (vref_mem_fun).mem_fun.
const_mem_fun_explicit and mem_fun_explicitThese extractors were provided as a workaround for MSVC++ 6.0 and are now deprecated. Refer to a former version of Boost.MultiIndex for further information.
BOOST_MULTI_INDEX_CONST_MEM_FUN
and BOOST_MULTI_INDEX_MEM_FUNBOOST_MULTI_INDEX_CONST_MEM_FUN(Class,Type,MemberFunName) BOOST_MULTI_INDEX_MEM_FUN(Class,Type,MemberFunName)
Portability macros for usage of const_mem_fun and mem_fun.
Although no longer needed in modern compilers, some users might still decide to
resort to them as they provide a slightly more concise syntax.
"boost/multi_index/global_fun.hpp" synopsisnamespace boost{ namespace multi_index{ template<class Value,typename Type,Type (*PtrToFunction)(Value)> struct global_fun; } // namespace boost::multi_index } // namespace boost
global_fun
global_fun is a Key Extractor
based on a given global or static member function accepting the base type as argument
and returning the associated key.
template<class Value,typename Type,Type (*PtrToFunction)(Value)> struct global_fun { typedef typename remove_reference<Type>::type result_type; // Only provided under the following circumstances: // - If Value is a reference to a constant type, only provided // when const ChainedPtr& is not convertible to Value; // - if Value is a reference to a non-const type, only provided // when ChainedPtr& is not convertible to Value; // - else, only provided when const ChainedPtr& is not // convertible to const Value&. Type operator()(const ChainedPtr& x)const; // only provided if Value is a reference type Type operator()(Value x)const; // only provided if Value is not a reference type Type operator()(const Value& x)const; // only provided if Value is not a reference type Type operator()(const reference_wrapper<const Value>& x)const; // only provided if Value is a reference type Type operator()( const reference_wrapper< remove_reference<Value>::type>& x)const; // only provided if Value is not a reference type or is // a reference to a constant type Type operator()( const reference_wrapper< typename remove_const< typename remove_reference<Value>::type>::type>& x)const; };
PtrToFunction specifies the particular function used to extract
the key of type Type from some BaseType.
global_fun supports the following function signatures:
Type f(BaseType) (Value is BaseType),Type f(const BaseType&) (Value is const BaseType&),Type f(BaseType&) (Value is BaseType&).global_fun<Type,Value,PtrToFunction> is a model of:
Key Extractor
from reference_wrapper<BaseType>,Key Extractor
from any chained pointer
to BaseType.Value is BaseType or
const BaseType&,
global_fun<Type,Value,PtrToFunction> is also a model of:
Key Extractor
from BaseType,Key Extractor
from reference_wrapper<const BaseType>,Key Extractor
from any chained pointer
to const BaseType.global_fun memberstemplate<typename ChainedPtr> Type operator()(const ChainedPtr& x)const;
Requires:ChainedPtris a chained pointer type toValue.
Returns:PtrToFunction)(y), whereyis the object chained-pointed to byx.
Type operator()(Value x)const;
Returns: PtrToFunction(x).
Type operator()(const Value& x)const;
Returns: PtrToFunction(x).
Type operator()(const reference_wrapper<const Value>& x)const;
Returns: PtrToFunction(x.get()).
Type operator()(
const reference_wrapper<remove_reference<Value>::type>& x)const;
Returns: PtrToFunction(x.get()).
Type operator()(
const reference_wrapper<
typename remove_const<
typename remove_reference<Value>::type>::type>& x)const;
Returns: PtrToFunction(x.get()).
"boost/multi_index/composite_key.hpp" synopsisnamespace boost{ namespace multi_index{ template<typename Value,typename KeyFromValue0,...,typename KeyFromValuen> struct composite_key; template<typename CompositeKey> struct composite_key_result; // comparison operators for composite_key_result: // OP is any of ==,<,!=,>,>=,<= template<typename CompositeKey1,typename CompositeKey2> bool operator OP( const composite_key_result<CompositeKey1>& x, const composite_key_result<CompositeKey2>& y); template<typename CompositeKey,typename... Values> bool operator OP( const composite_key_result<CompositeKey>& x, const std::tuple<Values...>& y); template<typename CompositeKey,typename... Values> bool operator OP( const std::tuple<Values...>& x, const composite_key_result<CompositeKey>& y); template<typename CompositeKey,typename Value0,...,typename Valuen> bool operator OP( const composite_key_result<CompositeKey>& x, const boost::tuple<Value0,...,Valuen>& y); template<typename Value0,...,typename Valuen,typename CompositeKey> bool operator OP( const boost::tuple<Value0,...,Valuen>& x, const composite_key_result<CompositeKey>& y); // equality functors: template<typename Pred0,...,typename Predn> struct composite_key_equal_to; template<typename CompositeKeyResult> struct composite_key_result_equal_to; // deprecated // comparison functors: template<typename Compare0,...,typename Comparen> struct composite_key_compare; template<typename CompositeKeyResult> struct composite_key_result_less; // deprecated template<typename CompositeKeyResult> struct composite_key_result_greater; // deprecated // hash functors: template<typename Hash0,...,typename Hashn> struct composite_key_hash; template<typename CompositeKeyResult> struct composite_key_result_hash; // deprecated } // namespace boost::multi_index } // namespace boost // specializations of external functors for composite_key_result: namespace std{ template<typename CompositeKey> struct equal_to<boost::multi_index::composite_key_result<CompositeKey> >; template<typename CompositeKey> struct less<boost::multi_index::composite_key_result<CompositeKey> >; template<typename CompositeKey> struct greater<boost::multi_index::composite_key_result<CompositeKey> >; } // namespace std namespace boost{ template<typename CompositeKey> struct hash<boost::multi_index::composite_key_result<CompositeKey> >; } // namespace boost
composite_key
composite_key is a Key Extractor
returning the combined value of several key extractors whose type is specified
at compile time. The returned object is of type
composite_key_result<composite_key>.
template<typename Value,typename KeyFromValue0,...,typename KeyFromValuen> struct composite_key { typedef boost::tuple<KeyFromValue0,...,KeyFromValuen> key_extractor_tuple; typedef Value value_type; typedef composite_key_result<composite_key> result_type; composite_key( const KeyFromValue0& k0=KeyFromValue0(), ... const KeyFromValuen& kn=KeyFromValuen()); composite_key(const key_extractor_tuple& x); const key_extractor_tuple& key_extractors()const; key_extractor_tuple& key_extractors() // only provided if const ChainedPtr& is not convertible to const value_type& template<typename ChainedPtr> result_type operator()(const ChainedPtr& x)const; result_type operator()(const value_type& x)const; result_type operator()(const reference_wrapper<const value_type>& x)const; result_type operator()(const reference_wrapper<value_type>& x)const; };
KeyFromValue0, ... , KeyFromValuen are the types of
the key extractors combined into the composite key. Each of these types
must be a Key Extractor from
Value. At least a key extractor must be provided. The maximum
number of key extractors of a composite_key instantiation is
implementation defined. composite_key internally stores an
object of every constituent key extractor type.
composite_key<Value,KeyFromValue0,...,KeyFromValuen> is a model
of:
Key Extractor
from Value,Key Extractor
from reference_wrapper<const Value>,Key Extractor
from reference_wrapper<Value>,Key Extractor
from any chained pointer
to const Value,Key Extractor
from any chained pointer
to Value.composite_key memberscomposite_key(
const KeyFromValue0& k0=KeyFromValue0(),
...
const KeyFromValuen& kn=KeyFromValuen());
Effects: Constructs a composite_key that stores
copies of the key extractor objects supplied.
composite_key(const key_extractor_tuple& x);
Effects: Constructs acomposite_keythat stores copies of the key extractor objects supplied inx.
const key_extractor_tuple& key_extractors()const;
Returns: a constant reference to a tuple holding the
key extractors internally stored by the composite_key.
key_extractor_tuple& key_extractors();
Returns: a reference to a tuple holding the
key extractors internally stored by the composite_key.
template<typename ChainedPtr>
result_type operator()(const ChainedPtr& x)const;
Requires:ChainedPtris a chained pointer type toresult_type.
Returns: aresult_typeobject dependent on*thisandy, whereyis the object chained-pointed to byx.
result_type operator()(const value_type& x)const;
Returns: aresult_typeobject dependent on*thisandx.
result_type operator()(const reference_wrapper<const value_type>& x)const;
Returns: aresult_typeobject dependent on*thisandx.get().
result_type operator()(const reference_wrapper<value_type>& x)const;
Returns: aresult_typeobject dependent on*thisandx.get().
composite_key_result
This is an opaque type returned by composite_key
instantiations as their extracted key.
template<typename CompositeKey> struct composite_key_result { no public interface available }; // comparison: // OP is any of ==,<,!=,>,>=,<= template<typename CompositeKey1,typename CompositeKey2> bool operator OP( const composite_key_result<CompositeKey1>& x, const composite_key_result<CompositeKey2>& y); template<typename CompositeKey,typename... Values> bool operator OP( const composite_key_result<CompositeKey>& x, const std::tuple<Values...>& y); template<typename CompositeKey,typename... Values> bool operator OP( const std::tuple<Values...>& x, const composite_key_result<CompositeKey>& y); template<typename CompositeKey,typename Value0,...,typename Valuen> bool operator OP( const composite_key_result<CompositeKey>& x, const boost::tuple<Value0,...,Valuen>& y); template<typename Value0,...,typename Valuen,typename CompositeKey> bool operator OP( const boost::tuple<Value0,...,Valuen>& x, const composite_key_result<CompositeKey>& y);
CompositeKey is the composite_key instantiation to
which the composite_key_result type is associated. Objects of type
composite_key_result returned by a composite key must be always treated
as temporary, i.e. they should not be stored or copied.
composite_key_result is not guaranteed to be
DefaultConstructible or CopyAssignable.
Every object of type composite_key_result<CompositeKey> is
internally associated to the CompositeKey from which it is returned
and the object of type CompositeKey::value_type to which the
composite key was applied.
Given an x of type composite_key_result<CompositeKey>,
we use the following notation:
ck(x) is the CompositeKey object associated to
x,v(x) is the object of type CompositeKey::value_type
associated to x,ki(x) = ck(x).key_extractors().get<i>(),
that is, is the i-th key extractor of ck(x),xi = ki(x)(v(x)), that is, the
key extracted from v(x) by the i-th key extractor,length(x) is the number of key extractors of ck(x).y is an std::tuple or boost::tuple of values, we define:
yi=get<i>(y),length(y) is the number of elements of y.template<typename CompositeKey1,typename CompositeKey2>
bool operator==(
const composite_key_result<CompositeKey1>& x,
const composite_key_result<CompositeKey2>& y);
template<typename CompositeKey,typename... Values>
bool operator==(
const composite_key_result<CompositeKey>& x,
const std::tuple<Values...>& y);
template<typename CompositeKey,typename... Values>
bool operator==(
const std::tuple<Values...>& x,
const composite_key_result<CompositeKey>& y);
template<typename CompositeKey,typename Value0,...,typename Valuen>
bool operator==(
const composite_key_result<CompositeKey>& x,
const boost::tuple<Value0,...,Valuen>& y);
template<typename Value0,...,typename Valuen,typename CompositeKey>
bool operator==(
const boost::tuple<Value0,...,Valuen>& x,
const composite_key_result<CompositeKey>& y);
Requires:length(x)==length(y). The expressionxi==yiis valid for alliin[0,length(x)).
Returns:trueif and only ifComplexity: No more key extraction operations and comparisons are performed than those necessary for the evaluation of the expression above, starting atxi==yifor alliin[0,length(x)).i==0. The evaluation is short-circuited as soon as the result is determined to befalse.
template<typename CompositeKey1,typename CompositeKey2>
bool operator<(
const composite_key_result<CompositeKey1>& x,
const composite_key_result<CompositeKey2>& y);
template<typename CompositeKey,typename... Values>
bool operator<(
const composite_key_result<CompositeKey>& x,
const std::tuple<Values...>& y);
template<typename CompositeKey,typename... Values>
bool operator<(
const std::tuple<Values...>& x,
const composite_key_result<CompositeKey>& y);
template<typename CompositeKey,typename Value0,...,typename Valuen>
bool operator<(
const composite_key_result<CompositeKey>& x,
const boost::tuple<Value0,...,Valuen>& y);
template<typename Value0,...,typename Valuen,typename CompositeKey>
bool operator<(
const boost::tuple<Value0,...,Valuen>& x,
const composite_key_result<CompositeKey>& y);
Requires: The expressionsxi<yiandyi<xiare valid for alliin[0,min(length(x),length(y))).
Returns:trueif and only if there exists somejin the range[0,min(length(x),length(y)))such thatComplexity: No more key extraction operations and comparisons are performed than those necessary for the evaluation of the expression above, starting at!(xi<yi) && !(yi<xi)for alliin[0,j),
xj<yj.i==0. The evaluation is short-circuited as soon as the result is determined to befalse.
template<typename CompositeKey1,typename CompositeKey2>
bool operator OP(
const composite_key_result<CompositeKey1>& x,
const composite_key_result<CompositeKey2>& y);
template<typename CompositeKey,typename... Values>
bool operator OP(
const composite_key_result<CompositeKey>& x,
const std::tuple<Values...>& y);
template<typename CompositeKey,typename... Values>
bool operator OP(
const std::tuple<Values...>& x,
const composite_key_result<CompositeKey>& y);
template<typename CompositeKey,typename Value0,...,typename Valuen>
bool operator OP(
const composite_key_result<CompositeKey>& x,
const boost::tuple<Value0,...,Valuen>& y);
template<typename Value0,...,typename Valuen,typename CompositeKey>
bool operator OP(
const boost::tuple<Value0,...,Valuen>& x,
const composite_key_result<CompositeKey>& y);
(OP is any of !=, >,
>=, <=.)
Requires: The expressions given below are valid (for the particularOPconsidered.)
Returns:trueif and only if!(x==y)(OPis!=),
y< x(OPis>),
!(x< y)(OPis>=),
!(y< x)(OPis<=).
composite_key_equal_to
composite_key_equal_to tests for equality between
composite_key_result instantiations and between
these and tuples of values, using an internally stored
collection of elementary equality predicates.
template<typename Pred0,...,typename Predn> struct composite_key_equal_to { typedef boost::tuple<Pred0,...,Predn> key_eq_tuple; composite_key_equal_to( const Pred0& p0=Pred0(), ... const Predn& pn=Predn()); composite_key_equal_to(const key_eq_tuple& x); const key_eq_tuple& key_eqs()const; key_eq_tuple& key_eqs(); template<typename CompositeKey1,typename CompositeKey2> bool operator()( const composite_key_result<CompositeKey1> & x, const composite_key_result<CompositeKey2> & y)const; template<typename CompositeKey,typename... Values> bool operator()( const composite_key_result<CompositeKey>& x, const std::tuple<Values...>& y)const; template<typename CompositeKey,typename... Values> bool operator()( const std::tuple<Values...>& x, const composite_key_result<CompositeKey>& y)const; template<typename CompositeKey,typename Value0,...,typename Valuen> bool operator()( const composite_key_result<CompositeKey>& x, const boost::tuple<Value0,...,Valuen>& y)const; template<typename Value0,...,typename Valuen,typename CompositeKey> bool operator()( const boost::tuple<Value0,...,Valuen>& x, const composite_key_result<CompositeKey>& y)const; };
Pred0, ... , Predn are the types of the equality
binary predicates stored by composite_key_equal_to. Each of these predicates
must be CopyConstructible and CopyAssignable. At least an
equality predicate must be provided. The maximum number of equality predicates of
a composite_key_equal_to instantiation is implementation defined.
composite_key_equal_to is
CopyConstructible and CopyAssignable.
It is also DefaultConstructible
if each Predi is DefaultConstructible in its turn.
Note that formally it is not required that the Predi types
behave as equality predicates in any definite way. However, the
semantics of composite_key_equal_to is well defined if this
is the case, as explained in the section on the
semantics of composite_key_result.
In what follows we use the same notation
introduced for composite_key_result.
composite_key_equal_to memberscomposite_key_equal_to(
const Pred0& p0=Pred0(),
...
const Predn& pn=Predn());
Effects: Constructs a composite_key_equal_to that stores
copies of the equality predicates supplied.
composite_key_equal_to(const key_eq_tuple& x);
Effects: Constructs acomposite_key_equal_tothat stores copies of the equality predicate objects supplied inx.
const key_eq_tuple& key_eqs()const;
Returns: a constant reference to a tuple holding the
equality predicate objects internally stored by the
composite_key_equal_to.
key_eq_tuple& key_eqs();
Returns: a reference to a tuple holding the
equality predicate objects internally stored by the
composite_key_equal_to.
template<typename CompositeKey1,typename CompositeKey2>
bool operator()(
const composite_key_result<CompositeKey1> & x,
const composite_key_result<CompositeKey2> & y)const;
template<typename CompositeKey,typename Values...>
bool operator()(
const composite_key_result<CompositeKey>& x,
const std::tuple<Values...>& y)const;
template<typename CompositeKey,typename Values...>
bool operator()(
const std::tuple<Values...>& x,
const composite_key_result<CompositeKey>& y)const;
template<typename CompositeKey,typename Value0,...,typename Valuen>
bool operator()(
const composite_key_result<CompositeKey>& x,
const boost::tuple<Value0,...,Valuen>& y)const;
template<typename Value0,...,typename Valuen,typename CompositeKey>
bool operator()(
const boost::tuple<Value0,...,Valuen>& x,
const composite_key_result<CompositeKey>& y)const;
Requires:length(x)==length(y). The expressionskey_eqs().get<i>()(xi,yi)andkey_eqs().get<i>()(yi,xi)are valid for alliin[0,length(x)).
Returns:trueif and onlyComplexity: No more key extraction operations and comparisons are performed than those necessary for the evaluation of the expression above, starting atkey_eqs().get<i>()(xi,yi)for alliin[0,length(x)).
i==0. The evaluation is short-circuited as soon as the result is determined to befalse.
composite_key_result_equal_to
Deprecated. Use std::equal_to<CompositeKeyResult> instead.
std::equal_to for composite_key results
std::equal_to<CompositeKeyResult>, with CompositeKeyResult
an instantiation of composite_key_result,
behaves as a particularization of
composite_key_equal_to where all the comparison predicates supplied
are instantiations of std::equal_to.
namespace std{ template<typename CompositeKey> struct equal_to<boost::multi_index::composite_key_result<CompositeKey> > { typedef boost::multi_index::composite_key_result< CompositeKey> first_argument_type; typedef first_argument_type second_argument_type; typedef bool result_type; template<typename CompositeKey1,typename CompositeKey2> bool operator()( const boost::multi_index::composite_key_result<CompositeKey1> & x, const boost::multi_index::composite_key_result<CompositeKey2> & y)const; template<typename CompositeKey,typename... Values> bool operator()( const boost::multi_index::composite_key_result<CompositeKey>& x, const std::tuple<Values...>& y)const; template<typename CompositeKey,typename... Values> bool operator()( const std::tuple<Values...>& x, const boost::multi_index::composite_key_result<CompositeKey>& y)const; template<typename CompositeKey,typename Value0,...,typename Valuen> bool operator()( const boost::multi_index::composite_key_result<CompositeKey>& x, const boost::tuple<Value0,...,Valuen>& y)const; template<typename Value0,...,typename Valuen,typename CompositeKey> bool operator()( const boost::tuple<Value0,...,Valuen>& x, const boost::multi_index::composite_key_result<CompositeKey>& y)const; }; } // namespace std
CompositeKeyResult must be an instantiation of
composite_key_result for some type
composite_key<KeyFromValue0,...,KeyFromValuen>.
std::equal:to<CompositeKeyResult>::operator() is
then equivalent to
composite_key_equal_to<Pred0,...,Predn>::operator(), taking
Predi = std::equal_to<KeyFromValuei::result_type>for alli = 0,...,n.
In addition to the requirements on Predi imposed by
composite_key_equal_to, each of these types must be
DefaultConstructible. std::equal_to<CompositeKeyResult>
is DefaultConstructible, CopyConstructible and
CopyAssignable.
composite_key_compare
composite_key_compare compares composite_key_result
instantiations between them and with tuples of values using an internally stored
collection of elementary comparison predicates.
template<typename Compare0,...,typename Comparen> struct composite_key_compare { typedef boost::tuple<Compare0,...,Comparen> key_comp_tuple; composite_key_compare( const Compare0& c0=Compare0(), ... const Comparen& cn=Comparen()); composite_key_compare(const key_comp_tuple& x); const key_comp_tuple& key_comps()const; key_comp_tuple& key_comps(); template<typename CompositeKey1,typename CompositeKey2> bool operator()( const composite_key_result<CompositeKey1> & x, const composite_key_result<CompositeKey2> & y)const; template<typename CompositeKey,typename... Values> bool operator()( const composite_key_result<CompositeKey>& x, const std::tuple<Values...>& y)const; template<typename CompositeKey,typename... Values> bool operator()( const std::tuple<Values...>& x, const composite_key_result<CompositeKey>& y)const; template<typename CompositeKey,typename Value0,...,typename Valuen> bool operator()( const composite_key_result<CompositeKey>& x, const boost::tuple<Value0,...,Valuen>& y)const; template<typename Value0,...,typename Valuen,typename CompositeKey> bool operator()( const boost::tuple<Value0,...,Valuen>& x, const composite_key_result<CompositeKey>& y)const; template<typename CompositeKey,typename Value> bool operator()( const composite_key_result<CompositeKey>& x,const Value& y)const; template<typename Value,typename CompositeKey> bool operator()( const Value& x,const composite_key_result<CompositeKey>& y)const; };
Compare0, ... , Comparen are the types of the comparison
binary predicates stored by composite_key_compare. Each of these predicates must be
CopyConstructible and CopyAssignable. At least a
comparison predicate must be provided. The maximum number of comparison predicates of
a composite_key_compare instantiation is implementation defined.
composite_key_compare is
CopyConstructible and CopyAssignable.
It is also
DefaultConstructible
if each Comparei is DefaultConstructible in its turn.
Note that formally it is not required that the Comparei types
behave as comparison predicates in any definite way. However, the
semantics of composite_key_compare is well defined if this
is the case, as explained in the section on the
semantics of composite_key_result.
In what follows we use the same notation
introduced for composite_key_result.
composite_key_compare memberscomposite_key_compare(
const Compare0& c0=Compare0(),
...
const Comparen& cn=Comparen());
Effects: Constructs a composite_key_compare that stores
copies of the comparison predicates supplied.
composite_key_compare(const key_comp_tuple& x);
Effects: Constructs acomposite_key_comparethat stores copies of the comparison predicate objects supplied inx.
const key_comp_tuple& key_comps()const;
Returns: a constant reference to a tuple holding the
comparison predicate objects internally stored by the
composite_key_compare.
key_comp_tuple& key_comps();
Returns: a reference to a tuple holding the
comparison predicate objects internally stored by the
composite_key_compare.
template<typename CompositeKey1,typename CompositeKey2>
bool operator()(
const composite_key_result<CompositeKey1> & x,
const composite_key_result<CompositeKey2> & y)const;
template<typename CompositeKey,typename... Values>
bool operator()(
const composite_key_result<CompositeKey>& x,
const std::tuple<Values...>& y)const;
template<typename CompositeKey,typename... Values>
bool operator()(
const std::tuple<Values...>& x,
const composite_key_result<CompositeKey>& y)const;
template<typename CompositeKey,typename Value0,...,typename Valuen>
bool operator()(
const composite_key_result<CompositeKey>& x,
const boost::tuple<Value0,...,Valuen>& y)const;
template<typename Value0,...,typename Valuen,typename CompositeKey>
bool operator()(
const boost::tuple<Value0,...,Valuen>& x,
const composite_key_result<CompositeKey>& y)const;
Requires: The expressionskey_comps().get<i>()(xi,yi)andkey_comps().get<i>()(yi,xi)are valid for alliin[0,min(length(x),length(y))).
Returns:trueif and only if there exists somejin the range[0,min(length(x),length(y)))such thatComplexity: No more key extraction operations and comparisons are performed than those necessary for the evaluation of the expression above, starting at!key_comps().get<i>()(xi,yi) && !key_comps().get<i>()(yi,xi)for alliin[0,j),
key_comps().get<j>()(xj,yj).i==0. The evaluation is short-circuited as soon as the result is determined to befalse.
template<typename CompositeKey,typename Value>
bool operator()(
const composite_key_result<CompositeKey>& x,const Value& y)const;
Effects:return operator()(x,boost::make_tuple(boost::cref(y)));
template<typename Value,typename CompositeKey>
bool operator()(
const Value& x,const composite_key_result<CompositeKey>& y)const;
Effects:return operator()(boost::make_tuple(boost::cref(x)),y);
composite_key_result_less
Deprecated. Use std::less<CompositeKeyResult> instead.
composite_key_result_greater
Deprecated. Use std::greater<CompositeKeyResult> instead.
std::less for
composite_key results
std::less<CompositeKeyResult>, with CompositeKeyResult
an instantiation of composite_key_result, behaves as a particularization of
composite_key_compare where all the comparison predicates supplied are
instantiations of std::less.
namespace std{ template<typename CompositeKey> struct less<boost::multi_index::composite_key_result<CompositeKey> > { typedef boost::multi_index::composite_key_result< CompositeKey> first_argument_type; typedef first_argument_type second_argument_type; typedef bool result_type; template<typename CompositeKey1,typename CompositeKey2> bool operator()( const boost::multi_index::composite_key_result<CompositeKey1> & x, const boost::multi_index::composite_key_result<CompositeKey2> & y)const; template<typename CompositeKey,typename... Values> bool operator()( const boost::multi_index::composite_key_result<CompositeKey>& x, const std::tuple<Values...>& y)const; template<typename CompositeKey,typename... Values> bool operator()( const std::tuple<Values...>& x, const boost::multi_index::composite_key_result<CompositeKey>& y)const; template<typename CompositeKey,typename Value0,...,typename Valuen> bool operator()( const boost::multi_index::composite_key_result<CompositeKey>& x, const boost::tuple<Value0,...,Valuen>& y)const; template<typename Value0,...,typename Valuen,typename CompositeKey> bool operator()( const boost::tuple<Value0,...,Valuen>& x, const boost::multi_index::composite_key_result<CompositeKey>& y)const; template<typename CompositeKey,typename Value> bool operator()( const boost::multi_index::composite_key_result<CompositeKey>& x, const Value& y)const; template<typename Value,typename CompositeKey> bool operator()( const Value& x, const boost::multi_index::composite_key_result<CompositeKey>& y)const; }; } // namespace std
CompositeKeyResult must be an instantiation of
composite_key_result for some type
composite_key<KeyFromValue0,...,KeyFromValuen>.
std::less<CompositeKeyResult>::operator() is
then equivalent to
composite_key_compare<Compare0,...,Comparen>::operator(), taking
Comparei = std::less<KeyFromValuei::result_type>for alli = 0,...,n.
In addition to the requirements on Comparei imposed by
composite_key_compare, each of these types must be
DefaultConstructible. std::less<CompositeKeyResult>
is CopyConstructible, CopyConstructible
and CopyAssignable.
std::greater for
composite_key results
std::greater<CompositeKeyResult>, with CompositeKeyResult
an instantiation of composite_key_result, behaves as a particularization of
composite_key_compare where all the comparison predicates supplied
are instantiations of std::greater.
namespace std{ template<typename CompositeKey> struct greater<boost::multi_index::composite_key_result<CompositeKey> > { typedef boost::multi_index::composite_key_result< CompositeKey> first_argument_type; typedef first_argument_type second_argument_type; typedef bool result_type; template<typename CompositeKey1,typename CompositeKey2> bool operator()( const boost::multi_index::composite_key_result<CompositeKey1> & x, const boost::multi_index::composite_key_result<CompositeKey2> & y)const; template<typename CompositeKey,typename... Values> bool operator()( const boost::multi_index::composite_key_result<CompositeKey>& x, const std::tuple<Values...>& y)const; template<typename CompositeKey,typename... Values> bool operator()( const std::tuple<Values...>& x, const boost::multi_index::composite_key_result<CompositeKey>& y)const; template<typename CompositeKey,typename Value0,...,typename Valuen> bool operator()( const boost::multi_index::composite_key_result<CompositeKey>& x, const boost::tuple<Value0,...,Valuen>& y)const; template<typename Value0,...,typename Valuen,typename CompositeKey> bool operator()( const boost::tuple<Value0,...,Valuen>& x, const boost::multi_index::composite_key_result<CompositeKey>& y)const; template<typename CompositeKey,typename Value> bool operator()( const boost::multi_index::composite_key_result<CompositeKey>& x, const Value& y)const; template<typename Value,typename CompositeKey> bool operator()( const Value& x, const boost::multi_index::composite_key_result<CompositeKey>& y)const; }; } // namespace std
CompositeKeyResult must be an instantiation of
composite_key_result for some type
composite_key<KeyFromValue0,...,KeyFromValuen>.
std::greater<CompositeKeyResult>::operator() is
then equivalent to
composite_key_compare<Compare0,...,Comparen>::operator(), taking
Comparei = std::greater<KeyFromValuei::result_type>for alli = 0,...,n.
In addition to the requirements on Comparei imposed by
composite_key_compare, each of these types must be
DefaultConstructible. std::greater<CompositeKeyResult>
is CopyConstructible, CopyConstructible
and CopyAssignable.
composite_key_hash
composite_key_hash produces hash values for composite_key_result
instantiations based on a collection of elementary hash functors.
template<typename Hash0,...,typename Hashn> struct composite_key_hash { typedef boost::tuple<Hash0,...,Hashn> key_hasher_tuple; composite_key_hash( const Hash0& h0=Hash0(), ... const Hashn& hn=Hashn()); composite_key_hash(const key_hasher_tuple& x); const key_hasher_tuple& key_hash_functions()const; key_hasher_tuple& key_hash_functions(); template<typename CompositeKey> std::size_t operator()( const composite_key_result<CompositeKey>& x)const; template<typename... Values> std::size_t operator()( const std::tuple<Values...>& x)const; template<typename Value0,...,typename Valuen> std::size_t operator()( const boost::tuple<Value0,...,Valuen>& x)const; };
Hash0, ... , Hashn are the types of the hash unary function objects
stored by composite_key_hash. Each of these objects
must be CopyConstructible and CopyAssignable
and return a value of type std::size_t in the range
[0, std::numeric_limits<std::size_t>::max())).
At least a hash functor must be provided. The maximum number of hash functors of
a composite_key_hash instantiation is implementation defined.
composite_key_hash is
CopyConstructible and CopyAssignable.
It is also DefaultConstructible
if each Hashi is DefaultConstructible in its turn.
In what follows we use the same notation
introduced for composite_key_result.
composite_key_hash memberscomposite_key_hash(
const Hash0& h0=Hash0(),
...
const Hashn& hn=Hashn());
Effects: Constructs a composite_key_hash that stores
copies of the hash functors supplied.
composite_key_hash(const key_hasher_tuple& x);
Effects: Constructs acomposite_key_hashthat stores copies of the hash functors supplied inx.
const key_hasher_tuple& key_hash_functions()const;
Returns: a constant reference to a tuple holding the
hash functors internally stored by the
composite_key_hash.
key_hasher_tuple& key_hash_functions();
Returns: a reference to a tuple holding the
hash functors internally stored by the
composite_key_hash.
template<typename CompositeKey>
bool operator()(
const composite_key_result<CompositeKey>& x)const;
template<typename... Values>
bool operator()(
const std::tuple<Values...>& x)const;
template<typename Value0,...,typename Valuen>
bool operator()(
const boost::tuple<Value0,...,Valuen>& x)const;
Requires:length(x)==length(key_hash_functions()). The expressionkey_hash_functions().get<i>()(xi)is valid for alliin[0,length(x)).
Returns: A value in the range[0, std::numeric_limits<std::size_t>::max())that solely depends on the numerical tuple(key_hash_functions().get<0>()(x0), ... ,key_hash_functions().get<N>()(xN)), withN=length(x)-1.
composite_key_result_hash
Deprecated. Use boost::hash<CompositeKeyResult> instead.
boost::hash
for composite_key results
boost::hash<CompositeKeyResult>, with CompositeKeyResult
an instantiation of composite_key_result, behaves as a particularization of
composite_key_hash where all the hash functors supplied
are instantiations of
boost::hash.
namespace boost{ template<typename CompositeKey> struct hash<multi_index::composite_key_result<CompositeKey> > { typedef multi_index::composite_key_result<CompositeKey> argument_type; typedef std::size_t result_type; template<typename CompositeKey> std::size_t operator()( const multi_index::composite_key_result<CompositeKey>& x)const; template<typename... Values> std::size_t operator()( const std::tuple<Values...>& x)const; template<typename Value0,...,typename Valuen> std::size_t operator()( const boost::tuple<Value0,...,Valuen>& x)const; }; } // namespace boost
CompositeKeyResult must be an instantiation of
composite_key_result for some type
composite_key<KeyFromValue0,...,KeyFromValuen>.
boost::hash<CompositeKeyResult>::operator() is
then equivalent to
composite_key_hash<Hash0,...,Hashn>::operator(), taking
Hashi = boost::hash<KeyFromValuei::result_type>for alli = 0,...,n.
In addition to the requirements on Hashi imposed by
composite_key_hash, each of these types must be
DefaultConstructible. boost::hash<CompositeKeyResult> is
DefaultConstructible, CopyConstructible
and CopyAssignable.
composite_key_result
The design of equality, comparison and hash operations for
composite_key_result objects is based on the following rationale:
a composite_key_result is regarded as a "virtual" tuple, each
of its elements being the result of the corresponding elementary
key extractor. Accordingly, any given operation resolves to a
combination of the corresponding elementwise operations.
This mapping preserves the fundamental properties of the elementary operations
involved; for instance, it defines a true equivalence relation if the
basic predicates induce equivalence relations themselves.
We can state these facts in a formal way as follows.
Consider an instantiation of composite_key_equal_to
with types Pred0, ... , Predn such that each
Predi induces an equivalence relation on a certain type Ti,
and let CompositeKey be a type of the form
composite_key<Value,KeyFromValue0,...,KeyFromValuej>,
with j <= n, such that
Then,KeyFromValuei::result_type = Ti, for alli = 0,...,j.
composite_key_equal_to induces an equivalence relation
on elements of type composite_key_result<CompositeKey>;
such two objects are equivalent if all its elementary key extractor values
are also equivalent. Additionally, given an instantiation
composite_key_hash<Hash0,...,Hashj>, the following types are
Compatible Keys of
(composite_key_hash, composite_key_equal_to)
with respect to composite_key_result<CompositeKey>:
provided that eachtuple<Q0,...,Qj>,
composite_key_result<composite_key<K0,...,Kj> >, withKi::result_type = Qifor alli = 0,...,j.
Qi is either Ti or a
Compatible Key
of (Hashi, Predi).
As for comparison, consider an instantiation of composite_key_compare
with types Compare0, ... , Comparen such that each
Comparei induces a strict weak ordering
on the type Ti. Then, for a
CompositeKey type defined in the same manner as above,
composite_key_compare induces a strict weak ordering on elements of type
composite_key_result<CompositeKey>, and the order induced
is lexicographical. Also, the following types are
Compatible Keys of
composite_key_compare with respect to
composite_key_result<CompositeKey>:
provided thattuple<Q0,...,Qk>,k <= n
composite_key_result<composite_key<K0,...,Kk> >, withKi::result_type = Qifor alli = 0,...,k.
i = 0,...,min(j,k)-1, Qi is either Ti
or not coarser than Ti (Qi is a
Compatible Key
of Comparei and there are no two distinct elements of
Ti equivalent to one single element of Qi);
Qm (with m = min(j,k)) is either Tm or a
Compatible Key
of Comparem.
1+min(j,k) elements.
Analogous properties hold for the equality and comparison operators
of composite_key_result. Note, however,
that equality is only defined for objects of the same length, whilst
comparison takes the minimum length of the operands considered.
Therefore, the equivalence classes induced by x==y are
subsets of those associated to !(x<y)&&!(y<x).
"boost/multi_index/key.hpp" synopsis#include <boost/multi_index/member.hpp> #include <boost/multi_index/mem_fun.hpp> #include <boost/multi_index/global_fun.hpp> #include <boost/multi_index/composite_key.hpp> #if implementation defined /* auto non-type template parameters supported */ #define BOOST_MULTI_INDEX_KEY_SUPPORTED namespace boost{ namespace multi_index{ template<auto... Keys> using key=implementation defined; } /* namespace multi_index */ } /* namespace boost */ #endif
key
In C++17 compliant environments, key provides a very terse syntax for
the specification of Boost.MultiIndex predefined key extractors. The number of template
arguments passed to key must be greater than zero and not exceed the
maximum number of key extractors accepted by
composite_key. key<Key>
resolves to:
member<Class,Type,Key>
if Key is of type Type Class::*,
const_mem_fun<Class,Type,Key>
if Key is of type Type (Class::*)()const
(with ot without noexcept-specification),
cv_mem_fun<Class,Type,Key>
if Key is of type Type (Class::*)()const volatile
(with ot without noexcept-specification),
cref_mem_fun<Class,Type,Key>
if Key is of type Type (Class::*)()const&
(with ot without noexcept-specification),
cvref_mem_fun<Class,Type,Key>
if Key is of type Type (Class::*)()const volatile&
(with ot without noexcept-specification),
mem_fun<Class,Type,Key>
if Key is of type Type (Class::*)()
(with ot without noexcept-specification),
volatile_mem_fun<Class,Type,Key>
if Key is of type Type (Class::*)()volatile
(with ot without noexcept-specification),
ref_mem_fun<Class,Type,Key>
if Key is of type Type (Class::*)()&
(with ot without noexcept-specification),
vref_mem_fun<Class,Type,Key>
if Key is of type Type (Class::*)()volatile&
(with ot without noexcept-specification),
global_fun<Value,Type,Key>
if Key is of type Type (*)(Value)
(with ot without noexcept-specification),
key<Key0,...,Keyn> resolves to
composite_key<Value,KeyFromValue0,...,KeyFromValuen>,
with:
KeyFromValuei = key<Keyi>
for all i = 0,...,n
Value = std::decay_t<Value0> ⊗
std::decay_t<Value1> ⊗ ··· ⊗
std::decay_t<Valuen>,
Valuei corresponds to the associated Class or Value
type of KeyFromValuei, and T ⊗ Q
is the least generic type between T and Q, defined as
T if std::is_convertible_v<const T&,const Q&>,
Q if std::is_convertible_v<const Q&,const T&>,
and undefined otherwise.
Revised June 9th 2019
© Copyright 2003-2019 Joaquín M López Muñoz. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)