I have a key and I want to change the value of the key with another json object.
json newjs = ...;
json tempjs = ...;
newjs["key"] = tempjs["key"];
What will happen to the data existed in newjs["key"] previously?
Will nlohmann class automatically destroy it or is it a memory leak?
OR do I need to manually erase the key first and assign as above?
Internally it's a kept by an "ordered_map: a minimal map-like container that preserves insertion order".
The actual standard container used in this ordered_map is a std::vector<std::pair<const Key, T>, Allocator> and the assignment you do is performed via
T& operator[](const Key& key)
{
return emplace(key, T{}).first->second;
}
where emplace is defined as:
std::pair<iterator, bool> emplace(const key_type& key, T&& t)
{
for (auto it = this->begin(); it != this->end(); ++it)
{
if (it->first == key)
{
return {it, false};
}
}
Container::emplace_back(key, t);
return {--this->end(), true};
}
This means that operator[] tries to emplace a default initialized T into the internal map. If key isn't present in the map, it will succeed, otherwise it will fail.
Regardless of which, when emplace returns, there will be a T in the map and it's a reference to that T that is returned by operator[] and it's that you then copy assign to.
It's a "normal" copy assignment and no leaks should happen.