模版偏特化之std::enable_if

1 SFINAE。

2 条件特化。可用作额外的函数参数（不可应用于运算符重载）、返回类型（不可应用于构造函数与析构函数），或类模板或函数模板形参。

函数参数：

#include <iostream>
#include <type_traits>
using namespace std; template <typename T>
T  addT(T  value, typename std::enable_if<std::is_integral<T>::value, T>::type* = nullptr)
{return value + 10;
}template <typename T>
T  addT(T value, typename std::enable_if<std::is_floating_point<T>::value, T>::type* = nullptr)
{return value + 5;
}int  main()
{int d = 10;double f = 20;d = addT(d);f = addT(f);return 0;}

单独看这个例子是比较模糊，因为编译器做了自动补充：d = addT(d, nullptr);通过如下网站可以展开上面代码

C++ Insights

#include <iostream>
#include <type_traits>
using namespace std;template<typename T>
T addT(T value, typename std::enable_if<std::is_integral<T>::value, T>::type *)
{return operator+(value, 10);
}/* First instantiated from: insights.cpp:22 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
int addT<int>(int value, typename std::enable_if<std::is_integral<int>::value, int>::type *)
{return value + 10;
}
#endiftemplate<typename T>
T addT(T value, typename std::enable_if<std::is_floating_point<T>::value, T>::type *)
{return operator+(value, 5);
}/* First instantiated from: insights.cpp:23 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
double addT<double>(double value, typename std::enable_if<std::is_floating_point<double>::value, double>::type *)
{return value + 5;
}
#endifint main()
{int d = 10;double f = 20;d = addT(d, nullptr);f = addT(f, nullptr);return 0;
}

函数返回值：

#include <type_traits>
#include <iostream>template <typename T>
typename std::enable_if <std::is_integral<T>::value, T>::type  addT(T value)
{return value + 10;
}template <typename T>
typename std::enable_if <std::is_floating_point<T>::value, T>::type  addT(T value)
{return value + 5.0;
}int main()
{int d = 10;double f = 20;d = addT(d);  //d = 20f = addT(f);  //f = 25.000000000return 0;
}

std::is_integral 用法参考：https://cplusplus.com/reference/type_traits/is_integral/

编译展开后的代码如下：

#include <type_traits>
#include <iostream>template<typename T>
typename std::enable_if<std::is_integral<T>::value, T>::type addT(T value)
{return value + 10;
}/* First instantiated from: insights.cpp:20 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
typename std::enable_if<std::is_integral<int>::value, int>::type addT<int>(int value)
{return value + 10;
}
#endiftemplate<typename T>
typename std::enable_if<std::is_floating_point<T>::value, T>::type addT(T value)
{return value + 5.0;
}/* First instantiated from: insights.cpp:21 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
typename std::enable_if<std::is_floating_point<double>::value, double>::type addT<double>(double value)
{return value + 5.0;
}
#endifint main()
{int d = 10;double f = 20;d = addT(d);f = addT(f);return 0;
}

模版类特化：

#include <type_traits>
#include <iostream>template <typename T, typename Enable = void>
class MyTestClass
{// 默认实现，适用于所有类型
public:MyTestClass() {std::cout << "default";}
};template <typename T>
class MyTestClass<T, typename std::enable_if<std::is_integral<T>::value>::type>
{// 整型类型的特化实现
public:MyTestClass() {std::cout << "int";}
};template <typename T>
class MyTestClass<T, typename std::enable_if<std::is_floating_point<T>::value>::type>
{// 浮点型类型的特化实现
public:MyTestClass() {std::cout << "float";}
};
int main()
{MyTestClass<std::string>  x1;// OK ：匹配初等模板MyTestClass<int>  x2;        // OK ：匹配偏特化 int MyTestClass<double>  x3;     // OK ：匹配偏特化 doublereturn 0;
}

模版展开后代码如下：

#include <type_traits>
#include <iostream>template<typename T, typename Enable = void>
class MyTestClass
{public: inline MyTestClass(){std::operator<<(std::cout, "default");}};/* First instantiated from: insights.cpp:35 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
class MyTestClass<std::basic_string<char>, void>
{public: inline MyTestClass(){std::operator<<(std::cout, "default");}};#endif
/* First instantiated from: insights.cpp:36 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
class MyTestClass<int, void>
{public: inline MyTestClass(){std::operator<<(std::cout, "int");}};#endif
/* First instantiated from: insights.cpp:37 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
class MyTestClass<double, void>
{public: inline MyTestClass(){std::operator<<(std::cout, "float");}};#endiftemplate<typename T>
class MyTestClass<T, typename std::enable_if<std::is_integral<T>::value>::type>
{public: inline MyTestClass(){std::operator<<(std::cout, "int");}};template<typename T>
class MyTestClass<T, typename std::enable_if<std::is_floating_point<T>::value>::type>
{public: inline MyTestClass(){std::operator<<(std::cout, "float");}};int main()
{MyTestClass<std::basic_string<char>, void> x1 = MyTestClass<std::basic_string<char>, void>();MyTestClass<int, void> x2 = MyTestClass<int, void>();MyTestClass<double, void> x3 = MyTestClass<double, void>();return 0;
}

参考文档：

https://cplusplus.com/reference/type_traits/is_integral/

std::enable_if - cppreference.com

C++之std::enable_if_std enable if-CSDN博客

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