Unchecked access: operator[]¶

Overview¶

Elements in a JSON object and a JSON array can be accessed via operator[] similar to a std::map and a std::vector, respectively.

Read access

Consider the following JSON value:

{
    "name": "Mary Smith",
    "age": 42,
    "hobbies": ["hiking", "reading"]
}

Assume the value is parsed to a json variable j.

expression	value
`j`	`{"name": "Mary Smith", "age": 42, "hobbies": ["hiking", "reading"]}`
`j["name"]`	`"Mary Smith"`
`j["age"]`	`42`
`j["hobbies"]`	`["hiking", "reading"]`
`j["hobbies"][0]`	`"hiking"`
`j["hobbies"][1]`	`"reading"`

The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a null value is inserted which can be immediately be overwritten.

Write access

j["name"] = "John Smith";
j["maidenName"] = "Jones";

This code produces the following JSON value:

{
    "name": "John Smith",
    "maidenName": "Jones",
    "age": 42,
    "hobbies": ["hiking", "reading"]
}

When accessing an invalid index (i.e., an index greater than or equal to the array size), the JSON array is resized such that the passed index is the new maximal index. Intermediate values are filled with null.

Filling up arrays with null values

j["hobbies"][0] = "running";
j["hobbies"][3] = "cooking";

This code produces the following JSON value:

{
    "name": "John Smith",
    "maidenName": "Jones",
    "age": 42,
    "hobbies": ["running", "reading", null, "cooking"]
}

Notes¶

Design rationale

The library behaves differently to std::vector and std::map:

std::vector::operator[] never inserts a new element.
std::map::operator[] is not available for const values.

The type json wraps all JSON value types. It would be impossible to remove operator[] for const objects. At the same time, inserting elements for non-const objects is really convenient as it avoids awkward insert calls. To this end, we decided to have an inserting non-const behavior for both arrays and objects.

Info

The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no exception is thrown.

Danger

It is undefined behavior to access a const object with a non-existing key.
It is undefined behavior to access a const array with an invalid index.
In debug mode, an assertion will fire in both cases. You can disable assertions by defining the preprocessor symbol NDEBUG or redefine the macro JSON_ASSERT(x). See the documentation on runtime assertions for more information.

Exceptions

operator[] can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a basic_json::type_error is thrown.

Summary¶

scenario	non-const value	const value
access to existing object key	reference to existing value is returned	const reference to existing value is returned
access to valid array index	reference to existing value is returned	const reference to existing value is returned
access to non-existing object key	reference to newly inserted `null` value is returned	undefined behavior; runtime assertion in debug mode
access to invalid array index	reference to newly inserted `null` value is returned; any index between previous maximal index and passed index are filled with `null`	undefined behavior; runtime assertion in debug mode

Last update: August 5, 2022