Json Value

The json_value type is a lazy pull parser. It provides lookup and extraction mechanisms for when the regular mappings cannot work or doing data exploration. One such example is the variant_on_members.cpp where the structure of the member is used to determine which parser to use. For types that use a tag or member to discriminate this isn't necessary as there are json_variant_... mappings for that. The use of json_value is not encouraged except when in conjunction with the mappings such as the json_raw mapping when existing mappings do not work, or data discovery.

The general query interface to json_value is via operator[]. This can be used with a JSON Path to extract a new json_value. e.g

auto type_jv = jv["value.type"];
assert( type_kv );
auto type = as<int>( type_jv );

The example extracts the "type" member of "value" and returns a new json_value. If it is not found, the operator bool will evaluate to false. It is also false if the "type" member had a value of null. If one wants to know if it was found or not, has_more( ) will return false as the return is a default constructed json_value. Following that, one can use the as function to parse the value. from_json will, also, work here.

There are several ways to parse from a json_value. The most succinct is the T as<T>( json_value ) free function, but there is an json_value::as<T>( ) const member function too. The from_json interface used for parsing from string data works too. Finally, one can cast to their desired type.

One can also check the type of the current value via several members such as is_class, is_null, is_array, is_number, is_string, and is_bool. There is also type( ) member that returns an enum of the basic JSON types that can be switched over.

The json_value type is a range and one can iterate over classes or arrays. The value_type is a json_pair type that has a name and a value member. The name is an optional<string_view>. When iterating over an array, the name will be nullopt. The value member is a json_value. The json_pair, also, fulfills the tuple protocol and can get used with structured bindings.

for( auto [name,value]: my_jv_of_class ) {
    std::cout << "name:" << *name << " value: " << value.get_string_view( ) << '\n';
}

The following is equivalent

auto jv = other_jv[5]["a"]["b"][2];

auto jv = other_jv["[5].a.b[2]"];