json/docs/mkdocs/docs/features/json_view.md

# Zero-copy JSON views

`#!cpp <nlohmann/json_view.hpp>` adds a read-only, non-owning way to look at a parsed JSON text, as an alternative to
building a [`basic_json`](../api/basic_json/index.md) tree with [`parse()`](../api/basic_json/parse.md).

## The problem

[`basic_json::parse()`](../api/basic_json/parse.md) builds a tree of `basic_json` values: one allocation for every
array and object, and every string copied into its own `std::string`. That is the right trade-off when the program
goes on to read and write the value freely, but it does more work than necessary when only a small part of a large
JSON text is actually needed, or when the same text is parsed over and over (many small messages, for instance) and
most of the resulting tree is thrown away almost immediately.

## The idea

[`basic_json_document::parse()`](../api/basic_json_document/parse.md) parses the same JSON grammar, with the same
options, but instead of a tree it builds a flat index of the values it found: one
[16-byte entry](../home/architecture.md#node-index-of-json-views) per value (and one per object key), in document
order. Integers are converted while parsing and stored in their entry. Floating-point numbers and strings are
referenced in place in the source text: a float is converted when it is read, and only strings that contain escape
sequences are decoded, into one shared buffer owned by the document.

[`basic_json_view`](../api/basic_json_view/index.md) is a small, trivially copyable handle (two pointers) into that
index. It gives you the read-only, type-inspection part of the `basic_json` interface --
[`type()`](../api/basic_json_view/type.md) and the `is_*()` predicates,
[`size()`](../api/basic_json_view/size.md)/[`empty()`](../api/basic_json_view/empty.md) -- as well as element access
([`operator[]`](../api/basic_json_view/operator%5B%5D.md), [`at`](../api/basic_json_view/at.md),
[`front`](../api/basic_json_view/front.md)/[`back`](../api/basic_json_view/back.md)), lookup
([`find`](../api/basic_json_view/find.md), [`contains`](../api/basic_json_view/contains.md),
[`count`](../api/basic_json_view/count.md)), and iteration
([`begin`](../api/basic_json_view/begin.md)/[`end`](../api/basic_json_view/end.md),
[`items`](../api/basic_json_view/items.md)) -- without ever allocating a `basic_json` value. When you do need an
actual `basic_json` value for a subtree, [`materialize()`](../api/basic_json_view/materialize.md) builds exactly the
one [`parse()`](../api/basic_json/parse.md) would have produced for it.

## How to use it

Include `<nlohmann/json_view.hpp>` in addition to (or instead of) `<nlohmann/json.hpp>`. Parse into a
[`json_document`](../api/json_document.md), inspect its [`root()`](../api/basic_json_document/root.md), and
`materialize()` when you need a real value. `materialize()` works on any view, not just the root, so you can turn just
the subtree you need into a value:

??? example "Example: parse a document, inspect its root, and materialize it"

    ```cpp
    --8<-- "examples/json_document.cpp"
    ```

    Output:

    ```json
    --8<-- "examples/json_document.output"
    ```

[`ordered_json_document`](../api/ordered_json_document.md) is the equivalent for
[`ordered_json`](../api/ordered_json.md), just as [`ordered_json`](../api/ordered_json.md) is to
[`json`](../api/json.md).

## Ownership and lifetime

A document either **borrows** the text it was parsed from, or **owns** its own copy of it; call
[`owns_source()`](../api/basic_json_document/owns_source.md) to find out which happened.
[`parse()`](../api/basic_json_document/parse.md) decides this from the value category and type of its argument (an
lvalue `#!cpp std::string` is borrowed; an rvalue `#!cpp std::string` is moved in, owned without a copy; a stream is
read into an owned buffer; and so on -- see [`parse`'s Notes](../api/basic_json_document/parse.md#notes) for the full
table). [`parse_copy()`](../api/basic_json_document/parse_copy.md) always owns a copy, regardless of the input.

!!! warning "A borrowed document depends on your buffer"

    If a document borrows its text, that text **must outlive the document** (and every view taken from it) and must
    stay **unmodified** for as long as the document exists, since floating-point values and unescaped strings are
    read from it lazily. Reading or writing through a view after the underlying buffer is gone is undefined behavior,
    exactly as it would be for a dangling `#!cpp std::string_view`.

A view is valid only while all of the following hold:

- the document is alive,
- the document has not been re-parsed since the view was taken (with [`read()`](../api/basic_json_document/read.md)
  or [`parse()`](../api/basic_json_document/parse.md) into it),
- the document has not had [`shrink_to_fit()`](../api/basic_json_document/shrink_to_fit.md) called on it since, and
- if the document borrows its source text, that text is still alive and unmodified.

Moving the `basic_json_document<>` itself is fine and does **not** invalidate its views: the index is a separate heap
allocation that keeps its address across the move. Take a fresh view from [`root()`](../api/basic_json_document/root.md)
whenever any of the other conditions above was not met.

A view can dangle, exactly like a `#!cpp std::string_view`: nothing stops you from using it after its document is
gone. This is deliberate. A view is two pointers and trivially copyable; making it safe (a `#!cpp std::shared_ptr` in
the document and a `#!cpp std::weak_ptr` in each view) would make it larger, add reference counting to every copy,
and a `lock()` to every access -- exactly the costs the view exists to avoid. What is caught is the obvious case:
because a view dies with its document, [`root()`](../api/basic_json_document/root.md) is not callable on a temporary
document: `#!cpp auto v = json_document::parse(text).root();` does not compile. Give the document a name first.

??? example "Example: borrowed and owned documents, and when views become invalid"

    ```cpp
    --8<-- "examples/json_view_ownership.cpp"
    ```

    Output:

    ```json
    --8<-- "examples/json_view_ownership.output"
    ```

## What is the same as `parse()`

- **Accept/reject.** [`accept()`](../api/basic_json_document/accept.md) and
  [`parse()`](../api/basic_json_document/parse.md) accept and reject exactly the same inputs as
  [`basic_json::accept()`](../api/basic_json/accept.md)/[`basic_json::parse()`](../api/basic_json/parse.md), with the
  same `ignore_comments` and `ignore_trailing_commas` options.
- **Errors.** A failing parse throws the same exception -- the same id, message, and position -- because on failure
  the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and
  the replay builds no values: it costs about a lexing pass up to the error.
  `#!cpp allow_exceptions == false` skips the replay and gives a
  [discarded](../api/basic_json_document/is_discarded.md) document instead of throwing, just as it gives a discarded
  value for `#!cpp basic_json::parse()`.
- **Number classification.** An integer literal that does not fit into the 64-bit integer type becomes a
  floating-point value, exactly as it does for `#!cpp basic_json::parse()`.
- **Macros.** [`JSON_STRICT_NUL_HANDLING`](../api/macros/json_strict_nul_handling.md) and
  [`JSON_NOEXCEPTION`](../api/macros/json_noexception.md)/[`JSON_THROW_USER`](../api/macros/json_throw_user.md)
  behave the same way they do for `<nlohmann/json.hpp>`. These are the macros that change what the parser accepts or
  how it reports errors. The others either do not apply ([`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md)
  needs parent pointers in `basic_json` values) or only affect the values `materialize()` produces, through
  `BasicJsonType`.

## What is different

- **Only 64-bit integers.** `basic_json_document<BasicJsonType>` requires `BasicJsonType::number_integer_t` and
  `number_unsigned_t` to both be 64 bits wide; this is a compile-time `#!cpp static_assert`.
- **A 4 GiB input limit.** An input of 4294967280 bytes (4 GiB minus 16 bytes) or more throws
  [`out_of_range.416`](../home/exceptions.md#jsonexceptionout_of_range416), a limit
  `#!cpp basic_json::parse()` does not have. It keeps an index entry at 16 bytes: offsets and counts are 32 bits
  wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the
  end must still fit.
- **A stream is always read to its end.** There is no partial/streaming read of an `#!cpp std::istream`: the stream
  is read to its end into a buffer the document owns, and parsing starts after that.
- **No source positions on `materialize()`.** Even with
  [`JSON_DIAGNOSTIC_POSITIONS`](../api/macros/json_diagnostic_positions.md) enabled,
  [`materialize()`](../api/basic_json_view/materialize.md) does not set them. The index has the start offset of every
  value, but not the end of arrays, objects, and escaped strings, which the positions also need.
- **Objects iterate in document order.** [`begin()`](../api/basic_json_view/begin.md)/
  [`end()`](../api/basic_json_view/end.md) and [`items()`](../api/basic_json_view/items.md) visit an object's
  members in the order they appear in the source text. `basic_json`'s default `object_t` is a `std::map`, which
  sorts by key, so iterating a [`materialize()`](../api/basic_json_view/materialize.md)d value can print members in
  a different order than iterating the view they came from.
- **Chained access is safe.** [`operator[]`](../api/basic_json_view/operator%5B%5D.md) with a missing key, an index
  out of range, or an unresolvable JSON pointer returns a [discarded](../api/basic_json_view/is_discarded.md) view, and
  `operator[]` on a discarded view returns a discarded view without throwing: `#!cpp v["a"]["b"][0]` can be tested
  once at the end. Type errors on values that exist (a key on an array, an index on an object) still throw, and
  [`at`](../api/basic_json_view/at.md) throws for every missing value.
- **Duplicate keys: lookups find the first member, `parse()` keeps the last.** See [Duplicate keys](#duplicate-keys)
  below.
- **No [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) path.** Exceptions thrown by `basic_json_view`'s own
  element access and lookup functions never carry the JSON Pointer path `JSON_DIAGNOSTICS` would otherwise add: the
  view has no `basic_json` value to point at, so the exception is created without one, regardless of how
  `BasicJsonType` was built.
- **`dump()` and comparison are not (yet) provided** by `basic_json_view`. For now,
  [`materialize()`](../api/basic_json_view/materialize.md) is the way to get a value you can do those things with.

## Duplicate keys

!!! warning "A lookup in a view and in the parsed value can give different answers"

    If an object in the source text repeats a key, [`operator[]`](../api/basic_json_view/operator%5B%5D.md),
    [`at`](../api/basic_json_view/at.md), [`find`](../api/basic_json_view/find.md),
    [`value`](../api/basic_json_view/value.md), [`contains`](../api/basic_json_view/contains.md), and JSON pointer
    resolution all return the **first** member with that key. `basic_json::parse()` instead keeps the **last** value
    of a repeated key, so for `#!cpp {"a":1,"a":2}`, `#!cpp view["a"]` is `#!cpp 1` while `#!cpp parse(text)["a"]` is
    `#!cpp 2`. [`count`](../api/basic_json_view/count.md) counts every member with the key, so it is `#!cpp 2` here.

[`begin()`](../api/basic_json_view/begin.md)/[`end()`](../api/basic_json_view/end.md) and
[`items()`](../api/basic_json_view/items.md) visit *every* occurrence, in document order, and
[`size()`](../api/basic_json_view/size.md) counts all of them, so `#!cpp view.size()` can be larger than
`#!cpp view.materialize().size()`.

**Why the first?** A lookup can stop as soon as it finds a match. Returning the last member would force every lookup
to scan all members of the object, even when the key is found at the very first one: this made lookups in small
objects 1.6 to 3.4 times slower. Other zero-copy parsers that index the source text, such as yyjson and simdjson, also
return the first member. RFC 8259 only says that names within an object SHOULD be unique and that the behavior of a
receiver that sees duplicates is unpredictable, so neither choice is wrong.

**What stays the same as `parse()`?** [`materialize()`](../api/basic_json_view/materialize.md) and
[`get<std::map<...>>()`](../api/basic_json_view/get.md) replay every member in order, so they keep the *last* value
exactly like `#!cpp basic_json::parse()` (at the position of the first occurrence of the key, for an
[`ordered_json`](../api/ordered_json.md)).

**How do I get the value `parse()` would give?** Either call `#!cpp view.materialize()` and look the key up in the
result, or iterate the members and keep the last match:

```cpp
// the last member with the key "a", as parse() would keep it
json_view last;
for (const auto item : view.items())
{
    if (item.key() == "a")
    {
        last = item.value();
    }
}
```

If you cannot trust the source text to have unique keys, check [`size()`](../api/basic_json_view/size.md) against the
number of distinct keys, or reject duplicates when iterating.

## Getting values out without copying

[`get<T>()`](../api/basic_json_view/get.md) converts many `T` directly from the flat index, without ever building a
`basic_json` value for the conversion: `#!cpp bool`, arithmetic types, `#!cpp std::nullptr_t`,
`#!cpp std::string`/other `#!cpp std::basic_string`s (copied once), `basic_json`/`ordered_json` (via
[`materialize()`](../api/basic_json_view/materialize.md)), `basic_json_view` itself, `#!cpp std::vector<U>`, and
`#!cpp std::map`/`#!cpp std::unordered_map` with string-like keys. Every other type -- `#!cpp std::list`,
`#!cpp std::pair`, `#!cpp std::array`, enumerations, user types with a `from_json()` -- goes through
[`materialize()`](../api/basic_json_view/materialize.md)`.get<T>()` instead: the subtree is built into a real
`basic_json` value first, exactly as [`parse()`](../api/basic_json/parse.md) would, and converted from there.

Two conversions never copy at all:

- [`get_string()`](../api/basic_json_view/get_string.md) (equivalently, `#!cpp get<string_view_t>()`) returns a
  string as a `string_view_t` pointing into the document's [`source()`](../api/basic_json_document/source.md) text --
  or, for a string that contains escape sequences, into the document's own buffer of decoded strings -- instead of
  allocating a new `#!cpp std::string`.
- [`number_token()`](../api/basic_json_view/number_token.md) returns a number exactly as it was written in the
  source, e.g. `#!cpp "1.50"`, `#!cpp "1E2"`, or an integer with more digits than any number type holds, instead of
  rounding it into a `#!cpp double`/`#!cpp int64_t` the way `#!cpp get<T>()` (and
  [`basic_json::parse()`](../api/basic_json/parse.md)) would.

Both results are only valid as long as the view -- and, for a string with no escapes, the borrowed source text -- is.

## Choosing between `json`, `ordered_json`, the SAX interface, and `json_view`

| | [`json`](../api/json.md) / [`ordered_json`](../api/ordered_json.md) | [SAX interface](parsing/sax_interface.md) | [`json_document`](../api/json_document.md) / [`json_view`](../api/json_view.md) |
|---|---|---|---|
| **Ownership** | owns every value | owns nothing; you decide what to keep, in your handler | borrows or owns the *text*; the index is always owned by the document |
| **Mutability** | freely mutable | not applicable (a one-shot event stream) | read-only |
| **What you get** | a full tree you can read, write, and keep as long as you like | a sequence of callbacks; whatever your handler builds from them | a flat index plus, on demand, [`materialize()`](../api/basic_json_view/materialize.md)d `json`/`ordered_json` values for the parts you actually use |
| **Typical use** | general-purpose JSON handling: config, request/response bodies you build or modify, anything you hold onto | validating or projecting a text into your own data structure without ever holding the whole thing as JSON | large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives |

## Version history

- Added in version 3.13.0.