Request object by key returning an optional

[nlohmann]

I saw the following snippet (see below) being used to avoid repeated code like

if(j.contains("key")) { something(j["key"]) }

The idea is to add a getter function that returns std::nullopt_t in case an object key is not found. As this approach is very similar to the value function which returns a default value in this case, it may be a good addition to the public API.

---

/// Optional element access
/// @returns empty if element does not exist
std::optional<json> optional_value(const std::string& name) const
{
    auto it = find(name);
    return it != end() ? *it : std::optional<Njson>();
}

/// Optional element access with value type
/// @returns empty if element doesn't exist or does not have the given type
template<typename ValueType>
std::optional<ValueType> optional_value(const std::string& name) const
{
    auto it = find(name);
    return (it != end() && it->type() == nlohmann::json(ValueType()).type())
        ? it->get<ValueType>()
        : std::optional<ValueType>();
}

[gregmarr]

I can see utility in these functions, but I'm not sure about the naming. I'm not sure it's obvious that in the below snippet that v1 is json and v2 is int.

auto v1 = j.optional_value("foo");
auto v2 = j.optional_value<int>("foo");

The second is value() but returns an optional of the type, so that makes sense. Isn't the first get() but returns an optional json? So optional_get?

[nlohmann]

Yes - naming is important there. I am also not sure whether both functions should have the same name, because the second one is stricter and it may be surprising to not get a value even if the key is present (but the time is not matching).