partial template specialization

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

C++ language

 

Templates

parameters and arguments

class templates

function templates

class member templates

variable templates(C++14)

template argument deduction

explicit (full) specialization

partial specialization

dependent names

parameter packs(C++11)

sizeof...(C++11)

SFINAE

 

Allows customizing class templates for a given category of template arguments.

Syntax

template < parameter-list > class-key class-head-name < argument-list > declaration

where class-head-name identifies the name of a previously declared class template.

For example,

template<class T1, class T2, int I>
class A {}; // primary template
 
template<class T, int I>
class A<T, T*, I> {}; // partial specialization where T2 is a pointer to T1
 
template<class T, class T2, int I>
class A<T*, T2, I> {}; // partial specialization where T1 is a pointer
 
template<class T>
class A<int, T*, 5> {}; // partial specialization where T1 is int, I is 5,
                        // and T2 is a pointer
 
template<class X, class T, int I>
class A<X, T*, I> {}; // partial specialization where T2 is a pointer

Examples of partial specializations in the standard library include std::unique_ptr, which has a partial specialization for array types.

The argument list

The following restrictions apply to the argument-list of a partial template specialization:

1) The argument list cannot be identical to the non-specialized argument list (it must specialize something)

template<class T1, class T2, int I> class A {}; // primary template
template<class X, class Y, int N> class A<X,Y,N> {}; // error

2) Default arguments cannot appear in the argument list

3) The specialization has to be more specialized than the primary template template<int B, typename T1, typename... Ts> struct A; template<typename... Ts> struct A<0, Ts...> { }; // Error: not more specialized

(since C++14)

4) If any argument is a pack expansion, it must be the last argument in the list

5) Non-type argument expression cannot use the name of the template parameter except when it is exactly the name of the template parameter

template <int I, int J> struct A {}; // primary template
template <int I> struct A<I+5, I*2> {}; // error
template <int I> struct A<I, I> {}; // OK

6) Non-type template argument cannot specialize a template parameter whose type depends on a parameter of the specialization:

template <class T, T t> struct C {}; // primary template
template <class T> struct C<T, 1>; // error: type of the argument 1 is T,
                                   // which depends on the parameter T
 
template< int X, int (*array_ptr)[X] > class A {}; // primary template
int array[5];
template< int X > class A<X,&array> { }; // error: type of the argument &array
                             // is int(*)[X], which depends on the parameter X

Name lookup

Partial template specializations are not found by name lookup. Only if the primary template is found by name lookup, its partial specializations are considered. In particular, a using declaration that makes a primary template visible, makes partial specializations visible as well:

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; // name lookup finds N::A (the primary template),
               // the partial specialization with T = int is then used

Partial ordering

When a class template is instantiated, and there are partial specializations avaiable, the compiler has to decide if the primary template is going to be used or one of its partial specializations.

1) If only one specialization matches the template arguments, that specialization is used

2) If more than one specialization matches, partial order rules are used to determine which specialization is more specialized. The most specialized specialization is used, if it is unique (if it is not unique, the program cannot be compiled)

3) If no specializations match, the primary template is used

// given the template A as defined above
A<int, int, 1> a1;   // no specializations match, uses primary template
A<int, int*, 1> a2;  // uses partial specialization #1 (T=int, I=1)
A<int, char*, 5> a3; // uses partial specialization #3, (T=char)
A<int, char*, 1> a4; // uses partial specialization #4, (X=int, T=char, I=1)
A<int*, int*, 2> a5; // error: matches #2 (T=int, T2=int*, I=2)
                     //        matches #4 (X=int*, T=int, I=2)
                     // neither one is more specialized than the other

To establish more-specialized-than relationship between partial specializations, each is first converted to a fictitious function template as follows:

• the first function template has the same template parameters as the first partial specialization and has just one function parameter, whose type is a class template specialization with all the template arguments from the first partial specialization
• the second function template has the same template parameters as the second partial specialization and has just one function parameter whose type is a class template specialization with all the template arguments from the second partial specialization.

The function templates are then ranked as if for function template overloading.

template<int I, int J, class T> struct X { }; // primary template
template<int I, int J>          struct X<I, J, int> {
        static const int s = 1;
}; // partial specialization #1
// fictitious function template for #1 is
// template<int I, int J> void f(X<I, J, int>); #A
 
template<int I>                 struct X<I, I, int> {
        static const int s = 2;
}; // partial specialization #2
// fictitious function template for #2 is 
// template<int I>        void f(X<I, I, int>); #B
 
int main()
{
    X<2, 2, int> x; // both #1 and #2 match
// partial ordering for function templates:
// #A from #B: void(X<I,J,int>) from void(X<U1, U1, int>): deduction ok
// #B from #A: void(X<I,I,int>) from void(X<U1, U2, int>): deduction fails
// #B is more specialized
// #2 is the specialization that is instantiated
    std::cout << x.s << '\n'; // prints 2
}

Members of partial specializations

The template parameter list and the template argument list of a member of a partial specialization must match the parameter list and the argument list of the partial specialization.

Just like with members of primary templates, they only need to be defined if used in the program.

Members of partial specializations are not related to the members of the primary template.

Explicit (full) specialization of a member of a partial specialization is declared the same way as an explicit specialization of the primary template.

template<class T, int I>  // primary template
struct A {
    void f(); // member declaration
};
 
template<class T, int I>
void A<T,I>::f() { } // primary template member definition
 
// partial specialization
template<class T>
struct A<T,2> {
    void f();
    void g();
    void h();
};
 
// member of partial specialization
template<class T>
void A<T,2>::g() { }
 
// explicit (full) specialization
// of a member of partial specialization
template<>
void A<char,2>::h() {}
 
int main() {
    A<char,0> a0;
    A<char,2> a2;
    a0.f(); // OK, uses primary template’s member definition
    a2.g(); // OK, uses partial specialization's member definition
    a2.h(); // OK, uses fully-specialized definition of
            // the member of a partial specialization
    a2.f(); // error: no definition of f() in the partial
            // specialization A<T,2> (the primary template is not used)
}

If a class template is a member of another class template, and it has partial specializations, these specializations are members of the enclosing class template. If the enclosing template is instantiated, the declarations of each member partial specialization is instantiated as well (the same way declarations, but not definitions, of all other members of a template are instantiated)

If the primary member template is explicitly (fully) 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.

template<class T> struct A { // enclosing class template
  template<class T2>
  struct B {}; // primary member template
  template<class T2>
  struct B<T2*> {}; // partial specialization of member template
};
 
template<>
template<class T2>
struct A<short>::B {}; // full specialization of primary member template
                       // (will ignore the partial)
 
A<char>::B<int*> abcip; // uses partial specialization T2=int
A<short>::B<int*> absip; // uses full specialization of the primary (ignores partial)
A<char>::B<int> abci; // uses primary