c++ - 递归添加子树以 boost 属性树

Question

我想用 C++ 编写一个参数服务器，我可以在其中递归地将参数树转储到属性树中，然后将其写入 JSON 文件。转储函数如下所示:

 void Params::dump(string filename) {
   // Create a root
   pt::ptree root;

   // Fill the root with parameters
   mapToPt(curParams, root);

   // Write to cout
   pt::write_json(cout, root);
 }

mapToPt 应该递归地遍历我的参数服务器的层次结构，并在这样做时填充属性树:

  void Params::mapToPt(boost::shared_ptr<Params> curParams, pt::ptree &root) {
   // Fill current root with parameters from curParams ParameterMap
   map<string, boost::shared_ptr<Param> >::iterator it;
   for ( it = curParams->getParamMap().begin(); it != curParams-getParamMap().end(); it++ ) {
     root.put(it->first, it->second->getValue());
     cout << "Add Parameter: \n";
     cout << "Parameter name: " << it->first << "\n";
     cout << "Parameter value: " << it->second->getValue() << "\n";
   }

   // Recursively go through all children to do the same to them
   if(curParams->hasChildren()) { //ERROR LINE
     map<string, boost::shared_ptr<Params> >::iterator it;
     for ( it = curParams->getChildren().begin(); it != curParams-getChildren().end(); it++ ) {
       pt::ptree new_tree;
       root.add_child(it->second->getName(), new_tree);
       cout << "Add Child: \n";
       cout << "Child name: " << it->second->getName() << "\n";
       mapToPt(it->second, new_tree);
     }
   }
 }

我的问题是，一旦我进入递归，错误就出现在随机行上，而这不可能是错误的原因。 “basic_string::_M_construct null 无效”是错误消息。我相信我可能会访问已删除的内容，这可能是由于我遍历属性树子项的方式所致。我的做法是错误的还是有其他方法？

谢谢。

Answer 1

当 mapToPt 还需要一个指向 Params 实例的指针时，为什么它是一个成员？

哎呀，有点困惑。

在设计层面，您的 Params 类型看起来无法决定它是否是叶节点。此外，它还受到“准类”设计的影响， setter/getter 基本上保证没有类不变性。在这种情况下，最好只使用一个包含成员字段的结构。

Note, if you fail to return by reference from getParamMap() and getChildren() then you already have Undefined Behaviour in both loops, because the iterators then point into non-existent copies of containers.

You should check this. Also, see my working demo below

在实现层面，这会给您带来问题:

        pt::ptree new_tree;
        root.add_child(it->second->getName(), new_tree);

add_child 插入 new_tree 的拷贝。以后对 new_tree 的任何修改都无效。相反，写:

        pt::ptree& new_tree = root.add_child(it->second->getName(), {});

在这里，new_tree 成为对实际添加的树的引用。

尝试修复

风格仍然低于我的预期。就我个人而言，我会仔细检查这段代码中 shared_ptr 的使用。

但这可能会对您有所帮助:

Live On Coliru

#include <boost/make_shared.hpp>
#include <boost/property_tree/json_parser.hpp>
#include <iostream>
#include <map>

namespace pt = boost::property_tree;

struct Param {
    std::string getValue() const { return "42"; }
};

struct Params {
    using ParamMap = std::map<std::string, boost::shared_ptr<Param> >;
    using Children = std::map<std::string, boost::shared_ptr<Params> >;

    Params(std::string name = "") : _name(name) {}

    std::string getName() const         { return _name; }

    ParamMap& getParamMap()             { return _map; } 
    ParamMap const& getParamMap() const { return _map; } 

    bool hasChildren() const            { return !_children.empty(); }
    Children& getChildren()             { return _children; } 
    Children const& getChildren() const { return _children; } 

    static void mapToPt(boost::shared_ptr<Params> curParams, pt::ptree &root);

  private:
    std::string _name;
    ParamMap _map;
    Children _children;
};

void Params::mapToPt(boost::shared_ptr<Params> curParams, pt::ptree &root) {
    // Fill current root with parameters from curParams ParameterMap
    std::map<std::string, boost::shared_ptr<Param> >::iterator it;
    for (it = curParams->getParamMap().begin(); it != curParams->getParamMap().end(); it++) {
        root.put(it->first, it->second->getValue());
        //std::cout << "Add Parameter: \n";
        //std::cout << "Parameter name: " << it->first << "\n";
        //std::cout << "Parameter value: " << it->second->getValue() << "\n";
    }

    // Recursively go through all children to do the same to them
    if (curParams->hasChildren()) {
        for (auto it = curParams->getChildren().begin(); it != curParams->getChildren().end(); it++) {
            pt::ptree& new_tree = root.add_child(it->second->getName(), {});
            //std::cout << "Add Child: \n";
            //std::cout << "Child name: " << it->second->getName() << "\n";
            mapToPt(it->second, new_tree);
        }
    }
}

int main() {
    auto a = boost::make_shared<Params>("rootparams");

    a->getParamMap().emplace("one", boost::make_shared<Param>());
    a->getParamMap().emplace("two", boost::make_shared<Param>());
    a->getParamMap().emplace("three", boost::make_shared<Param>());

    a->getChildren().emplace("child1", boost::make_shared<Params>("child1-name"))
        .first->second->getParamMap().emplace("four", boost::make_shared<Param>());

    a->getChildren().emplace("child2", boost::make_shared<Params>("child2-name"))
        .first->second->getParamMap().emplace("five", boost::make_shared<Param>());

    pt::ptree root;
    a->mapToPt(a, root);

    write_json(std::cout, root);
}

打印

{
    "one": "42",
    "three": "42",
    "two": "42",
    "child1-name": {
        "four": "42"
    },
    "child2-name": {
        "five": "42"
    }
}