Value throws on null string

[p-groarke]

Take for example { "key" : null }.

Using value("key", "") on a null string throws [json.exception.type_error.302] type must be string, but is null.

Expected behavior : Use default value.

[nlohmann]

I do not fully understand your issue. Could you provide actual example code?

[p-groarke]

For example,

	nlohmann::json _j = nlohmann::json::parse("{\"test\" : null}");
	std::string val = _j.value("test", "");

[nlohmann]

I see. I fear this is a conceptual issue: the default value is only returned if the key is not present. If the key is present, the default value and the existing value must have the same type. See https://nlohmann.github.io/json/classnlohmann_1_1basic__json_adcf8ca5079f5db993820bf50036bf45d.html#adcf8ca5079f5db993820bf50036bf45d for more information.

[p-groarke]

I guessed that much, I would like to challenge that however ;)

In a perfect world where API makers would follow the JSON spec perfectly, the current solution would work great. However, reality is quite different. I don't see any downsides to modifying value to deal with this sort of issue. get<> offers a great alternative if you want to deal with exceptions.

Note that with the current behavior, one would need to check every single string for null. Which is madness when using huge APIs. Since if an end-point returns null for 1 empty string, it means any string may be null.

Willing to discuss changing value behavior?

[p-groarke]

Note that I am only talking about null, which is a special snowflake. Using value to parse an int into a string should throw.

[gregmarr]

Why should null be more special that any other type? If you don't control the source of the JSON and you don't know that the type of the value of "test"' is a string, then you can't use value() to retrieve it. You need to get check that the type is what you expect before you retrieve it.

Why should the first of these cases be okay and the second be an error?

nlohmann::json _j = nlohmann::json::parse("{\"test\" : null}");
std::string val = _j.value("test", "");

vs

nlohmann::json _j = nlohmann::json::parse("{\"test\" : 0}");
std::string val = _j.value("test", "");

[p-groarke]

Null is used to denote empty strings, 0 isn't. Documentation lets you know what to expect.

@nlohmann yes or no?

[nlohmann]

@p-groarke I am not sure what you mean in #1163 (comment). An empty string is "". null just means "no value", but has no type.

I agree with @gregmarr that null is not a special value and hence would not need special treatment, see the documentation of value. Changing the function's semantics would be a breaking change which I would not be willing to do, because it would mean for people to adjust their code. Adding a new function with special semantics for null values could be possible, but could also be realized with the public API.

[p-groarke]

Alright.

[2beens]

@nlohmann
Sorry for writing here again, but I have a similar problem with the following code:
std::map<std::string, std::vector<std::string>> parsedParams = nlohmann::json::parse(parametersJson);
where parametersJson is a string of value: { "eventToken" : [null] }.
The exception I get is:

abort_message: assertion "terminating with uncaught exception of type nlohmann::detail::type_error: [json.exception.type_error.302] type must be string, but is null" failed

Is there any way I can get this map, which has only one element (of key eventToken) and null value?
Can you please suggest something else if not?

Thanks a lot, and sorry for any inconveniences!

[nlohmann]

I fear there is no easier solution than to manually filter the array at key "eventToken" from elements that are not strings.

Example (using https://en.wikipedia.org/wiki/Erase%E2%80%93remove_idiom):

#include <iostream>
#include "json.hpp"

using json = nlohmann::json;

int main() {
    json j = {"foo", nullptr, "bar", 1};
    j.erase(std::remove_if(j.begin(), j.end(), [](json& el) { return not el.is_string(); }), j.end());
    std::cout << j << std::endl;
}

Output:

["foo","bar"]