如何在迭代时从 poco json 中删除密钥

How to remove key from poco json while iterating it?

本文关键字：json 删除 密钥 poco 迭代      更新时间：2023-10-16

如何在迭代时从 Poco json 中删除密钥？喜欢：

Poco::JSON::Object::Ptr poco_json;
for (auto& objs : *poco_json)
{
     // do something
     if (objs.first == "specific key")
         poco_json->remove(key);     
}

或

Poco::JSON::Object::Ptr poco_json;
for(auto it = poco_json->begin();it != poco_json->end();)
{
    // do something
    if (it->first == "specific key")
        it = poco_json->remove(it->first);//error : poco didn't have something like this
    else
        ++it;
}

问题是从 JSON 中删除密钥后，它将使迭代器失效。我知道在 std：：map 中，擦除返回下一次迭代的有效迭代器，但我找不到类似的东西 Poco json。

std::map::erase 将迭代器返回到自 C++11 以来的下一个项目，在 C++11 之前，您以这种方式擦除项目：

for (auto it = m.begin(); it != m.end(); ) {
    if (it->first == someKey)
      m.erase(it++); // use post-increment,pass copy of iterator, advance it
    else 
      ++it;
  }

您可以以类似的方式执行此操作，同时从Poco::JSON::Object中删除密钥。您在哪里读到remove使迭代器无效？

来自源代码的一些代码片段：

class JSON_API Object {
    typedef std::map<std::string, Dynamic::Var> ValueMap; // <--- map
    //...
    Iterator begin();
        /// Returns begin iterator for values.
    Iterator end();
        /// Returns end iterator for values.
    void remove(const std::string& key);
        /// Removes the property with the given key.
    ValueMap          _values; // <---
};
inline Object::Iterator Object::begin()
{
    return _values.begin();
}
inline Object::Iterator Object::end()
{
    return _values.end();
}
inline void Object::remove(const std::string& key)
{
    _values.erase(key); // <--- erase is called on map, so iteratos are not invalidated
    if (_preserveInsOrder)
    {
        KeyList::iterator it = _keys.begin();
        KeyList::iterator end = _keys.end();
        for (; it != end; ++it)
        {
            if (key == (*it)->first)
            {
                _keys.erase(it);
                break;
            }
        }
    }
    _modified = true;
}

您可以将循环重写为：

for(auto it = poco_json->begin();it != poco_json->end();)
{
    // do something
    if (it->first == "specific key")
    {
        auto copyIt = it++;
        poco_json->remove(copyIt->first);
    }
    else
        ++it;
}

编辑为什么你的代码在 range-for 循环中不起作用：

for (auto& objs : *poco_json)
{
     // do something
     if (objs.first == "specific key")
         poco_json->remove(key);     
}

它被翻译成

for (auto it = poco_json->begin(); it != poco_json->end(); ++it)
{
     // do something
     if (it->first == "specific key")
         poco_json->remove(it->first);     
    // remove is called, it is erased from inner map
    // ++it is called on iterator which was invalidated, 
    // code crashes
}

您可以在 Poco 中修改此代码：

inline Iterator Object::remove(const std::string& key)
{
    auto ret_it = _values.erase(key);
    if (_preserveInsOrder)
    {
        KeyList::iterator it = _keys.begin();
        KeyList::iterator end = _keys.end();
        for (; it != end; ++it)
        {
            if (key == (*it)->first)
            {
                _keys.erase(it);
                break;
            }
        }
    }
    _modified = true;
  return ret_it;
}
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