Merge develop into fix/ubjson-valueless-budget and avoid MSVC C4127

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-11-13 07:33:26 +01:00
commit a2cc727dd5
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@@ -67,7 +67,11 @@ basic_json(basic_json&& other) noexcept;
- **objects**: [`object_t`](object_t.md) and all kinds of compatible associative containers such as `std::map`,
`std::unordered_map`, `std::multimap`, and `std::unordered_multimap` with a `key_type` compatible to `string_t`
and a `value_type` from which a `basic_json` value can be constructed.
- **strings**: `string_t`, string literals, and all compatible string containers can be used.
- **strings**: `string_t`, string literals, and all compatible string containers can be used. Since version 3.13.0,
this includes `#!cpp std::u8string` (C++20): its code units are stored as a JSON string and
[`get<std::u8string>()`](get.md) reads it back (in version 3.12.0, a `#!cpp std::u8string` was stored as an array
of numbers; `get<std::u8string>()` still accepts such an array). `#!cpp std::u8string_view` and `#!cpp char8_t`
arrays are not strings; they are stored as arrays of code units.
!!! note "Rvalue containers are moved from"
@@ -93,10 +97,18 @@ basic_json(basic_json&& other) noexcept;
The constructor tries to convert the internal `m_value` of the parameter. Each member value (object, array, string,
etc.) is serialized via the corresponding `to_json()` overload. For objects and strings, the conversion requires
that the *target* `basic_json` type's `object_t::key_type` (or `string_t`) be directly constructible from the
*source* type's corresponding member type via `is_constructible`. If this requirement is not met, the conversion
does not fail to compile; instead, it silently falls back to the array-conversion path, which represents objects
as arrays of `[key, value]` pairs and strings as arrays of character codes. This is a known limitation tracked in
[issue #3425](https://github.com/nlohmann/json/issues/3425).
*source* type's corresponding member type via `is_constructible`. If this requirement is not met, the result
depends on the *source* string type:
- If it is a range of characters (it has `begin()` and `end()`, as `std::string` does), the conversion falls back to
the array-conversion path, which represents objects as arrays of `[key, value]` pairs and strings as arrays of
character codes. This result has a different type than the source value, so the constructor's
[`JSON_ASSERT`](../macros/json_assert.md) fails; with its default definition, `assert`, the program aborts unless
`NDEBUG` is defined, in which case the array is returned.
- Otherwise, the conversion does not compile, even though `#!cpp std::is_constructible` reports `#!cpp true`; for
example, when converting a `basic_json` whose `string_t` is the `alt_string` of the unit tests to `json`.
This is a known limitation tracked in [issue #3425](https://github.com/nlohmann/json/issues/3425).
5. Creates a JSON value of type array or object from the passed initializer list `init`. In case `type_deduction` is
`#!cpp true` (default), the type of the JSON value to be created is deducted from the initializer list `init`
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@@ -130,6 +130,12 @@ overload (3).
`#!cpp json(true).get<double>()` throw. This is the behavior of all 3.x versions;
[`basic_json_view::get<T>()`](../basic_json_view/get.md) behaves the same.
!!! note "`std::u8string` conversions"
Since version 3.13.0, `#!cpp get<std::u8string>()` (C++20) reads a JSON string; it also accepts an array of numbers,
the form in which version 3.12.0 stored a `#!cpp std::u8string`. Any other JSON type throws
[`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) ("type must be string or array").
!!! note "`std::optional` conversions"
Prior to version 3.13.0, `#!cpp get<std::optional<T>>()` (and other conversions to `std::optional<T>`) failed to
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@@ -102,9 +102,12 @@ objects.
When converting an object from one `basic_json` specialization to another via the
[converting constructor](basic_json.md) (overload 4), the target `object_t`'s `key_type` must be
directly constructible from the source `basic_json`'s `string_t` type (or more generally, from the
source object's key type). If this requirement is not met, the conversion does not fail; instead,
the object is silently converted as an array of key-value pairs, which is incorrect. See
directly constructible from the source object's key type (with the default `std::map`, the source `basic_json`'s
`string_t`). If this requirement is not met and the source string type is a range of characters, the
object is converted as an array of key-value pairs, which is incorrect: the converting constructor's
[`JSON_ASSERT`](../macros/json_assert.md) fails; with its default definition, `assert`, the program aborts unless
`NDEBUG` is defined, in which case the array is returned. If the source string type is not a range of characters, the
conversion does not compile at all. See [`basic_json`](basic_json.md) and
[issue #3425](https://github.com/nlohmann/json/issues/3425) for details and an example.
## Examples
@@ -27,6 +27,16 @@ reference operator+=(initializer_list_t init);
`init` is converted into an object element and added using `operator+=(const typename object_t::value_type&)`.
Otherwise, `init` is converted to a JSON value and added using `operator+=(basic_json&&)`.
!!! note "Single braced JSON value"
For a call such as `#!cpp arr += {obj}`, where the initializer list contains a single JSON value `obj`,
compilers may disagree unless
[`JSON_BRACE_INIT_COPY_SEMANTICS`](../macros/json_brace_init_copy_semantics.md) is defined to `1`: GCC, for
example, selects this overload and appends a single-element array `[obj]`, whereas Clang selects the
`basic_json` overload and appends a copy of `obj`. With the macro, GCC and Clang both append a copy of `obj`.
To get a portable result without the macro, write `#!cpp arr += obj` to append a copy, or
`#!cpp arr += json::array({obj})` to append a single-element array.
## Iterator invalidation
For all cases where an element is added to an **array**, a reallocation can happen, in which case all iterators (including
@@ -60,6 +60,14 @@ Linear.
2. Comparing a `NaN` with another `NaN`.
3. Comparing a `NaN` and any other number.
For arrays and objects, the elements are compared in order. If the comparison reaches a `NaN` and a number (or
another `NaN`) before any other difference, the result depends on the language standard. Before C++20, the pair
counts as equivalent and the following elements (or the sizes) decide: for instance, `[NaN] < [1, NaN]` and
`[1] <= [NaN]` are `#!cpp true`. Since C++20, the whole comparison is unordered, and `<`, `<=`, `>`, and `>=` all
yield `#!cpp false`. Otherwise (for example `[1, NaN] < [2, NaN]`, or objects whose keys differ), the result is the
same for all language standards. The same program can therefore order such values differently when it is compiled
with a different language standard.
!!! note "Operator overload resolution"
Since C++20 overload resolution will consider the _rewritten candidate_ generated from
@@ -60,6 +60,14 @@ Linear.
2. Comparing a `NaN` with another `NaN`.
3. Comparing a `NaN` and any other number.
For arrays and objects, the elements are compared in order. If the comparison reaches a `NaN` and a number (or
another `NaN`) before any other difference, the result depends on the language standard. Before C++20, the pair
counts as equivalent and the following elements (or the sizes) decide: for instance, `[NaN] < [1, NaN]` and
`[1] <= [NaN]` are `#!cpp true`. Since C++20, the whole comparison is unordered, and `<`, `<=`, `>`, and `>=` all
yield `#!cpp false`. Otherwise (for example `[1, NaN] < [2, NaN]`, or objects whose keys differ), the result is the
same for all language standards. The same program can therefore order such values differently when it is compiled
with a different language standard.
!!! note "Operator overload resolution"
Since C++20 overload resolution will consider the _rewritten candidate_ generated from
@@ -61,6 +61,14 @@ Linear.
2. Comparing a `NaN` with another `NaN`.
3. Comparing a `NaN` and any other number.
For arrays and objects, the elements are compared in order. If the comparison reaches a `NaN` and a number (or
another `NaN`) before any other difference, the result depends on the language standard. Before C++20, the pair
counts as equivalent and the following elements (or the sizes) decide: for instance, `[NaN] < [1, NaN]` and
`[1] <= [NaN]` are `#!cpp true`. Since C++20, the whole comparison is unordered, and `<`, `<=`, `>`, and `>=` all
yield `#!cpp false`. Otherwise (for example `[1, NaN] < [2, NaN]`, or objects whose keys differ), the result is the
same for all language standards. The same program can therefore order such values differently when it is compiled
with a different language standard.
!!! note "Operator overload resolution"
Since C++20 overload resolution will consider the _rewritten candidate_ generated from
@@ -71,6 +71,14 @@ Linear.
2. Comparing a `NaN` with another `NaN`.
3. Comparing a `NaN` and any other number.
For arrays and objects, the elements are compared in order. If the comparison reaches a `NaN` and a number (or
another `NaN`) before any other difference, the result depends on the language standard. Before C++20, the pair
counts as equivalent and the following elements (or the sizes) decide: for instance, `[NaN] < [1, NaN]` and
`[1] <= [NaN]` are `#!cpp true`. Since C++20, the whole comparison is unordered, and `<`, `<=`, `>`, and `>=` all
yield `#!cpp false`. Otherwise (for example `[1, NaN] < [2, NaN]`, or objects whose keys differ), the result is the
same for all language standards. The same program can therefore order such values differently when it is compiled
with a different language standard.
!!! note "Operator overload resolution"
Since C++20 overload resolution will consider the _rewritten candidate_ generated from
@@ -55,6 +55,10 @@ Linear.
1. Comparing a `NaN` with itself.
2. Comparing a `NaN` with another `NaN`.
3. Comparing a `NaN` and any other number.
- For arrays and objects, the elements are compared in order. If the comparison reaches a `NaN` and a number (or
another `NaN`) before any other difference, the result is `std::partial_ordering::unordered`. (Without C++20, the
comparison operators `<`, `<=`, `>`, and `>=` use the comparison of the containers, which treats such a pair as
equivalent; see [**operator<**](operator_lt.md).)
## Examples
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@@ -130,11 +130,13 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
Contiguous inputs (strings, string views, vectors, arrays, pointer ranges) are the fastest to parse. A
`#!cpp std::istream` comes close: the parser scans the characters its stream buffer holds in bulk, without reading
ahead, so the stream is left exactly where it would be after reading one character at a time. A `#!cpp FILE*` is
read one character at a time, but the file is locked only once, so other threads that use the same `FILE` object
wait until parsing is done. See [input sources and performance](../../features/parsing/index.md#input-sources-and-performance),
[`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md), and
[`JSON_DISABLE_UNLOCKED_FILE_IO`](../macros/json_disable_unlocked_file_io.md).
ahead, so the stream is left exactly where it would be after reading one character at a time. With glibc
and the Apple C library, the characters the `#!cpp FILE` holds in its buffer are scanned in bulk like a stream's;
elsewhere a `#!cpp FILE*` is read one character at a time. Where the C library has unlocked stdio functions, the
file is locked only once, so other threads that use the same `FILE` object wait until parsing is done, unless
[`JSON_DISABLE_UNLOCKED_FILE_IO`](../macros/json_disable_unlocked_file_io.md) is defined to `1`. See
[input sources and performance](../../features/parsing/index.md#input-sources-and-performance) and
[`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md).
## Examples
@@ -311,6 +313,10 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
it as end of input; planned to become the default in version 4.0.0.
- Extended empty-input detection to also cover an `std::istream` without a stream buffer, and fixed a crash
(`std::terminate`) when parsing from an `std::istream` with `eofbit` in its exception mask, in version 3.13.0.
- Fixed the position that a [`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101) reports for an
unexpected number that is followed by a line feed in version 3.13.0: for the input `1`, line feed, `2`, line feed,
version 3.12.0 reported "line 2, column 0" and version 3.13.0 reports "line 2, column 1"; for `1 2` and a line feed,
the report was "line 1, column 0" and is now "line 1, column 3".
- Parsing from an `std::istream` scans the characters in its stream buffer in bulk in version 3.13.0
([`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md) restores reading one character at a
time).
@@ -27,6 +27,16 @@ void push_back(initializer_list_t init);
`init` is converted into an object element and added using `push_back(const typename object_t::value_type&)`.
Otherwise, `init` is converted to a JSON value and added using `push_back(basic_json&&)`.
!!! note "Single braced JSON value"
For a call such as `#!cpp arr.push_back({obj})`, where the initializer list contains a single JSON value `obj`,
compilers may disagree unless
[`JSON_BRACE_INIT_COPY_SEMANTICS`](../macros/json_brace_init_copy_semantics.md) is defined to `1`: GCC, for
example, selects this overload and appends a single-element array `[obj]`, whereas Clang selects the
`basic_json` overload and appends a copy of `obj`. With the macro, GCC and Clang both append a copy of `obj`.
To get a portable result without the macro, write `#!cpp arr.push_back(obj)` to append a copy, or
`#!cpp arr.push_back(json::array({obj}))` to append a single-element array.
## Iterator invalidation
For all cases where an element is added to an **array**, a reallocation can happen, in which case all iterators (including
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@@ -124,9 +124,10 @@ consumer `sax` has a super-linear complexity.
A UTF-8 byte order mark is silently ignored.
When reading from a `#!c FILE*`, the file is locked while it is parsed. The lock is recursive, so the SAX handler may
use the same file with stdio functions, but it must not wait for another thread that uses the same `FILE` object; see
[`JSON_DISABLE_UNLOCKED_FILE_IO`](../macros/json_disable_unlocked_file_io.md).
When reading from a `#!c FILE*` where the C library has unlocked stdio functions, the file is locked while it is parsed,
unless [`JSON_DISABLE_UNLOCKED_FILE_IO`](../macros/json_disable_unlocked_file_io.md) is defined to `1`.
The lock is recursive, so the SAX handler may use the same file with stdio functions, but it must not wait for another
thread that uses the same `FILE` object.
## Examples
@@ -67,3 +67,7 @@ Constant.
## Version history
- Added in version 3.12.0.
- Fixed in version 3.13.0: in version 3.12.0, the start position of a string that contains escape sequences (for
example `#!cpp "\n"` or `#!cpp "\u00e4"`) was too large, by the number of bytes the escape sequences add to the
source text. The position is now always that of the opening `"`. The "(bytes a-b)" range in the messages of
[`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) was wrong in the same way.
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@@ -60,9 +60,11 @@ String values are stored as pointers in a `basic_json` type. That is, for any ac
When converting a string value from one `basic_json` specialization to another via the
[converting constructor](basic_json.md) (overload 4), the target `string_t` must be directly
constructible from the source `basic_json`'s `string_t` type. If this requirement is not met, the
conversion does not fail; instead, the string is silently converted as an array of character codes,
which is incorrect. See [issue #3425](https://github.com/nlohmann/json/issues/3425) for details
and an example.
conversion depends on the source string type. If it is a range of characters, the string is converted as an array of
character codes, which is incorrect: the converting constructor's [`JSON_ASSERT`](../macros/json_assert.md) fails; with
its default definition, `assert`, the program aborts unless `NDEBUG` is defined, in which case the array is returned.
If the source string type is not a range of characters, the conversion does not compile at all. See
[issue #3425](https://github.com/nlohmann/json/issues/3425) for details and an example.
## Examples
@@ -111,8 +111,9 @@ points apply to every edit:
the end, and, with [`number_format::source`](../basic_json_view/number_format.md), keeps the spelling of every
number that was not itself edited — see [Editing a document](../../features/json_view/editing.md) for
why this matters.
- [`read()`](read.md) discards all edits, [`shrink_to_fit()`](shrink_to_fit.md) does not move the node index once
there are edits, and [`memory_usage()`](memory_usage.md) includes the memory edits use.
- [`read()`](read.md) discards all edits (so its `input` must not be a string obtained from this document),
[`shrink_to_fit()`](shrink_to_fit.md) does not move the node index once there are edits, and
[`memory_usage()`](memory_usage.md) includes the memory edits use.
[`source_offset()`](../basic_json_view/source_offset.md) of a value introduced by an edit is
`#!cpp static_cast<std::size_t>(-1)`, the same value it reports for a decoded string.
@@ -113,11 +113,12 @@ How thoroughly `load()` validates `image` before trusting it.
`full` (the default)
: Checks everything the parser itself guarantees: structure and bounds; that every string is valid UTF-8 (and, for a
string still in the source text, that it contains no quote, backslash, or control character); and that every number
token is well-formed and matches the value stored for it. Strings and float tokens may share a range only if the
ranges are identical (as nodes that share a value do), and never overlap otherwise. Reading and serializing a
checked image is safe and always produces valid JSON, exactly as for a parsed document. This is the right choice for
an image from anything you do not fully control — a file, a cache shared with other processes, a peer on the
network.
token is well-formed and matches the value stored for it, and has the type the parser gives it (a float node only
for a token with a fraction or an exponent, or an integer that does not fit 64 bits). Strings and float tokens may
share a range only if the ranges are identical (as nodes that share a value do), and never overlap otherwise.
Reading and serializing a checked image is safe and always produces valid JSON, exactly as for a parsed document.
This is the right choice for an image from anything you do not fully control — a file, a cache shared with other
processes, a peer on the network.
`bounds`
: Checks structure and bounds only: that every offset and count in the node index stays inside the image. So
@@ -46,6 +46,34 @@ Linear in the length of the input.
Every view taken from `#!cpp *this` before the call — including the previous [`root()`](root.md) — is invalidated,
whether or not the new parse succeeds; take fresh views from [`root()`](root.md) afterward.
!!! warning "`input` must not point into the document itself"
`read()` releases the document's previous text, its decoded strings, and (for an editable document) its edits
*before* it parses `input`. An `input` that is, or points into, any of them violates a precondition: it may seem to
work, parse empty or garbled text, be reported by a sanitizer, or read freed memory (undefined behavior).
This applies to every `string_view` obtained from the document whose characters live in memory that `read()`
releases:
- [`source()`](source.md) of a document that owns its text;
- [`get_string()`](../basic_json_view/get_string.md) (also as `get<string_view_t>()` or a `string_view` member of a
decoded struct) of a string with escapes, of a string of a document that owns its text, and of an edited string;
- [`number_token()`](../basic_json_view/number_token.md) of a number of a document that owns its text, and of a
number written by an edit;
- the key of an object member (`key()`) if it has escapes, was written by an edit, or the document owns its text.
It does not apply to a text that the caller owns and the document borrows, nor to the strings, keys, and tokens that
are read from such a text unchanged, nor to the decoded strings of a borrowed [image](load.md), which live in the
caller's image: `read()` never touches them. To parse JSON that a document holds in a string of its own (for
example, JSON embedded in a string value), copy the string first, for example with
`#!cpp std::string inner(doc.root()["payload"].get_string()); doc.read(inner);`.
A debug build (one without `NDEBUG`) checks this with `assert()`: an `input` in the document's text, decoded
strings, or image, or in the most recent buffer of strings written by edits, ends the program. (Strings written
by earlier edits are not covered.) A release build does not check, to keep the cost of reusing a document at a
minimum. The check uses `assert()` directly, not [`JSON_ASSERT`](../macros/json_assert.md), which `json.hpp`
undefines at its end.
`input` is borrowed or owned by the same rules as [`parse()`](parse.md#notes); a document can borrow on one call and
own on the next, since ownership is decided freshly each time. An input that is copied (see the table in
[`parse()`](parse.md#notes)) is written into the document's previous owned copy, reusing its memory; a moved-in
@@ -24,6 +24,9 @@ Constant.
For a borrowed document, `source()` points directly into the caller's buffer, so it is only valid while that buffer
is; see [`owns_source`](owns_source.md).
The text of a document that owns it must not be passed to [`read()`](read.md) of the same document: `read()` releases
an owned text before it parses its input. Copy it first. (A borrowed text belongs to the caller and is not affected.)
## Examples
??? example
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@@ -48,8 +48,11 @@ Strong exception safety: if an exception is thrown, there are no changes to the
May throw `#!cpp std::bad_alloc` if allocating the output string fails. Unlike
[`BasicJsonType::dump`](../basic_json/dump.md), there is no `error_handler` parameter and no
[`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316): the view only ever holds text the parser
already validated as UTF-8, so there is nothing to replace or ignore.
[`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316): the parser validated the text the view
holds as UTF-8, so there is nothing to replace or ignore. Only a document loaded with
[`image_check::bounds` or `image_check::none`](../basic_json_document/load.md#image_check) can still throw it: these do
not check the strings, so `dump()` writes such a string as it is, which can be invalid UTF-8 or invalid JSON, and
`dump()` with `ensure_ascii` set to `#!cpp true` throws `type_error.316` for invalid UTF-8.
## Complexity
@@ -46,6 +46,10 @@ The returned [`string_view_t`](index.md#member-types) is valid exactly as long a
source text, which for a borrowed document must remain valid **and unmodified** for as long as the document exists,
since floating-point values and unescaped strings are read from the source lazily.
A string with escapes, a string of a document that owns its text, and an edited string must not be passed to
[`read()`](../basic_json_document/read.md) of the document it came from: `read()` releases them before it parses its
input. Copy the string first. (A string read unchanged from a borrowed text is not affected.)
## Examples
??? example
@@ -38,9 +38,14 @@ by the grammar; trailing zeros in the fraction; the case and sign of the exponen
useful precisely where that form must survive — a price or an identifier that must be reproduced exactly, or a
number too large for any of `BasicJsonType`'s number types to hold without loss.
The returned [`string_view_t`](index.md#member-types) always points into the document's
[`source()`](../basic_json_document/source.md) text — numbers are never decoded into the document's separate string
buffer — and is valid exactly as long as that text is, see the [validity rules](index.md) of `basic_json_view`.
The returned [`string_view_t`](index.md#member-types) points into the document's
[`source()`](../basic_json_document/source.md) text or, for a number written by an
[edit](../basic_json_document/index.md#edits), into storage owned by the document (the token of an edited `NaN` is
`nan`). It is valid exactly as long as that text or storage is, see the [validity rules](index.md) of
`basic_json_view`. Numbers are never decoded into the document's separate string buffer.
For the same reason, the token must not be passed to [`read()`](../basic_json_document/read.md) of the document it came
from if that document owns its text or the number was written by an edit: `read()` releases both before it parses.
## Examples
@@ -57,7 +57,9 @@ compared. Two arrays of different [`size()`](size.md) are rejected without visit
Only a single number, boolean, or `#!cpp null` value is ever materialized into a `BasicJsonType`, to reuse its
`operator==` — for numbers, so that values written differently in the source text but equal in value (e.g. an
integer and a floating-point literal) still compare equal, following the same rules `BasicJsonType` does for special
values such as `#!cpp NaN`. Constructing one of these scalars never allocates. Strings are compared directly, without
values such as `#!cpp NaN`: a `#!cpp NaN` that an [edit](../basic_json_document/set.md) wrote is not equal to itself, so
a view of it is not equal to itself either, nor is a view of an array or object that contains one, as for
`BasicJsonType`. Constructing one of these scalars never allocates. Strings are compared directly, without
allocating, either from the source text on both sides or, for overload 2, against `BasicJsonType`'s own string.
Arrays and objects are never materialized at all; only their elements or members are visited, one pair at a time.
+2
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@@ -40,6 +40,8 @@ header. See also the [macro overview page](../../features/macros.md).
- [**JSON_NO_IO**](json_no_io.md) - switch off functions relying on certain C++ I/O headers
- [**JSON_NO_SIMD**](json_no_simd.md) - use only portable code instead of vector instructions (SSE2, SSSE3, NEON)
- [**JSON_NO_THREAD_LOCAL**](json_no_thread_local.md) - switch off the use of `thread_local` storage
- [**JSON_RECURSION_DEPTH_LIMIT**](json_recursion_depth_limit.md) - set how many nesting levels the operations on a value
recurse into on the call stack (default 128)
- [**JSON_SKIP_UNSUPPORTED_COMPILER_CHECK**](json_skip_unsupported_compiler_check.md) - do not warn about unsupported compilers
- [**JSON_USE_GLOBAL_UDLS**](json_use_global_udls.md) - place user-defined string literals (UDLs) into the global namespace
- [**JSON_USE_SIMDUTF**](json_use_simdutf.md) - use the simdutf library to accelerate UTF-8 validation
@@ -27,9 +27,13 @@ The default value is `0` (disabled — existing behavior is preserved).
json j{obj};
```
creates a single-element **array** `[{"key":"value"}]` instead of a copy of `obj`. This behavior is
compiler-dependent for older compilers (GCC wrapped, Clang did not), but starting from Clang 20, both compilers
behave the same way.
creates a single-element **array** `[{"key":"value"}]` instead of a copy of `obj`. For this constructor, the
behavior was compiler-dependent for older compilers (GCC wrapped, Clang did not), but starting from Clang 20, both
compilers behave the same way.
The calls `#!cpp arr.push_back({obj})` and `#!cpp arr += {obj}` are still compiler-dependent without the macro,
even with Clang 20 and later: GCC converts `{obj}` to a single-element array and appends that array, whereas Clang
appends a copy of `obj`. With the macro defined to `1`, both compilers append a copy of `obj`.
Enabling this macro opts into copy/move semantics for this case
(see [#5074](https://github.com/nlohmann/json/issues/5074)).
@@ -125,3 +125,5 @@ When the macro is not defined, the library will define it to its default value.
## Version history
- Added in version 3.12.0.
- Fixed in version 3.13.0: in version 3.12.0, [`start_pos()`](../basic_json/start_pos.md) of a string that contains
escape sequences was too large; it is now the position of the opening `"`.
@@ -45,6 +45,7 @@ Copying and comparing fall back to working without the call stack there, as they
## See also
- [`JSON_RECURSION_DEPTH_LIMIT`](json_recursion_depth_limit.md) - set how many nesting levels the operations recurse into
- [FAQ: Thread safety](../../home/faq.md#thread-safety) - thread-safety guarantees of the library
- [Supported Macros](../../features/macros.md) - guide to all supported macros
@@ -0,0 +1,307 @@
# JSON_RECURSION_DEPTH_LIMIT
```cpp
#define JSON_RECURSION_DEPTH_LIMIT /* value */
```
This macro sets how many nesting levels the operations that walk a value recurse into on the call stack. Beyond that,
they continue with an explicit stack on the heap. The value is an integer constant from `0` to `254`; the default is
`128`.
The operations are copying a value, comparing two values, serializing with [`dump`](../basic_json/dump.md) or
[`operator<<`](../operator_ltlt.md), hashing with `std::hash`, [`merge_patch`](../basic_json/merge_patch.md),
[`update`](../basic_json/update.md) with `merge_objects`, [`diff`](../basic_json/diff.md),
[`merge_diff`](../basic_json/merge_diff.md), converting a value to another specialization of `basic_json`, destroying a
value, and the binary writers [`to_cbor`](../basic_json/to_cbor.md),
[`to_msgpack`](../basic_json/to_msgpack.md), [`to_ubjson`](../basic_json/to_ubjson.md),
[`to_bjdata`](../basic_json/to_bjdata.md), and [`to_bon8`](../basic_json/to_bon8.md). They share this one limit; none
has
a limit of its own. Recursing is the fastest way to walk a value, but a value nested deeply enough would exhaust the
call
stack, so the library recurses only as far as the limit and finishes the levels below with an explicit stack. This is
what makes all of them safe for untrusted input
(see [Nesting depth](../../features/parsing/untrusted_input.md#nesting-depth)). Parsing, the binary readers,
[`flatten`](../basic_json/flatten.md), [`unflatten`](../basic_json/unflatten.md), and
[`to_bson`](../basic_json/to_bson.md) always use an explicit stack and do not depend on the macro. The lockstep
comparison of two [`json_view`](../basic_json_view/index.md) documents has a fixed bound of its own, 48 levels,
and does not depend on it either.
The call stack that these operations need grows with the limit: roughly *a few kilobytes plus a cost per level times the
limit*, where the cost per level is up to about 2.3 KiB in an unoptimized and up to about 0.5 KiB in an optimized build
(see [Stack needed](#stack-needed)). The default of `128` levels is tuned for threads with ordinary stacks of a few
hundred kilobytes or more. Set a lower limit if your threads have small stacks (RTOS tasks, fibers, threads of the musl
C
library, which has a default stack of 128 KiB), or a higher limit to trade stack for speed on deeply nested values.
The macro changes only how much stack is used and how fast the operations are. It never changes a result: every
operation returns the same value for every setting of the limit, for values nested less deeply than the limit as well as
for values nested more deeply.
## Default definition
The default value is `128`.
```cpp
#define JSON_RECURSION_DEPTH_LIMIT 128
```
When the macro is not defined, the library defines it to its default value.
## Notes
!!! note "Allowed values"
The value must be an integer constant expression from `0` to `254`; any other value (a negative number, a number
above `254`, a floating-point number such as `254.0`, or `true` or `false`) is rejected at compile time with an
error message that names the macro. A character of any character type (`'a'`, `L'a'`, `u8'a'`, ...) is rejected,
too, although it is an integer.
- `0` is allowed, and means that every operation uses its explicit stack from the first level on and no operation
recurses over the nesting of a value at all. That needs a constant amount of call stack (at most 16 KiB in the
measurements below, for a value nested 20,000 levels deep) and is the setting for tiny stacks, at the price of
speed (see [Speed](#speed)).
- `254` is the largest allowed value. Copying a value and comparing two values (with `<=>`, or with the operators on
objects and arrays of a custom type) cannot pass the depth down as an argument, because the signatures of the
copy constructor and the operators are fixed. They count the levels they have descended into in a variable of type
`#!cpp std::uint8_t` that exists once per thread (see [`JSON_NO_THREAD_LOCAL`](json_no_thread_local.md)). Only
the levels that are descended into are counted, so the count never exceeds the limit, not even when user code (a
copy constructor of a custom base class, say) copies a value while a copy is going on. A byte holds `255` at
most, and the limit stays below that, so that the count cannot wrap around. (More levels would hardly be
useful: `254` levels need up to 576 KiB of stack in an unoptimized build already, twice as much as the
default.)
### Stack needed
The tables show the smallest stack that an operation needs, in KiB, at several limits, on a value that is nested
more deeply than the limit (limit + 10 levels). Each cell shows the result for Clang (Apple clang 21) and for
GCC 16, separated by a slash; both measured on macOS with arm64, compiling in C++20 mode. The operations run in a
thread whose stack has the size given, and are not given any stack that they do not need: the value is built, and
the result destroyed, outside of that thread. Stacks can only be set in steps of 16 KiB (the page size), so `16`
means "16 KiB or less". A row stands for all operations listed; the table shows the largest value among them.
Unoptimized (`-O0`):
| Operation | limit 16 | limit 32 | limit 64 | limit 128 | limit 254 |
|---|---:|---:|---:|---:|---:|
| `dump()` and `dump(indent)`, `operator<<` | 48 / 32 | 80 / 64 | 144 / 128 | 288 / 256 | 576 / 512 |
| copy construction and assignment | 32 / 16 | 48 / 32 | 112 / 80 | 224 / 144 | 432 / 304 |
| `==` | 16 / 16 | 16 / 16 | 16 / 16 | 48 / 32 | 96 / 48 |
| `<=>` (C++20) | 16 / 16 | 16 / 16 | 32 / 32 | 64 / 64 | 144 / 128 |
| `std::hash` | 16 / 16 | 16 / 16 | 32 / 32 | 80 / 64 | 160 / 128 |
| `merge_patch`, `update(j, true)`, `patch` | 32 / 16 | 48 / 32 | 112 / 80 | 224 / 144 | 432 / 304 |
| `diff` | 16 / 16 | 16 / 16 | 32 / 32 | 64 / 48 | 144 / 96 |
| `merge_diff` | 16 / 16 | 48 / 32 | 80 / 48 | 176 / 112 | 352 / 224 |
| `to_cbor`, `to_msgpack` | 16 / 16 | 32 / 16 | 64 / 32 | 112 / 80 | 224 / 160 |
| `to_ubjson`, `to_bjdata` | 16 / 16 | 16 / 16 | 48 / 32 | 96 / 64 | 176 / 112 |
| `to_bon8` | 16 / 16 | 16 / 16 | 16 / 16 | 32 / 16 | 64 / 48 |
| conversion to another `basic_json` type | 32 / 16 | 64 / 32 | 112 / 80 | 224 / 144 | 448 / 304 |
| destruction | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 16 | 32 / 32 |
Optimized (`-O2`):
| Operation | limit 16 | limit 32 | limit 64 | limit 128 | limit 254 |
|---|---:|---:|---:|---:|---:|
| `dump()` and `dump(indent)`, `operator<<` | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 16 | 32 / 32 |
| copy construction and assignment | 16 / 16 | 16 / 16 | 16 / 16 | 32 / 32 | 64 / 48 |
| `==` | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 16 |
| `<=>` (C++20) | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 16 | 48 / 16 |
| `std::hash` | 16 / 16 | 16 / 16 | 16 / 16 | 32 / 48 | 80 / 80 |
| `merge_patch`, `update(j, true)`, `patch` | 16 / 16 | 16 / 16 | 16 / 16 | 32 / 32 | 64 / 48 |
| `diff` | 16 / 16 | 16 / 16 | 16 / 32 | 48 / 48 | 96 / 112 |
| `merge_diff` | 16 / 16 | 16 / 16 | 16 / 16 | 48 / 32 | 96 / 64 |
| `to_cbor`, `to_msgpack` | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 16 |
| `to_ubjson`, `to_bjdata` | 16 / 16 | 16 / 16 | 16 / 16 | 32 / 32 | 64 / 64 |
| `to_bon8` | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 32 | 32 / 64 |
| conversion to another `basic_json` type | 16 / 16 | 16 / 16 | 16 / 16 | 32 / 48 | 64 / 96 |
| destruction | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 16 | 16 / 16 |
With a limit of `0`, every operation in the tables ran in a 16 KiB stack at `-O0` (Clang and GCC), on a value nested
20,000 levels deep. The operations that do not depend on the macro (`flatten`, `unflatten`, `to_bson`) needed
16 KiB or less at every limit.
The rule of thumb is
> stack needed ≈ a few KiB + cost per level × limit
with a cost per level of up to 2.3 KiB (`dump` at `-O0` with Clang; 2.1 KiB with GCC), 1.7 KiB for copying and merging
(1.3 KiB with GCC), and 0.1 to 0.5 KiB for most other operations at `-O0`; and 0.5 KiB or less at `-O2`. The numbers
come from the difference between the limits `128` and `254` in the tables, so they are rough: other compilers,
other targets (the frames of x86-64 code are about as large), other standard libraries, `JSON_DIAGNOSTICS`, and
sanitizers change them. Measure with your own tool chain (see the example below) rather than relying on the tables.
The stack that your own code and the operating system need comes on top of these numbers.
Which limit to choose, for the stack that you can spare for the library (the largest limit of those measured
for which the slowest operation needs at most that much):
| Stack for the library | optimized (`-O2`) | unoptimized (`-O0`) |
|---|---:|---:|
| 32 KiB | 64 | 0 |
| 64 KiB | 128 | 16 |
| 128 KiB (the default stack of threads on musl) | 254 | 32 |
| 256 KiB | 254 | 64 |
| 512 KiB | 254 | 128 |
| 1 MiB or more | 254 | 254 |
### Speed
Operations are faster the more of a value they handle with plain recursion. The levels below the limit are handled with
an explicit stack, which has to be kept on the heap and costs more per level. The macro therefore trades speed against
stack.
The table shows the time of copying a value (including destroying the copy), of `==` on two equal values, of `dump`, and
of `std::hash` for several limits, relative to the default of `128` (`1.00` means the same time; more is slower). It was
measured with Clang 21 at `-O2` in C++17 mode on macOS with arm64, as the best of seven runs of each setting (two
rounds, best of both). The documents are ordinary JSON files that are nested 7 (`canada.json`), 8
(`citm_catalog.json`), 10 (`twitter.json`), and 2 levels deep (`jeopardy.json`); the other values are one array or object
with one member per level, nested 1,000 and 100,000 levels deep. The absolute time at the limit `128` is given in the
third column. Differences of up to about 0.1 are noise: a different limit changes the layout of the compiled code, and
timings of the same binary vary by about as much. The slow columns (limits 0 and 1) vary more: repeating `std::hash` on
`canada.json` gave 1.27 to 1.30 for limit 0 and 1.4 to 1.9 for limit 1, so read the cells of those two columns as
accurate to about 0.3.
| Value | Operation | limit 128 (absolute) | limit 0 | limit 1 | limit 16 | limit 254 |
|---|---|---:|---:|---:|---:|---:|
| `canada.json` (2.1 MB) | copy | 3.5 ms | 1.80 | 1.80 | 0.99 | 1.00 |
| `canada.json` (2.1 MB) | `==` | 350.5 µs | 2.36 | 2.38 | 1.00 | 0.99 |
| `canada.json` (2.1 MB) | `dump` | 2.4 ms | 1.11 | 1.11 | 1.00 | 1.00 |
| `canada.json` (2.1 MB) | `std::hash` | 920.2 µs | 1.33 | 1.59 | 1.00 | 1.00 |
| `citm_catalog.json` (1.7 MB) | copy | 1.8 ms | 1.38 | 1.36 | 1.00 | 0.99 |
| `citm_catalog.json` (1.7 MB) | `==` | 275.5 µs | 1.31 | 1.31 | 0.99 | 1.00 |
| `citm_catalog.json` (1.7 MB) | `dump` | 314.6 µs | 1.24 | 1.23 | 1.00 | 1.00 |
| `citm_catalog.json` (1.7 MB) | `std::hash` | 309.1 µs | 1.77 | 1.42 | 1.06 | 1.05 |
| `twitter.json` (0.6 MB) | copy | 865.7 µs | 1.45 | 1.47 | 1.01 | 1.00 |
| `twitter.json` (0.6 MB) | `==` | 135.8 µs | 1.26 | 1.23 | 1.00 | 1.00 |
| `twitter.json` (0.6 MB) | `dump` | 224.6 µs | 1.12 | 1.12 | 1.01 | 1.00 |
| `twitter.json` (0.6 MB) | `std::hash` | 132.9 µs | 1.28 | 1.27 | 1.03 | 1.04 |
| `jeopardy.json` (53 MB) | copy | 111.1 ms | 1.22 | 1.25 | 1.00 | 1.00 |
| `jeopardy.json` (53 MB) | `==` | 23.5 ms | 1.29 | 1.53 | 1.07 | 1.00 |
| `jeopardy.json` (53 MB) | `dump` | 33.2 ms | 1.14 | 1.34 | 1.03 | 1.01 |
| `jeopardy.json` (53 MB) | `std::hash` | 19.9 ms | 1.17 | 1.52 | 1.05 | 1.00 |
| arrays, 1,000 levels | copy | 69.9 µs | 0.95 | 0.98 | 0.92 | 0.99 |
| arrays, 1,000 levels | `==` | 10.4 µs | 1.02 | 1.01 | 0.96 | 1.00 |
| arrays, 1,000 levels | `dump` | 9.6 µs | 0.97 | 0.96 | 0.97 | 1.06 |
| arrays, 1,000 levels | `std::hash` | 16.9 µs | 1.00 | 1.01 | 1.01 | 0.98 |
| objects, 1,000 levels | copy | 77.1 µs | 0.98 | 0.97 | 0.97 | 1.03 |
| objects, 1,000 levels | `==` | 12.0 µs | 0.96 | 0.95 | 0.95 | 1.07 |
| objects, 1,000 levels | `dump` | 12.6 µs | 0.97 | 0.95 | 0.98 | 1.01 |
| objects, 1,000 levels | `std::hash` | 17.3 µs | 0.99 | 1.01 | 0.98 | 1.00 |
| arrays, 100,000 levels | copy | 6.4 ms | 1.00 | 1.00 | 1.00 | 1.00 |
| arrays, 100,000 levels | `==` | 1.1 ms | 1.01 | 1.03 | 1.00 | 1.06 |
| arrays, 100,000 levels | `dump` | 917.9 µs | 1.01 | 1.01 | 1.02 | 1.01 |
| arrays, 100,000 levels | `std::hash` | 1.7 ms | 1.04 | 1.05 | 0.99 | 0.99 |
| objects, 100,000 levels | copy | 7.5 ms | 0.99 | 1.02 | 1.01 | 0.99 |
| objects, 100,000 levels | `==` | 1.3 ms | 1.02 | 1.09 | 1.08 | 0.96 |
| objects, 100,000 levels | `dump` | 1.2 ms | 1.06 | 0.99 | 1.01 | 0.99 |
| objects, 100,000 levels | `std::hash` | 1.8 ms | 1.00 | 1.00 | 1.00 | 1.00 |
The results are as follows.
- A limit of `16` or more does not change the speed of ordinary documents (differences are within noise), because they
are not nested more deeply than that anyway. A document nested more deeply than the limit is walked partly with the
explicit stack.
- A limit of `0` or `1` makes the operations on ordinary documents 1.1 to 2.4 times as slow, because even their first
levels are handled with the explicit stack. The slowdown is largest for `==` on a document of many small containers
(`canada.json`: 2.4 times) and for copying (up to 1.8 times).
- For values that are nested far more deeply than any limit (1,000 and 100,000 levels deep), the limit makes no
measurable difference: almost all of the levels are handled with the explicit stack for every setting.
Raising the limit gains nothing for them, and lowering it costs nothing.
!!! note "Other macros"
- With [`JSON_NO_THREAD_LOCAL`](json_no_thread_local.md) defined (or for Clang targeting MinGW, where the library
defines it by itself), copying and comparing never descend, so for them the limit does not matter. The other
operations still recurse as many levels as the limit says. The range of the limit is the same.
- The macro is not part of the [ABI tag](../../features/namespace.md), as `JSON_NO_THREAD_LOCAL` is not, and it is
not undefined at the end of `json.hpp`.
!!! warning "Use the same value in every translation unit"
All translation units of a program that include the library must define the same value. Otherwise, the program
violates the One Definition Rule: the function `nlohmann::detail::recursion_depth_limit()` and all the functions
that use it are defined differently in different translation units. Which limit a call uses then depends on which
functions the compiler inlined (an inlined copy uses the value of its own translation unit) and which out-of-line
copy of each remaining function the linker kept, which is not specified. A function compiled with one value can
call a function compiled with another, so a single operation can mix both limits.
Mixing values cannot corrupt the byte per thread that counts the levels of copying and comparing: each limit
is at most `254`, and all of them stop counting at their own limit, so the count never exceeds `254`, whichever
translation unit's definition runs. The counter exists once per thread and `basic_json` specialization, and is
shared by all translation units. Nor can mixing values change any result, as no result depends on the limit. What
changes is how much stack is used and how fast the operations are. Nevertheless, avoid it: the program has undefined
behavior as far as the C++ standard is concerned.
## Examples
??? example "Example: lower the limit"
The example nests an array 5,000 levels deep. With the limit set to 16, the library recurses over the first 16
levels
and over the rest uses an explicit stack. The results are those of the default limit.
```cpp
--8<-- "examples/json_recursion_depth_limit.cpp"
```
Output:
```
--8<-- "examples/json_recursion_depth_limit.output"
```
??? example "Example: find out how much stack an operation needs"
The program runs `dump()` on a value nested `depth` levels deep in a thread with a stack of `kib` KiB. Compile it
with the options and the limit that you use, e.g. `-DJSON_RECURSION_DEPTH_LIMIT=64`, run it with `depth` set to the
limit plus 10 and decreasing values of `kib` (the stack size must be a multiple of the page size, often 4 or 16
KiB),
and note the smallest stack with which it still finishes. That is how the tables above were measured.
```cpp
#include <cstdlib>
#include <iostream>
#include <string>
#include <utility>
#include <pthread.h>
#include <nlohmann/json.hpp>
static nlohmann::json value;
static void* run(void*)
{
std::cout << "dump() ok, " << value.dump().size() << " bytes\n";
return nullptr;
}
int main(int argc, char** argv)
{
const int depth = std::atoi(argv[1]);
const std::size_t kib = static_cast<std::size_t>(std::atoi(argv[2]));
// built on the main thread, without recursion
value = 1;
for (int i = 0; i < depth; ++i)
{
nlohmann::json outer = nlohmann::json::array();
outer.push_back(std::move(value));
value = std::move(outer);
}
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_attr_setstacksize(&attr, kib * 1024);
pthread_t thread;
pthread_create(&thread, &attr, run, nullptr);
pthread_join(thread, nullptr);
}
```
## See also
- [Nesting depth](../../features/parsing/untrusted_input.md#nesting-depth) - how the library handles deeply nested
values
- [`JSON_NO_THREAD_LOCAL`](json_no_thread_local.md) - switch off the use of `thread_local` storage
- [Supported Macros](../../features/macros.md) - guide to all supported macros
## Version history
- Added in version 3.13.0.
@@ -93,7 +93,7 @@ The default value is `0` (disabled, the behavior of version 3.12.0 and earlier i
## See also
- [Binary formats](../../features/binary_formats/index.md) - overview of the supported binary formats
- [`from_cbor`](../basic_json/from_cbor.md) - also affected when parsing CBOR strings
- [`from_cbor`](../basic_json/from_cbor.md) - not affected by the macro; pass `error_handler` to validate CBOR strings
- [`to_cbor`](../basic_json/to_cbor.md) - create a CBOR serialization of a JSON value
- [`to_ubjson`](../basic_json/to_ubjson.md) - create a UBJSON serialization of a JSON value
- [`to_bjdata`](../basic_json/to_bjdata.md) - create a BJData serialization of a JSON value