nlohmann提供了成员函数type()，用于返回当前的json数据类型：

constexpr value_t type() const noexcept
{    return m_type;
}using value_t = detail::value_t;enum class value_t : std::uint8_t
{null,             ///< null valueobject,           ///< object (unordered set of name/value pairs)array,            ///< array (ordered collection of values)string,           ///< string valueboolean,          ///< boolean valuenumber_integer,   ///< number value (signed integer)number_unsigned,  ///< number value (unsigned integer)number_float,     ///< number value (floating-point)binary,           ///< binary array (ordered collection of bytes)discarded         ///< discarded by the parser callback function
};

可以根据type进行类型检查：

#include <iostream>
#include <nlohmann/json.hpp>
using namespace std;
using js