1. 简介

局部特化（partial specialization）容许为给定类别的模板实参自定义类模板和变量模板.

2. 语法

template<parameter-list> class ClassName<argument-list> declaration

其中，argument-list 能够蕴含具体的类型（对应类型参数）、具体的值（对应非类型参数），也能够蕴含在 <parameter-list> 中指定的模板形参.

如，

// primary templatetemplate<class T1, class T2, int I>class A {};// #1: partial specialization where T2 is a pointer to T1template<class T, int I>class A<T, T*, I> {};// #2: partial specialization where T1 is a pointertemplate<class T, class T2, int I>class A<T*, T2, I> {};// #3: partial specialization where T1 is int, I is 5, and T2 is a pointertemplate<class T>class A<int, T*, 5> {}; // #4: partial specialization where T2 is a pointertemplate<class X, class T, int I>class A<X, T*, I> {};

具体例子：移除援用

#include <iostream>template <typename T>struct RemoveReference{    using type = T;};template <typename T>struct RemoveReference<T&>{    using type = T;};template <typename T>struct RemoveReference<T&&>{    using type = T;};int main(){    std::cout << std::boolalpha << std::is_reference<int&>::value << '\n';    std::cout << std::boolalpha << std::is_reference<int&&>::value << '\n';    std::cout << "---------------------\n";    std::cout << std::boolalpha << std::is_reference<RemoveReference<int>::type>::value << '\n';    std::cout << std::boolalpha << std::is_reference<RemoveReference<int&>::type>::value << '\n';    std::cout << std::boolalpha << std::is_reference<RemoveReference<int&&>::type>::value << '\n';}
truetrue---------------------falsefalsefalse

3. 局部特化的成员

（1）成员的模板形参列表、模板实参列表必须和局部特化的形参列表、实参列表统一.

// primary templatetemplate<class T, int I>struct A {    void f();};// primary template member definitiontemplate<class T, int I>void A<T, I>::f() { }// partial specializationtemplate<class T>struct A<T, 2> {    void f();    void g();    void h();};// member of partial specializationtemplate<class T>void A<T, 2>::g() { }

（2）局部特化的成员也能够进一步显式特化.

// explicit (full) specialization of a member of partial specializationtemplate<>void A<char, 2>::h() {}

（3）外围类模板的特化要优先于内层类模板的特化.

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