boost::ptree 找到？或者如何访问深度数组？ C++

Question

我现在已经尝试了太多时间，从 boost 库中访问 json_reader ptree。

我有一个经常被封装的 json 文件：(pseudo-json:)

"Foo": {
  "nameofFoo:"foofoo"
  "Bar": [{
    "BarFoo": 
      { BarFooDeep: { 
           BarFooDeepDeep: { 
            "BarFooValue1": 123 
            "BarFooValue2" : 456
          }
        }
      }
     "FooBar": [ {
        "FooBarDeep" :[ {
           FooBarDeepDeep:[ {
              FooBarValue1: "ineedthis"
              FooBarValue2: "andthis"
              } ]
           FooBarDeepDeep1:[ {
              FooBarValue1: "ineedthis"
              FooBarValue2: "andthis"
              } ]
        "FooBarDeep" :[ {
           FooBarDeepDeep2:[ {
              FooBarValue1: "ineedthis"
              FooBarValue2: "andthis"
              } ]
           FooBarDeepDeep3:[ {
              FooBarValue1: "ineedthis"
              FooBarValue2: "andthis"
              } ]
and so on .... won t complete this now...

现在我只需要获取所有FooBar的FooBarValue1和FooBarValue2。

我知道 ptree 将数组与空子 ("") 放在一起

我可以通过递归遍历所有子节点来访问所有成员。

但是没有更好的方法来访问特殊值吗？

ptree find 是如何工作的？我总是遇到编译器错误...

ptree jsonPT;
read_json( JSON_PATH, jsonPT);
ptree::const_iterator myIT = jsonPT.find("FooBarValue1");
double mlat = boost::lexical_cast<int>(myIT->second.data());

错误：从 'boost::property_tree::basic_ptree, std::basic_string >::assoc_iterator' 到非标量类型 'boost::property_tree::basic_ptree, std::basic_string >::const_iterator' 请求 ptree::const_iterator myIT = jsonPT.find("FooBarValue1");

谁能给我一个有用的提示如何访问这个 ptree？！？

Answer 1

正如我评论的链接答案 (Boost.PropertyTree subpath processing) 中所暗示的，您可以编写自己的“选择器”查询，因此您可以编写如下内容：

read_json("input.txt", pt);

std::ostream_iterator<std::string> out(std::cout, ", ");

std::cout << "\nSpecific children but in arrays: ";
enumerate_path(pt, "Foo.Bar..FooBar..FooBarDeep1..FooBarDeepDeep6..FooBarValue2", out);

std::cout << "\nSingle wildcard: ";
enumerate_path(pt, "Foo.Bar..FooBar..FooBarDeep1..*..FooBarValue2", out);

std::cout << "\nTwo wildcards: ";
enumerate_path(pt, "Foo.Bar..FooBar..*..*..FooBarValue2", out);

enumerate_path 函数不需要太复杂，它可以接受任何输出迭代器（所以你也可以back_inserter(some_vector)）：

template <typename Tree, typename Out, typename T = std::string>
Out enumerate_path(Tree const& pt, typename Tree::path_type path, Out out) {
    if (path.empty())
        return out;

    if (path.single()) {
        *out++ = pt.template get<T>(path);
    } else {
        auto head = path.reduce();
        for (auto& child : pt) {
            if (head == "*" || child.first == head) {
                out = enumerate_path(child.second, path, out);
            }
        }
    }

    return out;
}

作为简单的工作演示打印：

Specific children but in arrays: andthis6, 
Single wildcard: andthis6, andthis7, andthis8, andthis9, 
Two wildcards: andthis1, andthis2, andthis3, andthis4, andthis6, andthis7, andthis8, andthis9, 

那就是下面的input.txt：

{
    "Foo": {
        "nameofFoo": "foofoo",
        "Bar": [{
            "BarFoo": {
                "BarFooDeep": {
                    "BarFooDeepDeep": {
                        "BarFooValue1": 123,
                        "BarFooValue2": 456
                    }
                }
            },
            "FooBar": [{
                "FooBarDeep0": [{
                    "FooBarDeepDeep1": [{
                        "FooBarValue1": "ineedthis1",
                        "FooBarValue2": "andthis1"
                    }],
                    "FooBarDeepDeep2": [{
                        "FooBarValue1": "ineedthis2",
                        "FooBarValue2": "andthis2"
                    }]
                },
                {
                    "FooBarDeepDeep3": [{
                        "FooBarValue1": "ineedthis3",
                        "FooBarValue2": "andthis3"
                    }],
                    "FooBarDeepDeep4": [{
                        "FooBarValue1": "ineedthis4",
                        "FooBarValue2": "andthis4"
                    }]
                }],
                "FooBarDeep1": [{
                    "FooBarDeepDeep6": [{
                        "FooBarValue1": "ineedthis6",
                        "FooBarValue2": "andthis6"
                    }],
                    "FooBarDeepDeep7": [{
                        "FooBarValue1": "ineedthis7",
                        "FooBarValue2": "andthis7"
                    }]
                },
                {
                    "FooBarDeepDeep8": [{
                        "FooBarValue1": "ineedthis8",
                        "FooBarValue2": "andthis8"
                    }],
                    "FooBarDeepDeep9": [{
                        "FooBarValue1": "ineedthis9",
                        "FooBarValue2": "andthis9"
                    }]
                }]
            }]
        }]
    }
}

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#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>
#include <iostream>

template <typename Tree, typename Out, typename T = std::string>
Out enumerate_path(Tree const& pt, typename Tree::path_type path, Out out) {
    if (path.empty())
        return out;

    if (path.single()) {
        *out++ = pt.template get<T>(path);
    } else {
        auto head = path.reduce();
        for (auto& child : pt) {
            if (head == "*" || child.first == head) {
                out = enumerate_path(child.second, path, out);
            }
        }
    }

    return out;
}

int main() {

    std::ostream_iterator<std::string> out(std::cout, ", ");
    using namespace boost::property_tree;

    ptree pt;
    read_json("input.txt", pt);

    std::cout << "\nSpecific children but in arrays: ";
    enumerate_path(pt, "Foo.Bar..FooBar..FooBarDeep1..FooBarDeepDeep6..FooBarValue2", out);

    std::cout << "\nSingle wildcard: ";
    enumerate_path(pt, "Foo.Bar..FooBar..FooBarDeep1..*..FooBarValue2", out);

    std::cout << "\nTwo wildcards: ";
    enumerate_path(pt, "Foo.Bar..FooBar..*..*..FooBarValue2", out);
}

Answer 2

find() 用于按键检索子节点；它不会搜索整个 ptree。它返回一个assoc_iterator（或const_assoc_iterator），您可以通过父级上的to_iterator() 方法将其转换为iterator：

ptree::const_assoc_iterator assoc = jsonPT.find("FooBarValue1");
ptree::const_iterator myIT = jsonPT.to_iterator(assoc);

要搜索 ptree，您需要递归地迭代它：

struct Searcher {
    struct Path { std::string const& key; Path const* prev; };
    void operator()(ptree const& node, Path const* path = nullptr) const {
        auto it = node.find("FooBarValue1");
        if (it == node.not_found()) {
            for (auto const& child : node) {  // depth-first search
                Path next{child.first, path};
                (*this)(child.second, &next);
            }
        } else {    // found "FooBarValue1"
            double mlat = boost::lexical_cast<int>(myIT->second.data());
            // ...
            std::cout << "Mlat: " << mlat << std::endl;
            std::cout << "Path (reversed): ";
            for (Path const* p = path; p != nullptr; p = p->prev)
                std::cout << p << ".";
            std::cout << std::endl;
        }
    }
};
Searcher{}(jsonPT);

编写递归遍历的替代方法是 C++14 通用 lambda，或在 C++11 中使用std::function 的类型擦除具体 lambda。