Boost属性树不是JSON库。
如果已经包含了nlohmann::json，为什么还要使用它呢？
你的选择器路径可能与JSON不匹配，这很难说，因为你展示的例子工作正常：

**一个

图纸

{
    "pi": "3.141",
    "temp": "3.141",
    "happy": "true",
    "name": "Niels",
    "nothing": "null",
    "answer": {
        "everything": "42"
    },
    "list": [
        "1",
        "0",
        "2"
    ],
    "object": {
        "currency": "USD",
        "value": "42.99"
    }
}
Yay 3.141 and 3.141

没问题。
但是，请注意所有类型信息是如何丢失的，这只是触及PropertyTree中“限制”的表面：https://www.boost.org/doc/libs/1_75_0/doc/html/property_tree/parsers.html#property_tree.parsers.json_parser

使用Boost JSON

严肃点：

#include <boost/json.hpp>
#include <iostream>

int main() {
    auto root = boost::json::parse(R"({
    "pi": 3.141,
    "temp": 3.141,
    "happy": true,
    "name": "Niels",
    "nothing": null,
    "answer": {
        "everything": 42
    },
    "list": [
        1,
        0,
        2
    ],
    "object": {
        "currency": "USD",
        "value": 42.99
    }
})");

    std::cout << root << "\n";
    auto valPi   = root.at("pi").get_double();
    auto valtemp = root.at("temp").get_double();

    std::cout << "Yay " << valPi << " and " << valtemp << "\n";
}

图纸

{"pi":3.141E0,"temp":3.141E0,"happy":true,"name":"Niels","nothing":null,"answer":{"everything":42},"list":
[1,0,2],"object":{"currency":"USD","value":4.299E1}}
Yay 3.141 and 3.141

使用nlohmann::json

一个月一次

#include <nlohmann/json.hpp>
#include <iostream>

int main() {
    auto root = nlohmann::json::parse(R"({
    "pi": 3.141,
    "temp": 3.141,
    "happy": true,
    "name": "Niels",
    "nothing": null,
    "answer": {
        "everything": 42
    },
    "list": [
        1,
        0,
        2
    ],
    "object": {
        "currency": "USD",
        "value": 42.99
    }
})");

    std::cout << root << "\n";
    auto valPi   = root["pi"].get<double>();
    auto valtemp = root["temp"].get<double>();

    std::cout << "Yay " << valPi << " and " << valtemp << "\n";
}

图纸

{"answer":{"everything":42},"happy":true,"list":[1,0,2],"name":"Niels","nothing":null,"object":{"currency"
:"USD","value":42.99},"pi":3.141,"temp":3.141}
Yay 3.141 and 3.141

string valPi = root.get<string>("pi");

应将其更改为：

std::string valPi =root.get<std::string>(pt::ptree::path_type{"pi"});

如果您想要第2级：

int answer_everything =root.get<int>(pt::ptree::path_type{"answer.everthing"});

或者更喜欢使用另一种定界符：

int answer_everything =root.get<int>(pt::ptree::path_type{"answer\neverthing","\n"});