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### jsoncons::basic_json::operator[]
```cpp
proxy_type operator[](const string_view_type& key); (1) (until 1.0.0)
reference operator[](const string_view_type& key); (since 1.0.0)
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) Unitl 1.0.0, returns a "reference-like" proxy object that can be used to access
or assign to the underlying keyed value.
Since 0.179, returns a reference to the value mapped to `key`,
inserting a default constructed value mapped to `key` if no such key already exists
(a default constructed value is an empty object.)
(2) Until 1.0.0, if `key` exists, returns a const reference to the `basic_json` value mapped to `key`, otherwise throws.
Since 1.0.0, returns a const reference to the `basic_json` value mapped to `key`, returning a const reference to a default
constructed `basic_json` value with static storage duration if no such key already exists.
(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.
Exceptions
(1) Throws `std::domain_error` if not an object. Until 1.0.0, throws a `std::out_of_range` if
assigning to a `basic_json` and the key does not exist. Since 1.0.0, inserts a default constructed
key-value pair if the key does not exist.
(2) Throws `std::domain_error` if not an object. Until 1.0.0, throws a `std::out_of_range` if
the key does not exist. Since 1.0.0, does
(3) Throws `std::domain_error` if not an array.
(4) Throws `std::domain_error` if not an 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 an object when the key does not exist
```cpp
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"]` doesnt 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.
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