How to remove key from poco json while iterating it? - c++

Question

How do I remove key from a Poco json while iterating it? Like:
Poco::JSON::Object::Ptr poco_json;
for (auto& objs : *poco_json)
{
// do something
if (objs.first == "specific key")
poco_json->remove(key);
}
or
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;
}
the problem is after remove a key from the json, it will invalidate the iterators. I know that in std::map, erase return the valid iterator for next iteration, but I cant find something similar for Poco json.

Answer 1

std::map::erase returns iterator to next item since C++11, before c++11 you erase items in this way:
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;
}
and you can do it in similar way while erasing key from Poco::JSON::Object. Where did you read that remove invalidates iterators?
Some snippet code from source:
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;
}
You could rewrite your loop into:
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;
}
EDIT
Why your code doesn't work on range-for loop:
for (auto& objs : *poco_json)
{
// do something
if (objs.first == "specific key")
poco_json->remove(key);
}
it is translated into
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
}

Answer 2

You can modify this code in Poco:
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;
}