Best way to catch errors when querying json

[apivovarov]

I need to create API to return item name if item is found or nullptr if not.
What is the best way to do it if all json layers are optional? Should I check if every single layer exist and array index is in range? Or should I try to query json and catch particular exception(s) which indicate that item does not exist or array index is our of range in json

// Returns nullptr if name is not in json or index is out of range
const char* GetName(const int index) const {
    return json["Product"]["Suppliers"][index]["name"]
        .get_ref<const std::string&>()
        .c_str();
}

[nlohmann]

I would do this (though not tested):

// Returns nullptr if name is not in json or index is out of range
const char* GetName(const int index) const {
	try
	{
	    return json.at("Product").at("Suppliers").at(index).at("name")
	        .get_ref<const std::string&>()
	        .c_str();
	}
	except (json::exception&)
	{
		return nullptr;
	}
}

[apivovarov]

is except Microsoft extension of C++?

[nlohmann]

No, it must be catch - I’m using too much Python these days.

[dota17]

The try-except statement is a Microsoft extension that supports structured exception handling in the C and C++ languages. This extension is Microsoft-specific.

More details are in this link: https://docs.microsoft.com/en-us/cpp/cpp/try-except-statement?view=vs-2019

[apivovarov]

is there any diff btw the following?

json.at("Product").at("Suppliers").at(index).at("name")
// and
json["Product"]["Suppliers"][index]["name"]

Why you used longer at notation instead of shorter square bracket op []?

[dota17]

As it stated in readme:

We designed the JSON class to behave just like an STL container.

at() will check the bound, but operator[] not. so at() is more safe.

[nlohmann]

Using operator[] on a nonexisting key/invalid index is undefined behavior. You must not do this if you are not sure the values exist.

Do you need further assistance?

[apivovarov]

Niels, Chen, thank you for great answers! Vielen Dank!