nlohmann::detail::type_error: [json.exception.type_error.302

[mohammedalasmar]

This might be a simple question.

When I receive a json object and try to read a field in this object but that field doesn't exist, I get nlohmann::detail::type_error: [json.exception.type_error.302]

Is there any way to just get null or something like that instead of "terminating with uncaught exception"? Is there any error handler example available?

Example:

// create json obj and send it
json jObj;
jObj["name"] = "nick";
..

// received  jObj
std::string n =  jObj["username"]; // username is not right so I get the terminating error

Many thanks!
Mo

[nlohmann]

You can use contains (https://json.nlohmann.me/api/basic_json/contains/) to check if a key is present.

Also note a type error (302) is not thrown when a key is missing, but rather when you were able to retrieve a key, but the value type is not fitting the receiver type.

[mohammedalasmar]

Many thanks for the prompt reply!
contains works for me. I also found is_null() which does what I wanted as well.

bool isFound = jObj.contains("name");
or
bool isFound = jObj["name"].is_null();

[nlohmann]

Note the two options are not the same:

jObj.contains("name");

will never change jObj and will work on const and non-const values.

jObj["name"].is_null();

only returns true, because operator[] will insert a null value at key name in case there is no such key. When jObj is const, accessing a non-existing key is undefined behavior. See https://json.nlohmann.me/features/element_access/unchecked_access/ for more information.

tl;dr: The upper code is right. Don't use the lower one.

[tienviitta]

I got the same error but I want different behavior, e.g., how to handle null in JSON? For example I have this normally in my input JSON:

... "tv": { "disable": null, "enable": null, "on": false } ...

Those disable and enable are std::vector<std::string>s but those are specified only when on is true.

struct Tv {
    std::vector<std::string> disable;
    std::vector<std::string> enable;
    bool on;
};

void from_json(const nlo::json &j, Tv &tv) {
    j.at("disable").get_to(tv.disable);
    j.at("enable").get_to(tv.enable);
    j.at("on").get_to(tv.on);
}

So, should I check if the disable or enable is null with .is_null() in this case and if then set this as nullptr?

[tienviitta]

And this of course works when disable and enable are list of strings.

[tienviitta]

This would be my guess:

void from_json(const nlo::json &j, Tv &tv) {
    if (not j.at("disable").is_null())
        j.at("disable").get_to(tv.disable);
    if (not j.at("enable").is_null())
        j.at("enable").get_to(tv.enable);
    j.at("on").get_to(tv.on);
}