operator_at.md

jsoncons::basic_json::operator[]

proxy_type operator[](const string_view_type& key); (1)

const_reference operator[](const string_view_type& key) const; (2)

reference operator[](std::size_t i); (3)

const_reference operator[](std::size_t i) const; (4)

(1) Returns a "reference-like" proxy object that can be used to access or assign to the underlying keyed value. Type proxy_type supports the basic_json interface and conversion to basic_json&. If accessing and the key exists, evaluates to a reference to the keyed value.If accessing and the key does not exist, noexcept accessors return a default value, e.g. the is_xxx functions return false, other accessors throw. If assigning, inserts or updates with the new value. Throws std::domain_error if not an object. If read, throws std::out_of_range if the object does not have a member with the specified key.

(2) If key matches the key of a member in the basic_json object, returns a reference to the basic_json object, otherwise throws. Throws std::domain_error if not an object. Throws std::out_of_range if the object does not have a member with the specified key.

(3) Returns a reference to the value at index i in a basic_json object or array. Throws std::domain_error if not an object or array.

(4) Returns a const_reference to the value at index i in a basic_json object or array. Throws std::domain_error if not an object or array.

Notes

Unlike std::map::operator[], a new element is never inserted into the container when this operator is used for reading but the key does not exist.

Examples

Assigning to a proxy when the key does not exist

int main()
{
    json image_formats(json_array_arg, {"JPEG","PSD","TIFF","DNG"});

    json color_spaces(json_array_arg);
    color_spaces.push_back("sRGB");
    color_spaces.push_back("AdobeRGB");
    color_spaces.push_back("ProPhoto RGB");

    json export_settings;
    export_settings["File Format Options"]["Color Spaces"] = std::move(color_spaces);
    export_settings["File Format Options"]["Image Formats"] = std::move(image_formats);

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

Output:

{
    "File Format Options": {
        "Color Spaces": ["sRGB", "AdobeRGB", "ProPhoto RGB"],
        "Image Formats": ["JPEG", "PSD", "TIFF", "DNG"]
    }
}

Note that if file_export["File Format Options"] doesn’t exist, the statement

file_export["File Format Options"]["Color Spaces"] = std::move(color_spaces)

creates "File Format Options" as an object and puts "Color Spaces" in it.