How to check if key existed

[leeqicheng]

Hi to all is there a method to check if the key exist ,

what is the close to

Java JSON

indivFB.has("object_id")

Thanks

[nlohmann]

Just like for a map: find().

[leeqicheng]

Hi thanks for the reply but im using iterator

any idea why is causing this problem ?

Value of type 'iterator' (aka 'nlohmann::detail::iter_impl<nlohmann::basic_json<std::map, std::vector, std::__1::basic_string, bool, long long, unsigned long long, double, std::allocator, adl_serializer> >') is not contextually convertible to 'bool'

[nlohmann]

I would need to see a code example for that.

[leeqicheng]

Hi thanks for time,

The problem i face is that sometimes the json result doesnt have the key so im trying to check for the key

[nlohmann]

When you iterate over an object (which I assume - I have not seen the JSON), doing this with a range for only gives you access to the values, not to the keys. You need to use items() for this.

The exception comes, because you try to assign a number (from ...["total_count"]) to a string (totallike). Try to output ...["total_count"] directly and use a number type for totallike.

[leeqicheng]

Sorry wrong image it should be this

[nlohmann]

Then use find:

if (json_field_data[p]["reactions"]["summary"].find("total_count") != json_field_data[p]["reactions"]["summary"].end())
   // now you can be sure json_field_data[p]["reactions"]["summary"]["total_count"] exists

Alternatively, you could use value and provide a default value (e.g., 0):

totallike = json_field_data[p]["reactions"]["summary"].value("total_count", 0);

[leeqicheng]

Thanks for the library and the help

[Tudyx]

In case if we are not sure if the keys before the nested key exist, should we use a nested if ? Because in the documentation it's wrote that is an undefined behavior if the element with key idx accessing by operator[] does not exist.

if (json_field_data[p].find("reactions") != json_field_data[p].end()){
    if(json_field_data[p]["reactions"].find("summary") != json_field_data[p]["reactions"].end()){
        if(json_field_data[p]["reactions"]["summary"].find("total_count") != json_field_data[p]["reactions"]["summary"].end())
    }
}

[nlohmann]

Yes, you need to check for each level. Note there is a contains function that could simplify your code: https://json.nlohmann.me/api/basic_json/contains/

Also note that that JSON Pointer could help you, see #2585 (comment)

[Tudyx]

Thank you! I chose a JSON Pointer option like in this comment #1449 (comment)
If it can be useful for someone :

if( json_field_data[p].contains(json::json_pointer("/reactions/summary/total_count")) == true ){

}