Check value for existence by json_pointer

[3galki]

I have found no proper way to check element existence.
I can use json.at("..."_json_pointer) but it will throw an exception if it is missed.
I can use json.value() method. But I have to do it twice to check all cases (or use default that will not used in my json).

[nlohmann]

Internally, the JSON Pointer implementation also relies on exceptions. That is, there would be no real difference between implementing a find(const json_pointer&) interface as it would be literally the same as a calling at and coping with possible exceptions.

[3galki]

yep, now I have looked in code. So, looks like I have to avoid such logics...

[nlohmann]

Related: #1017

[ArnCarveris]

@nlohmann I need this json.find("..."_json_pointer)

[3galki]

json.find("..."_json_pointer)

Just one question, what value should be for end() in this case?

[ArnCarveris]

@nlohmann Hmm good question, in mind answer is json.end(), but this is way not obvious.

Problem is what json.value("..."_json_pointer, /*default*/) throws exception when object in path not found.

I was forced to write this on my side:

std::istringstream s(key);
std::string k;

const Data* ptr = nullptr;

while (std::getline(s, k, '/'))
{
	if (k.empty())
	{
		ptr = &m_cData;
	}
	else if (ptr == nullptr)
	{
		return false;
	}
	else
	{
		auto it = ptr->find(k);
		
		if (it == ptr->end())
		{
			return false;
		}
		else
		{
			ptr = &it.value();
		}
	}
}

[nlohmann]

The issue is that iterators of different containers cannot be compared as this would be undefined behavior.

Therefore, writing code like

if (j.find("/foo/bar"_json_pointer) != j.end())
  ...

is not possible.

[ArnCarveris]

@nlohmann Ok, maybe this: json.search<std::string>("/foo/bar"_json_pointer, [](const auto& value) {});?

[nlohmann]

Passing a lambda feels strange. I'd rather pass a std::pair<bool, json::iterator> with the bool indicating whether the search was successful (asserting the iterator can be dereferenced).

[ArnCarveris]

@nlohmann You said that with iterator no possible. Maybe this:

std::string value;
if (json.try_get(/foo/bar"_json_pointer, value))
{
 //do something when found
}

[nlohmann]

What would value mean in this situation?

[3galki]

You may try to get value() work without exceptions. It will help in most cases. Additional, you can add some value, that can't be found in json, and use it as default.
Or you can look at std::optional

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.