A primary class template declaration is one in which the class template name is an identifier. A template declaration in which the class template name is a simple-template-id is a partial specialization of the class template named in the simple-template-id. A partial specialization of a class template provides an alternative definition of the template that is used instead of the primary definition when the arguments in a specialization match those given in the partial specialization ([temp.class.spec.match]). The primary template shall be declared before any specializations of that template. A partial specialization shall be declared before the first use of a class template specialization that would make use of the partial specialization as the result of an implicit or explicit instantiation in every translation unit in which such a use occurs; no diagnostic is required.
Each class template partial specialization is a distinct template and definitions shall be provided for the members of a template partial specialization ([temp.class.spec.mfunc]).
[ Example:
template<class T1, class T2, int I> class A { }; // #1
template<class T, int I> class A<T, T*, I> { }; // #2
template<class T1, class T2, int I> class A<T1*, T2, I> { }; // #3
template<class T> class A<int, T*, 5> { }; // #4
template<class T1, class T2, int I> class A<T1, T2*, I> { }; // #5
The first declaration declares the primary (unspecialized) class template. The second and subsequent declarations declare partial specializations of the primary template. — end example ]
The template parameters are specified in the angle bracket enclosed list that immediately follows the keyword template. For partial specializations, the template argument list is explicitly written immediately following the class template name. For primary templates, this list is implicitly described by the template parameter list. Specifically, the order of the template arguments is the sequence in which they appear in the template parameter list. [ Example: the template argument list for the primary template in the example above is <T1, T2, I>. — end example ] [ Note: The template argument list shall not be specified in the primary template declaration. For example,
template<class T1, class T2, int I> class A<T1, T2, I> { }; // error
— end note ]
A class template partial specialization may be declared or redeclared in any namespace scope in which its definition may be defined ([temp.class] and [temp.mem]). [ Example:
template<class T> struct A {
struct C {
template<class T2> struct B { };
};
};
// partial specialization of A<T>::C::B<T2>
template<class T> template<class T2>
struct A<T>::C::B<T2*> { };
A<short>::C::B<int*> absip; // uses partial specialization
— end example ]
Partial specialization declarations themselves are not found by name lookup. Rather, when the primary template name is used, any previously-declared partial specializations of the primary template are also considered. One consequence is that a using-declaration which refers to a class template does not restrict the set of partial specializations which may be found through the using-declaration. [ Example:
namespace N {
template<class T1, class T2> class A { }; // primary template
}
using N::A; // refers to the primary template
namespace N {
template<class T> class A<T, T*> { }; // partial specialization
}
A<int,int*> a; // uses the partial specialization, which is found through
// the using declaration which refers to the primary template
— end example ]
A non-type argument is non-specialized if it is the name of a non-type parameter. All other non-type arguments are specialized.
Within the argument list of a class template partial specialization, the following restrictions apply:
A partially specialized non-type argument expression shall not involve a template parameter of the partial specialization except when the argument expression is a simple identifier. [ Example:
template <int I, int J> struct A {};
template <int I> struct A<I+5, I*2> {}; // error
template <int I, int J> struct B {};
template <int I> struct B<I, I> {}; // OK
— end example ]
The type of a template parameter corresponding to a specialized non-type argument shall not be dependent on a parameter of the specialization. [ Example:
template <class T, T t> struct C {};
template <class T> struct C<T, 1>; // error
template< int X, int (*array_ptr)[X] > class A {};
int array[5];
template< int X > class A<X,&array> { }; // error
— end example ]
The argument list of the specialization shall not be identical to the implicit argument list of the primary template.
The template parameter list of a specialization shall not contain default template argument values.140
An argument shall not contain an unexpanded parameter pack. If an argument is a pack expansion ([temp.variadic]), it shall be the last argument in the template argument list.
There is no way in which they could be used.
When a class template is used in a context that requires an instantiation of the class, it is necessary to determine whether the instantiation is to be generated using the primary template or one of the partial specializations. This is done by matching the template arguments of the class template specialization with the template argument lists of the partial specializations.
If exactly one matching specialization is found, the instantiation is generated from that specialization.
If more than one matching specialization is found, the partial order rules ([temp.class.order]) are used to determine whether one of the specializations is more specialized than the others. If none of the specializations is more specialized than all of the other matching specializations, then the use of the class template is ambiguous and the program is ill-formed.
If no matches are found, the instantiation is generated from the primary template.
A partial specialization matches a given actual template argument list if the template arguments of the partial specialization can be deduced from the actual template argument list ([temp.deduct]). [ Example:
A<int, int, 1> a1; // uses #1 A<int, int*, 1> a2; // uses #2, T is int, I is 1 A<int, char*, 5> a3; // uses #4, T is char A<int, char*, 1> a4; // uses #5, T1 is int, T2 is char, I is 1 A<int*, int*, 2> a5; // ambiguous: matches #3 and #5
— end example ]
A non-type template argument can also be deduced from the value of an actual template argument of a non-type parameter of the primary template. [ Example: the declaration of a2 above. — end example ]
In a type name that refers to a class template specialization, (e.g., A<int, int, 1>) the argument list shall match the template parameter list of the primary template. The template arguments of a specialization are deduced from the arguments of the primary template.
For two class template partial specializations, the first is at least as specialized as the second if, given the following rewrite to two function templates, the first function template is at least as specialized as the second according to the ordering rules for function templates ([temp.func.order]):
the first function template has the same template parameters as the first partial specialization and has a single function parameter whose type is a class template specialization with the template arguments of the first partial specialization, and
the second function template has the same template parameters as the second partial specialization and has a single function parameter whose type is a class template specialization with the template arguments of the second partial specialization.
[ Example:
template<int I, int J, class T> class X { };
template<int I, int J> class X<I, J, int> { }; // #1
template<int I> class X<I, I, int> { }; // #2
template<int I, int J> void f(X<I, J, int>); // A
template<int I> void f(X<I, I, int>); // B
The partial specialization #2 is more specialized than the partial specialization #1 because the function template B is more specialized than the function template A according to the ordering rules for function templates. — end example ]
The template parameter list of a member of a class template partial specialization shall match the template parameter list of the class template partial specialization. The template argument list of a member of a class template partial specialization shall match the template argument list of the class template partial specialization. A class template specialization is a distinct template. The members of the class template partial specialization are unrelated to the members of the primary template. Class template partial specialization members that are used in a way that requires a definition shall be defined; the definitions of members of the primary template are never used as definitions for members of a class template partial specialization. An explicit specialization of a member of a class template partial specialization is declared in the same way as an explicit specialization of the primary template. [ Example:
// primary template template<class T, int I> struct A { void f(); }; template<class T, int I> void A<T,I>::f() { } // class template partial specialization template<class T> struct A<T,2> { void f(); void g(); void h(); }; // member of class template partial specialization template<class T> void A<T,2>::g() { } // explicit specialization template<> void A<char,2>::h() { } int main() { A<char,0> a0; A<char,2> a2; a0.f(); // OK, uses definition of primary template's member a2.g(); // OK, uses definition of // partial specialization's member a2.h(); // OK, uses definition of // explicit specialization's member a2.f(); // ill-formed, no definition of f for A<T,2> // the primary template is not used here }
— end example ]
If a member template of a class template is partially specialized, the member template partial specializations are member templates of the enclosing class template; if the enclosing class template is instantiated ([temp.inst], [temp.explicit]), a declaration for every member template partial specialization is also instantiated as part of creating the members of the class template specialization. If the primary member template is explicitly specialized for a given (implicit) specialization of the enclosing class template, the partial specializations of the member template are ignored for this specialization of the enclosing class template. If a partial specialization of the member template is explicitly specialized for a given (implicit) specialization of the enclosing class template, the primary member template and its other partial specializations are still considered for this specialization of the enclosing class template. [ Example:
template<class T> struct A {
template<class T2> struct B {}; // #1
template<class T2> struct B<T2*> {}; // #2
};
template<> template<class T2> struct A<short>::B {}; // #3
A<char>::B<int*> abcip; // uses #2
A<short>::B<int*> absip; // uses #3
A<char>::B<int> abci; // uses #1
— end example ]