add_if_absent.md

jsoncons::jsonpointer::add_if_absent

Adds a value to an object or inserts it into an array at the target location, if a value does not already exist at that location.

#include <jsoncons_ext/jsonpointer/jsonpointer.hpp>

template<class Json, class T>
void add_if_absent(Json& target, 
                    const basic_json_pointer<Json::char_type>& location, 
                    T&& value, 
                    bool create_if_missing = false);                     (1) 

template<class Json, class T>
void add_if_absent(Json& target, 
                   const basic_json_pointer<Json::char_type>& location, 
                   T&& value, 
                   std::error_code& ec);                                 (2) 

template<class Json, class T>
void add_if_absent(Json& target, 
                   const basic_json_pointer<Json::char_type>& location, 
                   T&& value, 
                   bool create_if_missing, 
                   std::error_code& ec);                                 (3) 

template<class Json, class StringSource, class T>
void add_if_absent(Json& target, 
                    const StringSource& location_str, 
                    T&& value, 
                    bool create_if_missing = false);                     (4) 

template<class Json, class StringSource, class T>
void add_if_absent(Json& target, 
                   const StringSource& location_str, 
                   T&& value, 
                   std::error_code& ec);                                 (5) 

template<class Json, class StringSource, class T>
void add_if_absent(Json& target, 
                   const StringSource& location_str, 
                   T&& value, 
                   bool create_if_missing, 
                   std::error_code& ec);                                 (6) 

Inserts a value into the target at the specified location, if the location doesn't specify an object member that already has the same key.

• If location specifies an array index, a new value is inserted into the array at the specified index.

• If location specifies an object member that does not already exist, a new member is added to the object.

Parameters

target	JSON value
location	A basic_json_pointer
location_str	A JSON Pointer provided as a string, string view, or C-string
value	New value
create_if_missing (since 0.162.0)	Create key-object pairs when object key is missing
ec	out-parameter for reporting errors in the non-throwing overload

Return value

None

Exceptions

(1) Throws a jsonpointer_error if add_if_absent fails.

(2) Sets the out-parameter ec to the jsonpointer_error_category if add_if_absent fails.

Examples

Add a member to a target location that does not already exist

#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpointer/jsonpointer.hpp>

// for brevity
using jsoncons::json;
namespace jsonpointer = jsoncons::jsonpointer;

int main()
{
    auto target = json::parse(R"(
        { "foo": "bar"}
    )");

    std::error_code ec;
    jsonpointer::add_if_absent(target, "/baz", json("qux"), ec);
    if (ec)
    {
        std::cout << ec.message() << std::endl;
    }
    else
    {
        std::cout << target << std::endl;
    }
}

Output:

{"baz":"qux","foo":"bar"}

Add an element to the second position in an array

#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpointer/jsonpointer.hpp>

using jsoncons::json;
namespace jsonpointer = jsoncons::jsonpointer;

int main()
{
    auto target = json::parse(R"(
        { "foo": [ "bar", "baz" ] }
    )");

    std::error_code ec;
    jsonpointer::add_if_absent(target, "/foo/1", json("qux"), ec);
    if (ec)
    {
        std::cout << ec.message() << std::endl;
    }
    else
    {
        std::cout << target << std::endl;
    }
}

Output:

{"foo":["bar","qux","baz"]}

Add a value at the end of an array

#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpointer/jsonpointer.hpp>

using jsoncons::json;
namespace jsonpointer = jsoncons::jsonpointer;

int main()
{
    auto target = json::parse(R"(
        { "foo": [ "bar", "baz" ] }
    )");

    std::error_code ec;
    jsonpointer::add_if_absent(target, "/foo/-", json("qux"), ec);
    if (ec)
    {
        std::cout << ec.message() << std::endl;
    }
    else
    {
        std::cout << target << std::endl;
    }
}

Output:

{"foo":["bar","baz","qux"]}

Try to add an object member at a location that already exists

#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpointer/jsonpointer.hpp>

using jsoncons::json;
namespace jsonpointer = jsoncons::jsonpointer;

int main()
{
    auto target = json::parse(R"(
        { "foo": "bar", "baz" : "abc"}
    )");

    std::error_code ec;
    jsonpointer::add_if_absent(target, "/baz", json("qux"), ec);
    if (ec)
    {
        std::cout << ec.message() << std::endl;
    }
    else
    {
        std::cout << target << std::endl;
    }
}

Output:

Key already exists

Try to add a value to a location in an array that exceeds the size of the array

#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpointer/jsonpointer.hpp>

using jsoncons::json;
namespace jsonpointer = jsoncons::jsonpointer;

int main()
{
    auto target = json::parse(R"(
    { "foo": [ "bar", "baz" ] }
    )");

    std::error_code ec;
    jsonpointer::add(target, "/foo/3", json("qux"), ec);
    if (ec)
    {
        std::cout << ec.message() << std::endl;
    }
    else
    {
        std::cout << target << std::endl;
    }
}

Output:

Index exceeds array size

Add a value to a location after creating objects when missing object keys

#include <iostream>
#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpointer/jsonpointer.hpp>

using jsoncons::json;
namespace jsonpointer = jsoncons::jsonpointer;

int main()
{
    std::vector<std::string> keys = { "foo","bar","baz" };

    jsonpointer::json_pointer location;
    for (const auto& key : keys)
    {
        location /= key;
    }

    json doc;
    jsonpointer::add_if_absent(doc, location, "str", true);

    std::cout << pretty_print(doc) << "\n\n";
}

Output:

{
    "foo": {
        "bar": {
            "baz": "str"
        }
    }
}