json_filter.md

jsoncons::json_filter

typedef basic_json_filter<char> json_filter

The json_filter class is an instantiation of the basic_json_filter class template that uses char as the character type.

json_filter is noncopyable and nonmoveable.

Header

#include <jsoncons/json_filter.hpp>

Base classes

json_input_handler

Member types

Member type	Definition
string_view_type	A non-owning view of a string, holds a pointer to character data and length. Supports conversion to and from strings. Will be typedefed to the C++ 17 string view if JSONCONS_HAS_STRING_VIEW is defined in jsoncons_config.hpp, otherwise proxied.

Constructors

json_filter(json_input_handler& handler)

All JSON events that pass through the json_filter go to the specified json_input_handler (i.e. another filter.) You must ensure that the handler exists as long as does json_filter, as json_filter holds a pointer to but does not own this object.

json_filter(json_output_handler& handler)

All JSON events that pass through the json_filter go to the specified json_output_handler. You must ensure that the handler exists as long as does json_filter, as json_filter holds a pointer to but does not own this object.

Accessors

operator json_output_handler&() 

Adapts a json_filter to a json_output_handler

json_input_handler& downstream_handler()

Returns a reference to the JSON handler that sends json events to downstream handlers.

Examples

Rename object member names with the built in filter rename_object_member_filter

#include <sstream>
#include <jsoncons/json.hpp>
#include <jsoncons/json_filter.hpp>

using namespace jsoncons;

int main()
{
    std::string s = R"({"first":1,"second":2,"fourth":3,"fifth":4})";    

    json_serializer serializer(std::cout);

    // Filters can be chained
    rename_object_member_filter filter2("fifth", "fourth", serializer);
    rename_object_member_filter filter1("fourth", "third", filter2);

    // A filter can be passed to any function that takes
    // a json_input_handler ...
    std::cout << "(1) ";
    std::istringstream is(s);
    json_reader reader(is, filter1);
    reader.read();
    std::cout << std::endl;

    // or a json_output_handler    
    std::cout << "(2) ";
    ojson j = ojson::parse(s);
    j.dump(filter1);
    std::cout << std::endl;
}

Output:

(1) {"first":1,"second":2,"third":3,"fourth":4}
(2) {"first":1,"second":2,"third":3,"fourth":4}

Fix up names in an address book JSON file

Example address book file (address-book.json):

{
    "address-book" : 
    [
        {
            "name":"Jane Roe",
            "email":"[email protected]"
        },
        {
             "name":"John",
             "email" : "[email protected]"
         }
    ]
}

Suppose you want to break the name into a first name and last name, and report a warning when name does not contain a space or tab separated part.

You can achieve the desired result by subclassing the json_filter class, overriding the default methods for receiving name and string value events, and passing modified events on to the parent json_input_handler (which in this example will forward them to a json_serializer.)

#include <jsoncons/json_serializer.hpp>
#include <jsoncons/json_filter.hpp>
#include <jsoncons/json_reader.hpp>

using namespace jsoncons;


class name_fix_up_filter : public json_filter
{
public:
    name_fix_up_filter(json_output_handler& handler)
        : json_filter(handler)
    {
    }

private:
    void do_name(const string_view_type& name, 
                 const parsing_context& context) override
    {
        member_name_ = name;
        if (member_name_ != "name")
        {
            this->downstream_handler().name(name, context);
        }
    }

    void do_string_value(const string_view_type& s, 
                         const parsing_context& context) override
    {
        if (member_name_ == "name")
        {
            size_t end_first = val.find_first_of(" \t");
            size_t start_last = val.find_first_not_of(" \t", end_first);
            this->downstream_handler().name("first-name", context);
            string_view_type first = val.substr(0, end_first);
            this->downstream_handler().value(first, context);
            if (start_last != string_view_type::npos)
            {
                this->downstream_handler().name("last-name", context);
                string_view_type last = val.substr(start_last);
                this->downstream_handler().value(last, context);
            }
            else
            {
                std::cerr << "Incomplete name \"" << s
                   << "\" at line " << context.line_number()
                   << " and column " << context.column_number() << std::endl;
            }
        }
        else
        {
            this->downstream_handler().value(s, context);
        }
    }

    std::string member_name_;
};

Configure a rename_object_member_filter to emit json events to a json_serializer.

std::ofstream os("output/new-address-book.json");
json_serializer serializer(os, true);
name_fix_up_filter filter(serializer);

Parse the input and send the json events into the filter ...

std::cout << "(1) ";
std::ifstream is("input/address-book.json");
json_reader reader(is, filter);
reader.read();
std:: << "\n";

or read into a json value and write to the filter

std::cout << "(2) ";
json j;
is >> j;
j.dump(filter);
std:: << "\n";

Output:

(1) Incomplete name "John" at line 9 and column 26 
(2) Incomplete name "John" at line 0 and column 0

Note that when filtering json events written from a json value to an output handler, contexual line and column information in the original file has been lost.

The new address book (`address-book-new.json`) with name fixes is
```json
{
    "address-book":
    [
        {
            "first-name":"Jane",
            "last-name":"Roe",
            "email":"[email protected]"
        },
        {
            "first-name":"John",
            "email":"[email protected]"
        }
    ]
}