trying to log message for missing keyword, difference between ["foo"] and at("foo")

[cwreynolds]

I want to print a helpful message when a required keyword is missing from a json file. I mimicked the example code given here to print a message about a caught exception. Instead of printing “key 'foo' not found” it continued to print the less helpful “type must be number, but is null”. I eventually realized that the difference was not related to handling the exception—the default handler was fine—but it came down to how I wrote the keyword lookup. This produces “type must be number, but is null”:

float foo = properties["foo"];

While this produces what I wanted, “key ‘foo’ not found”:

float foo = properties.at("foo");

Is this an intentional/expected difference between [] and at()? And is the solution to my desire for more specific “keyword not found” error messages simply to use at() instead of []? Or is something more subtle going on?

(I’m using version 3.6.1)

[gregmarr]

Yes, properties["foo"] creates a new null entry if the entry isn't found and properties is not const. If it is const, then it asserts (I think). If you aren't sure if it's there, you need to use find() first to check, or at() with exception handling.

[cwreynolds]

Thanks @gregmarr for the explanation. Performance of json reading is not a key issue in my application, it only happens at initialization. So I will replace [] with at() on nlohmann::json objects in order to get better error messages “for free.”