c++17引入了any 和 variant，可以将任意数据类型统一用any或variant类型表示，在开发中还是能够带来很多便利的。在c++11版本中，可以用下面这个例子，仿照实现一个Any类型。
#include <iostream>
#include <stdexcept>
#include <memory>class UserData{
public:UserData(){}std::string operator()(){return m_name;}
private:std::string m_name="world";
};
class Any
{
public:template<typename T>Any(const T& val) noexcept : content(new Holder<T>(val)) {}Any(const Any& other) noexcept : content(other.content ? other.content->Copy() : nullptr) {}Any(Any&& other) noexcept :content(std::move(other.content)) {}Any& operator=(const Any& other){if (this == &other)return *this;content.reset(other.content ? other.content->Copy() : nullptr);return *this;}template<typename T>T& get() const{if(content.get() == nullptr){throw std::exception_ptr();}if (typeid(T).hash_code() != content->TypeID()){throw std::bad_cast();}return static_cast<Holder<T>*>(content.get())->value;}std::string TypeName() const{if(content)return content->TypeName();return "";}size_t TypeID(){if(content)return content->TypeID();return 0;}
private:struct Base {virtual ~Base() {}virtual Base* Copy() const = 0;virtual size_t TypeID() const = 0;virtual std::string TypeName() const = 0;};template<typename T>struct Holder :public Base{Holder<T>() = delete;Holder<T>(const T& val) : value(val), typid(typeid(T).hash_code()) {}~Holder<T>() {}Base* Copy() const override { return new Holder<T>(value); }size_t TypeID() const override { return typid; }std::string TypeName() const override{ return typeid(T).name();}T value;size_t typid;};std::unique_ptr<Base> content;
};int main()
{int iVal = 10;Any param = iVal;int& iv = param.get<int>();iv = 100;std::cout<<param.TypeName()<<", val="<<param.get<int>()<<std::endl;std::string sVal="hello";param = sVal;std::cout<<param.TypeName()<<", val="<<param.get<std::string>()<<std::endl;double dVal = 101.21;Any paramd = dVal;param = paramd;std::cout<<param.TypeName()<<", val="<<param.get<double>()<<std::endl;UserData data;param = data;UserData& dt = param.get<UserData>();std::cout<<param.TypeName()<<", val="<<dt()<<std::endl;return 0;
}
