C++ 模板类类型限定

#include <iostream>
#include <type_traits>
using namespace std;
namespace
{class IAnimal{public:virtual void say() = 0;};class Dog : IAnimal{public:void say() override{cout << "我是 狗!!" << '\n';}};class Cat : IAnimal{public:void say() override{cout << "我是 猫!!" << '\n';}};class Horse{public:void say(){cout << "我是 马!!" << '\n';}};
}template <typename T,typename = typename std::enable_if<std::is_base_of<IAnimal, T>::value>::type>class MyPet
{
public:void sayWhat(){m_pet.say();}
private:T m_pet;
};int main()
{MyPet<Dog> myDog;myDog.sayWhat();MyPet<Cat> myCat;myCat.sayWhat();// 放开注释 typename = typename std::enable_if ... ::type> type 会找不到而报错/* MyPet<Horse> myHorse;myHorse.sayWhat();*/system("pause");return EXIT_SUCCESS;
}

输出：

我是 狗!!
我是 猫!!

template <typename T,typename = typename std::enable_if<std::is_base_of<IAnimal, T>::value>::type>class MyPet
{
}

is_base_of 为 true 会展开成：

template <typename T,typename = voidclass MyPet
{
}

is_base_of 为 false 时会找不到 Type 而报错。