C++模板—部分特化

Posted 2021-08-21 gzming

1. 简介

部分特化（partial specialization）允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 可以包含具体的类型（对应类型参数）、具体的值（对应非类型参数），也可以包含在 <parameter-list> 中指定的模板形参.

如，

// primary template
template<class T1, class T2, int I>
class A {};

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

// #2: partial specialization where T1 is a pointer
template<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 pointer
template<class T>
class A<int, T*, 5> {}; 

// #4: partial specialization where T2 is a pointer
template<class X, class T, int I>
class A<X, T*, I> {};

具体例子：移除引用

#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\';
}

true
true
---------------------
false
false
false

3. 部分特化的成员

（1）成员的模板形参列表、模板实参列表必须和部分特化的形参列表、实参列表一致.

// primary template
template<class T, int I>
struct A {
    void f();
};

// primary template member definition
template<class T, int I>
void A<T, I>::f() { }

// 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() { }

（2）部分特化的成员也可以进一步显式特化.

// explicit (full) specialization of a member of partial specialization
template<>
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=int
A<short>::B<int*> absip;       // uses full specialization of the primary (ignores partial)
A<char>::B<int> abci;          // uses primary