C++ boost：：如何检查空值/未定义值

我有一个boost:：any对象，我想检查它的类型

typedef boost::any Value;

Value a = 12;

if(a.type() == typeid(int)) {
    std::cout << boost::any_cast<int>(a) << std::endl;
}

typedef boost:：任意值；
a值=12；
如果（a.type（）==typeid（int））{
如果没有值，std:：cout将返回
true

std：：any has has_value（）方法，我希望boost也有相同的方法。
Value b;

if(b is undefined) {
   std::cout << "b is not defined" << std::endl;
}

#include "boost/any.hpp"
#include <iostream>

int main()
{
    boost::any a = 42;
    if (!a.empty())
        std::cout << "a has a value\n";

    boost::any b;
    if (b.empty())
        std::cout << "b does not have a value\n";
}

boost::any a = 42;
std::cout << std::boolalpha << (a.type() == typeid(int)) << std::endl; // true

boost::any b;
std::cout << std::boolalpha << (b.type() == typeid(void)) << std::endl; // true