contains(Key&&, const T& cmp_value)

[DerAlbiG]

Hi,
when dealing with external json-data, it is not certain that a given key exists.
I frequently have constructs like these:
if (json.contains("key") && json["key"] == expected_value) { ... }

This looks up "key" twice and repeats "key" as a potential copy&paste-error.

I therefore propose to add a simple contains-overload to basic_json:

template<typename T, class KeyType, detail::enable_if_t<
                 detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
    bool contains(KeyType && key, const T& compare_value) const
    {
        auto it = find(std::forward<KeyType>(key));
        if (it != end())
            return it->template get<T>() == compare_value;
        return false;
    }

Of course this should be [[nodiscard]], however adhering to the lib-style, I left it as it is.
Overall, I think this overload is

• useful syntactic sugar
• its naming is intuitive enough
• it improves code-quality
• reduces potential errors.

I understand that this situation can be solved by using value<T>(Key, default_value) == expected_value, however since this constructs the default_value just to compare it, may be less efficient. It also is error-prone in the sense that default_value must be chosen to not be expected_value.