部分模板专业化

template<class T1, class T2, int I>
class A {};            // primary template
 
template<class T, int I>
class A<T, T*, I> {};  // #1: partial specialization where T2 is a pointer to T1
 
template<class T, class T2, int I>
class A<T*, T2, I> {}; // #2: partial specialization where T1 is a pointer
 
template<class T>
class A<int, T*, 5> {}; // #3: 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> {};   // #4: partial specialization where T2 is a pointer

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

template <int I, int J> struct A {};
template <int I> struct A<I+5, I*2> {}; // error, I is not deducible
 
template <int I, int J, int K> struct B {};
template <int I> struct B<I, I*2, 2> {};  // OK: first parameter is deducible

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 B {}; // primary template
int array[5];
template< int X > class B<X,&array> { }; // error: type of the argument &array
                             // is int(*)[X], which depends on the parameter X

namespace N {
    template<class T1, class T2> class Z { }; // primary template
}
using N::Z; // refers to the primary template
namespace N {
    template<class T> class Z<T, T*> { }; // partial specialization
}
Z<int,int*> z; // name lookup finds N::Z (the primary template),
               // the partial specialization with T = int is then 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

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
}

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)
}

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

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