api/basic_json_document/set.md

# <small>nlohmann::basic_json_document::</small>set

```cpp
// (1)
template<typename V>
view_type set(view_type target, V&& value);

// (2)
template<typename V>
view_type set(view_type object, string_view_t key, V&& value);

// (3)
template<typename I, typename V>
view_type set(view_type array, I idx, V&& value);

// (4)
template<typename V>
view_type set(const json_pointer& ptr, V&& value);
```

Only an **editable** document (`#!cpp view_mode::editable`, e.g. [`json_editable_document`](../json_editable_document.md))
has `set`; calling it on a read-only `basic_json_document` fails to compile (`#!cpp static_assert`).

1. Replaces the value `target` refers to with `value`.
2. Sets the member `key` of the object `object` to `value`: assigns it if `object` already has a member with this
   key — the first one, should the key occur more than once, and the later duplicates are then dropped (see the
   [Notes](#notes) below) — or appends a new member at the end otherwise. A [null](../basic_json_view/is_null.md)
   `object` first becomes an empty object.
3. Assigns `value` to the element at index `idx` of the array `array`, which must already exist (`#!cpp idx <
   array.size()`).
4. Sets the value the JSON pointer `ptr` refers to, relative to [`root()`](root.md), to `value`. The *parent* of the
   target must already exist: an object member is set as in 2. (added if it does not exist yet), an array element is
   assigned as in 3., and a last reference token of `#!cpp "-"`, or equal to the size of the array, appends `value`
   instead, exactly as [`push_back`](push_back.md) would. A [null](../basic_json_view/is_null.md) parent becomes what
   [`basic_json::operator[]`](../basic_json/operator%5B%5D.md) with a JSON pointer makes of it: an array if the last
   reference token is `#!cpp "-"` or consists of digits only (for an index beyond 0, the array is first filled with
   null values up to that index), an object otherwise. An empty `ptr` sets [`root()`](root.md) itself, as in 1.

In every overload, `value` is accepted three ways: a [`basic_json_view`](../basic_json_view/index.md) of *any*
document — read-only or editable, and it does not have to be `target`'s/`object`'s/`array`'s own document — which
is copied so that nothing is shared with the source document afterward; a `BasicJsonType` value; or anything
`BasicJsonType` can be constructed from (numbers, strings, `#!cpp bool`, `#!cpp nullptr`, containers, ...).

## Template parameters

`V`
:   the type of `value`, deduced; see above for what is accepted.

`I`
:   an integral type other than `#!cpp bool`, deduced (overloads taking a `#!cpp bool` or a non-integral type for
    `idx` do not participate in overload resolution).

## Parameters

`target` (in)
:   the value to replace

`object` (in)
:   the object (or null value) whose member to set

`array` (in)
:   the array whose element to assign

`key` (in)
:   the key of the member to set

`idx` (in)
:   the index of the element to assign; a negative value throws (see [Exceptions](#exceptions))

`ptr` (in)
:   a JSON pointer to the value to set, relative to `root()`

`value` (in)
:   the new value

## Return value

1. a view of `target`, now holding `value`
2. a view of the member `key` of `object`, now holding `value`
3. a view of the element `idx` of `array`, now holding `value`
4. a view of the value `ptr` refers to, now holding `value`

## Exception safety

Basic exception safety: `value` is fully encoded — including the checks below — into storage owned by the document
before anything already reachable from [`root()`](root.md) is touched, so a failure while encoding `value` (an
invalid argument, or `#!cpp std::bad_alloc`) leaves the document completely unchanged, other than memory allocated
for the encoding that is not reclaimed. A failure of a later allocation — while an edited array or object switches
from its parsed layout to a growable block, see [Notes](#notes) — can still leave a partial effect, such as a
[null](../basic_json_view/is_null.md) `object`/`array` argument already turned into an empty object/array even
though `value` itself was not linked in. Likewise, a failure of `value` in 4. leaves a null parent that is set with an
index beyond 0 already filled with the null values before the index.

## Exceptions

1. Throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if `value` is a
   [discarded](../basic_json_view/is_discarded.md) view, or a [discarded](../basic_json/is_discarded.md)
   `BasicJsonType` value (e.g. `#!cpp BasicJsonType(value_t::discarded)`) — an object or array `value`, of either
   kind, is fine and is encoded as a whole subtree.
2. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if `object` is neither an object
   nor null — the same message [`operator[]`](../basic_json_view/operator%5B%5D.md) throws for a string argument on
   such a value. Throws [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) if `key` is not
   valid UTF-8, with the same message [`BasicJsonType::dump()`](../basic_json/dump.md) gives for that string.
   Also throws what 1. throws for `value`.
3. Throws `type_error.305` if `array` is not an array — the same message `operator[]` throws for a numeric argument
   on such a value. Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if `idx` is
   negative, or if `#!cpp idx >= array.size()`. Also throws what 1. throws for `value`.
4. Throws what [`at`](../basic_json_view/at.md) throws (overload 3) for resolving `ptr`'s parent, except that a
   missing object member or an array index equal to the array's size at the very last reference token is not an
   error there (it becomes a new member or an appended element) instead of
   [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403)/[`out_of_range.402`](../../home/exceptions.md#jsonexceptionout_of_range402).
   For the last reference token itself: if the parent is an object (or a primitive value, where it throws
   `type_error.305`), throws what 2. throws; if the parent is an array, throws what 3. throws for an index that is
   out of range, or, for a token that is not a valid array index,
   [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) (a leading `#!cpp '0'`),
   [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) (not a number),
   [`out_of_range.410`](../../home/exceptions.md#jsonexceptionout_of_range410) (too large for `size_type`), or
   [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) (an empty token); the same errors are
   thrown for a null parent and a token of digits (the parent is not changed then), and
   [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if the index is 4294967295 or more.
   Also throws what 1. throws for `value`.

Every overload also throws [`type_error.319`](../../home/exceptions.md#jsonexceptiontype_error319) if `value` is (or
contains) a binary value — `BasicJsonType` can hold one, but a `json_document` cannot — and
[`invalid_iterator.202`](../../home/exceptions.md#jsonexceptioninvalid_iterator202) ("view does not belong to this
document") if `target`/`object`/`array` is a [discarded](../basic_json_view/is_discarded.md) view or a view of a
*different* document (overloads 1-3 only; overload 4 always starts from this document's own [`root()`](root.md)).

## Complexity

1. Linear in the size of `value` (encoding it into the document's storage): constant for a scalar, linear in the
   number of nested values for an array or object. If `target` is itself an array or object that spans more than one
   node in its parent's original, unedited layout, and `value` is a scalar, replacing it additionally costs time
   linear in the size of the document, the *first* time (the parent of `target` is looked up from
   [`root()`](root.md), and then switches to links) — see [Notes](#notes). Once the parent has links, `target` is
   replaced in constant time: setting every element of a large array one after the other is linear overall. To avoid
   the lookup altogether, use 3. (or 2. for an object), which know the parent.
2. Linear in the number of members of `object`, to find an existing member with `key`, plus the complexity of 1. for
   `value`.
3. Constant, plus the complexity of 1. for `value`.
4. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
   that level or the index into the array (as [`at`](../basic_json_view/at.md)), plus the complexity of 2. or 3. for
   the last token; for a null parent and an index, linear in the index.

## Notes

!!! info "Duplicate keys"

    If `object` already has more than one member with `key` (2.), the *first* one is assigned `value` and every
    later member with the same key is removed — so that a lookup, an iteration, and
    [`materialize()`](../basic_json_view/materialize.md) of `object` afterward all agree on a single value for
    `key`, the same way [`operator[]`](../basic_json_view/operator%5B%5D.md) already picks the first occurrence of a
    duplicate key for reading. See the [Notes on duplicate keys](../basic_json_view/operator%5B%5D.md#notes) of
    `operator[]`.

Setting a member (2.) or an element (3., through 4.) of an array or object whose elements have not been edited
before switches it from its parsed layout to a growable block holding links to its elements; a later
[`push_back`](push_back.md) or `set` on the same container reuses that block, growing it (amortized constant time)
only once it runs out of room. This never moves an element itself — only where the container's *links* to its
elements live — so a view of an element stays valid, but any iterator already taken over the container is
invalidated, since it was walking the old layout. See [Edits](index.md#edits) for what stays valid across an edit in
general.

The same switch happens, for the same reason, when overload 1. replaces a multi-node array/object value with a
scalar: the *parent's* element sequence is what has to switch to links, not `target` itself, because the parent
originally stepped over `target`'s whole subtree by its node count, which no longer applies once `target` is a
one-node scalar.

## Examples

??? example "Example: (1)/(2)/(3)/(4) replace a value, set a member, assign an element, set via a JSON pointer"

    The example below edits a small configuration document — replacing a value, adding an object member, assigning
    an array element, and reaching a field through a JSON pointer — and shows what
    [`dump()`](../basic_json_view/dump.md) preserves that is lost once the same edits are made on a `BasicJsonType`
    value instead: the order object members were written in, and the exact spelling of a number that was never
    touched.

    ```cpp
    #include <iostream>
    #include <nlohmann/json_view.hpp>
    
    using nlohmann::json;
    using nlohmann::json_editable_document;
    using nlohmann::json_editable_view;
    
    int main()
    {
        // a configuration file, as it might be read from disk -- "price" is
        // written with a trailing zero that has no effect on its value
        const std::string text = R"({
      "name": "cache",
      "host": "db1",
      "port": 6379,
      "price": 19.90,
      "replicas": ["db2", "db3"],
      "timeout": 30
    })";
    
        json_editable_document doc = json_editable_document::parse(text);
    
        doc.set(doc.root()["port"], 6380);                    // (1) replace a value
        doc.set(doc.root(), "region", "us-east");              // (2) add a member
        doc.set(doc.root()["replicas"], 0, "db4");             // (3) assign an element
        doc.set(json::json_pointer("/timeout"), 45);           // (4) via a JSON pointer
    
        // members stay in document order (the new one at the end), and a number
        // that was not itself edited keeps its exact spelling
        std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << "\n\n";
    
        // the same edits on a plain json value: object_t is a std::map, so
        // parsing already sorted the keys, and dump() rewrites every number to
        // its shortest form, even "price", which was never touched
        json plain = json::parse(text);
        plain["port"] = 6380;
        plain["region"] = "us-east";
        plain["replicas"][0] = "db4";
        plain[json::json_pointer("/timeout")] = 45;
        std::cout << plain.dump(2) << '\n';
    }
    
    ```

    Output:

    ```json
    {
      "name": "cache",
      "host": "db1",
      "port": 6380,
      "price": 19.90,
      "replicas": [
        "db4",
        "db3"
      ],
      "timeout": 45,
      "region": "us-east"
    }
    
    {
      "host": "db1",
      "name": "cache",
      "port": 6380,
      "price": 19.9,
      "region": "us-east",
      "replicas": [
        "db4",
        "db3"
      ],
      "timeout": 45
    }
    
    ```

## See also

- [`push_back`](push_back.md) - append to an array
- [`insert`](insert.md) - insert an element into an array
- [`erase`](erase.md) - remove an object member, an array element, or the value a JSON pointer refers to
- [`root`](root.md) - the view of the root value, the starting point of overload 4
- [`basic_json_view::dump`](../basic_json_view/dump.md) - serialize the document, keeping an untouched number's
  spelling with `#!cpp number_format::source`
- [Edits](index.md#edits) - what an edit guarantees, for every overload
- [Editing a document](../../features/json_view.md#editing-a-document) - why editable documents keep the source
  text's order and number spelling

## Version history

- Added in version 3.13.0.