boost::any 如何检查 null/undefined 值

Question

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;
}

定义类型时这很容易，但是当类型未定义时（即因为尚未设置其值）我将如何获得相同的结果。

Value b;

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

Answer 1

如果没有值，

boost::any::empty 将 return true。

Demo

#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";
}

或者，您可以像在第一个示例中那样使用 boost::any::type，如果没有值，它将 return typeid(void):

Demo 2

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

boost::any 如何检查 null/undefined 值

boost::any how to check for a null/undefined value

c++

null

boost

types

boost-any

Demo

Demo 2