mirror of
https://github.com/nlohmann/json.git
synced 2026-09-28 18:50:31 +00:00
Add JSON_PRECISE_STREAM_POSITION to leave the character that terminates a number in the stream (#5344)
* docs: qualify the operator>> stream positioning guarantee operator>>'s notes state that it leaves the stream positioned right after the parsed value, so that concatenated JSON values can be read back to back. That does not hold when the value is a number: a number is only terminated by the character that follows it, and the lexer's unget() is simulated (it rewinds only the lexer's own bookkeeping), so that character stays consumed from the stream. Document the actual behaviour: the guarantee holds for all value types except numbers, which must be followed by whitespace. Also qualify the cross-reference on the JSON Lines page, which repeated the unqualified claim. Documentation only; the behaviour itself is tracked in #5340. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * fix: restore the character that terminates a number (#5340) operator>> is documented to leave the stream positioned right after the parsed value, so that concatenated JSON values can be read back to back. That did not hold for numbers: a number is only terminated by the character following it, and lexer::scan_number() reads that character and calls unget() -- which is simulated and rewinds only the lexer's own bookkeeping. input_stream_adapter consumes via sbumpc() with no matching sungetc(), so the terminating character stayed consumed and the next extraction started one byte too late ('1true' left the stream at 'rue'). Propagating unget() to the adapter directly does not work: next_unget makes the following get() replay the cached character, so the terminator would be delivered twice. Instead, restore the still-pending character once at the end of a non-strict parse, where the input is handed back to the caller: - input_stream_adapter gains unget_character() (sungetc()) and advertises it via supports_unget, detected the same way as supports_seek. - lexer::restore_pending_unget() turns a pending simulated unget of a real (non-EOF) character into a real one and clears next_unget so the character is not also replayed. It is a no-op for adapters that cannot unget, and reports failure when sungetc() fails, in which case the input is left as it was before. - parser calls it on the three non-strict paths, i.e. for operator>> and sax_parse(strict = false). Strict parse()/accept() are unaffected: they require the input to end after the value, so the character is consumed by the end-of-input check anyway. Parse error messages and reported positions are unchanged. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * tests: fix CI failures in the #5340 test helpers Four CI failures, all in the new test code: - GCC (-Werror=useless-cast): drop the `json(...)` wrapper around `json::parse(...)`, which already returns a `json`. - GCC (-Werror=unused-result): assign the discarded `json::parse()` result to a dummy, the idiom used elsewhere in the test suite, and catch `json::parse_error&` for consistency. - clang-tidy (google-default-arguments): remove the default argument from the `pbackfail()` override; `sungetc()` supplies the base declaration's default. - MSVC (bad allocation): `no_putback_streambuf::underflow()` set a one-character get area without advancing `m_pos`, so an implementation whose `istream::get` peeks before it bumps re-read the same character forever. Keep no get area at all: `underflow()` peeks, `uflow()` consumes, and `sungetc()` still always lands in `pbackfail()`, which is what the test needs. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * fix: leave the character that terminates a number in the input Read the character following a number without consuming it, instead of consuming it and putting it back. input_stream_adapter now peeks with sgetc() and only steps over the character when the next one is requested or when the adapter is destroyed, so releasing it cannot fail - no putback position is required from the streambuf. Suggested by gregmarr in #5344. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: match the version history wording to the peek-based fix Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: drop the whitespace-separator caveat from the parsing pages The caveat added in #5343 describes the behavior this branch fixes: a number no longer consumes the character that terminates it, so concatenated values need no separator. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * refactor: split the strict and non-strict paths in parser Folding the release_lookahead() call into the existing strict check left the "in strict mode" comment on an else-if branch, and made the strict condition in sax_parse() redundant with the branch it followed. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Put the stream position fix behind JSON_PRECISE_STREAM_POSITION Leaving the character that terminates a number in the stream is observable: reading "1,2,3" with repeated operator>> works today only because the comma after each number is swallowed, and std::getline after a number skips the line break. Both break with the fix, so make it opt-in for 3.x, as suggested by @gregmarr in the review. - JSON_PRECISE_STREAM_POSITION (default 0) selects the peek-based input_stream_adapter. Without it, the adapter is the consuming one from develop and has no supports_lookahead, so lexer::release_lookahead() and the parser's calls to it compile to nothing. - The macro changes input_stream_adapter's layout and member functions, so it gets the ABI tag _psp, after _bics. The ABI config tests, the natvis generator, and nlohmann_json.natvis (regenerated) know the tag. - The tests for the fix move to unit-precise-stream-position.cpp, which defines the macro itself and runs in every build, and gain the two cases above. unit-deserialization.cpp pins the default behavior instead. - The docs describe the default behavior again and point to the new macro page; version history says "added in 3.13.0, planned default in 4.0.0". Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
@@ -69,7 +69,9 @@ The SAX event lister must follow the interface of [`json_sax`](../json_sax/index
|
||||
[`input_format_t`](input_format_t.md) for more information
|
||||
|
||||
`strict` (in)
|
||||
: whether the input has to be consumed completely (optional, `#!cpp true` by default)
|
||||
: whether the input has to be consumed completely (optional, `#!cpp true` by default); when `#!cpp false` and the
|
||||
input is a `#!cpp std::istream`, the character that terminates a number is consumed unless
|
||||
[`JSON_PRECISE_STREAM_POSITION`](../macros/json_precise_stream_position.md) is defined to `1`; see [`operator>>`](../operator_gtgt.md#notes)
|
||||
|
||||
`ignore_comments` (in)
|
||||
: whether comments should be ignored and treated like whitespace (`#!cpp true`) or yield a parse error
|
||||
@@ -136,6 +138,8 @@ A UTF-8 byte order mark is silently ignored.
|
||||
- Added `ignore_trailing_commas` in version 3.13.0.
|
||||
- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
|
||||
- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
|
||||
- `JSON_PRECISE_STREAM_POSITION` added in version 3.13.0 to optionally leave a `#!cpp std::istream` positioned right
|
||||
after the parsed value when `strict` is `#!cpp false`.
|
||||
|
||||
!!! warning "Deprecation"
|
||||
|
||||
|
||||
@@ -16,6 +16,8 @@ header. See also the [macro overview page](../../features/macros.md).
|
||||
|
||||
## Parsing
|
||||
|
||||
- [**JSON_PRECISE_STREAM_POSITION**](json_precise_stream_position.md) - opt in to leaving an input stream positioned
|
||||
right after a parsed number
|
||||
- [**JSON_STRICT_NUL_HANDLING**](json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input instead of
|
||||
treating it as end of input
|
||||
|
||||
|
||||
@@ -0,0 +1,131 @@
|
||||
# JSON_PRECISE_STREAM_POSITION
|
||||
|
||||
```cpp
|
||||
#define JSON_PRECISE_STREAM_POSITION /* value */
|
||||
```
|
||||
|
||||
When defined to `1`, [`operator>>`](../operator_gtgt.md) and [`sax_parse`](../basic_json/sax_parse.md) with
|
||||
`strict = false` leave a `#!cpp std::istream` positioned right after the parsed value for every value type. By default,
|
||||
the character that terminates a number is consumed as well.
|
||||
|
||||
The macro only affects reading from a `#!cpp std::istream` when the rest of the stream is not required to be consumed.
|
||||
[`parse`](../basic_json/parse.md), [`accept`](../basic_json/accept.md), and all other inputs (strings, iterators,
|
||||
containers, `#!cpp FILE*`) are never affected.
|
||||
|
||||
## Default definition
|
||||
|
||||
The default value is `0` (disabled — existing behavior is preserved).
|
||||
|
||||
```cpp
|
||||
#define JSON_PRECISE_STREAM_POSITION 0
|
||||
```
|
||||
|
||||
## Notes
|
||||
|
||||
!!! note "Background"
|
||||
|
||||
A number is the only JSON value whose end can be detected solely by reading the character that follows it. By
|
||||
default, that character is consumed and not put back, so the stream is left one byte too far after a number, and
|
||||
only after a number:
|
||||
|
||||
```cpp
|
||||
std::istringstream input("1true");
|
||||
json j;
|
||||
input >> j; // j == 1, but the stream now starts at "rue"
|
||||
```
|
||||
|
||||
With this macro, the character is only looked at and left in the stream, so the stream starts at `true`. This
|
||||
does not require the stream buffer to support putting a character back.
|
||||
|
||||
This was not changed unconditionally, because code can depend on the consumed character, even unknowingly (see
|
||||
[#5340](https://github.com/nlohmann/json/issues/5340)). Both of the following work by default only because the
|
||||
character after each number is swallowed, and behave differently with this macro:
|
||||
|
||||
```cpp
|
||||
std::istringstream input("1,2,3");
|
||||
json j1, j2, j3;
|
||||
input >> j1 >> j2 >> j3; // default: 1, 2, 3
|
||||
// with the macro: throws parse_error.101 at the ','
|
||||
```
|
||||
|
||||
```cpp
|
||||
std::istringstream input("42\nfoo");
|
||||
json j;
|
||||
std::string line;
|
||||
input >> j;
|
||||
std::getline(input, line); // default: "foo"
|
||||
// with the macro: "" (like after reading an int with >>)
|
||||
```
|
||||
|
||||
In both cases, the behavior with the macro is what you already get today when the value is not a number: `"a","b"`
|
||||
fails at the `,`, and `std::getline` after `{}` returns an empty string. This macro offers an opt-in path to
|
||||
the consistent behavior ahead of version 4.0.0, where it is planned to become the default.
|
||||
|
||||
!!! warning "Opt-in only"
|
||||
|
||||
This macro must be defined **before** including `<nlohmann/json.hpp>`. Defining it after the include has no
|
||||
effect.
|
||||
|
||||
!!! note "ABI compatibility"
|
||||
|
||||
The value of this macro is encoded in the [namespace](../../features/namespace.md) (tag `_psp`), resulting in
|
||||
distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program
|
||||
without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.
|
||||
|
||||
!!! tip "Workaround without the macro"
|
||||
|
||||
Separate the values in the stream with whitespace. The character consumed after a number is then the separator,
|
||||
and whitespace before the next value is skipped anyway.
|
||||
|
||||
## Examples
|
||||
|
||||
??? example "Default behavior (macro not defined)"
|
||||
|
||||
Without the macro, the character after a number is consumed:
|
||||
|
||||
```cpp
|
||||
#include <iostream>
|
||||
#include <sstream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
std::istringstream input("1true");
|
||||
json j1, j2;
|
||||
input >> j1; // j1 == 1
|
||||
input >> j2; // throws parse_error.101: the stream now starts at "rue"
|
||||
}
|
||||
```
|
||||
|
||||
??? example "Opt-in precise stream position (macro defined to 1)"
|
||||
|
||||
With the macro, the stream is positioned right after the number:
|
||||
|
||||
```cpp
|
||||
#define JSON_PRECISE_STREAM_POSITION 1
|
||||
#include <iostream>
|
||||
#include <sstream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
std::istringstream input("1true");
|
||||
json j1, j2;
|
||||
input >> j1; // j1 == 1
|
||||
input >> j2; // j2 == true
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [**operator>>**](../operator_gtgt.md) - deserialize from stream
|
||||
- [**sax_parse**](../basic_json/sax_parse.md) - generate SAX events
|
||||
|
||||
## Version history
|
||||
|
||||
- Added in version 3.13.0.
|
||||
- Planned to become the default (with the macro removed) in version 4.0.0.
|
||||
@@ -67,7 +67,9 @@ input >> j2; // parses the next value
|
||||
Only numbers are affected. Values ending in a self-delimiting character do not read past themselves, so
|
||||
`truefalse`, `[1][2]`, `{"a":1}{"b":2}`, and `"a""b"` can be read back to back without a separator.
|
||||
|
||||
This is tracked in [#5340](https://github.com/nlohmann/json/issues/5340).
|
||||
Define [`JSON_PRECISE_STREAM_POSITION`](macros/json_precise_stream_position.md) to `1` to leave the terminating character in the stream
|
||||
instead, so that the stream is positioned right after the value for every value type and no separator is
|
||||
needed. This is tracked in [#5340](https://github.com/nlohmann/json/issues/5340).
|
||||
|
||||
Note that reading concatenated values does **not** work for [JSON Lines](../features/parsing/json_lines.md)
|
||||
(newline-delimited JSON) input -- see that page for why and for the recommended alternative.
|
||||
@@ -107,9 +109,12 @@ being read.
|
||||
- [parse](basic_json/parse.md) - deserialize from a compatible input
|
||||
- [`JSON_STRICT_NUL_HANDLING`](macros/json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input
|
||||
instead of treating it as end of input
|
||||
- [`JSON_PRECISE_STREAM_POSITION`](macros/json_precise_stream_position.md) - opt in to leaving the stream positioned right after a number
|
||||
|
||||
## Version history
|
||||
|
||||
- Added in version 1.0.0.
|
||||
- `JSON_STRICT_NUL_HANDLING` added in version 3.13.0 to optionally reject a NUL byte in the input instead of treating
|
||||
it as end of input; planned to become the default in version 4.0.0.
|
||||
- `JSON_PRECISE_STREAM_POSITION` added in version 3.13.0 to optionally leave the character that terminates a number in
|
||||
the stream; planned to become the default in version 4.0.0.
|
||||
|
||||
@@ -98,6 +98,15 @@ rather than descending into a bounded number of levels first, which is slower bu
|
||||
|
||||
See [full documentation of `JSON_NO_THREAD_LOCAL`](../api/macros/json_no_thread_local.md).
|
||||
|
||||
## `JSON_PRECISE_STREAM_POSITION`
|
||||
|
||||
When defined to `1`, [`operator>>`](../api/operator_gtgt.md) and non-strict
|
||||
[`sax_parse`](../api/basic_json/sax_parse.md) leave an input stream positioned right after the parsed value, instead of
|
||||
also consuming the character that terminates a number. The default value is `0`, which preserves the existing behavior;
|
||||
this is planned to become the default in version 4.0.0.
|
||||
|
||||
See [full documentation of `JSON_PRECISE_STREAM_POSITION`](../api/macros/json_precise_stream_position.md).
|
||||
|
||||
## `JSON_SKIP_LIBRARY_VERSION_CHECK`
|
||||
|
||||
When defined, the library will not create a compiler warning when a different version of the library was already
|
||||
|
||||
@@ -18,6 +18,7 @@ The complete default namespace name is derived as follows:
|
||||
- [`JSON_DIAGNOSTIC_POSITIONS`](../api/macros/json_diagnostic_positions.md) defined non-zero appends `_dp`.
|
||||
- [`JSON_BRACE_INIT_COPY_SEMANTICS`](../api/macros/json_brace_init_copy_semantics.md) defined non-zero appends
|
||||
`_bics`.
|
||||
- [`JSON_PRECISE_STREAM_POSITION`](../api/macros/json_precise_stream_position.md) defined non-zero appends `_psp`.
|
||||
- The inline namespace ends with the suffix `_v` followed by the 3 components of the version number separated by
|
||||
underscores. To omit the version component, see [Disabling the version component](#disabling-the-version-component)
|
||||
below.
|
||||
|
||||
@@ -41,7 +41,8 @@ document followed by trailing bytes" is accepted rather than rejected. If you ar
|
||||
reject any input that is not exactly one JSON document, prefer `parse`.
|
||||
|
||||
When using `operator>>` to read several concatenated values this way, a value that is a number must be followed by
|
||||
whitespace, because `operator>>` consumes the character that terminates a number — see the
|
||||
whitespace, because `operator>>` consumes the character that terminates a number, unless
|
||||
[`JSON_PRECISE_STREAM_POSITION`](../../api/macros/json_precise_stream_position.md) is defined to `1` — see the
|
||||
[`operator>>` notes](../../api/operator_gtgt.md#notes) for details and examples.
|
||||
|
||||
## SAX vs. DOM parsing
|
||||
|
||||
@@ -293,6 +293,7 @@ nav:
|
||||
- 'JSON_NOEXCEPTION': api/macros/json_noexception.md
|
||||
- 'JSON_NO_IO': api/macros/json_no_io.md
|
||||
- 'JSON_NO_THREAD_LOCAL': api/macros/json_no_thread_local.md
|
||||
- 'JSON_PRECISE_STREAM_POSITION': api/macros/json_precise_stream_position.md
|
||||
- 'JSON_SKIP_LIBRARY_VERSION_CHECK': api/macros/json_skip_library_version_check.md
|
||||
- 'JSON_SKIP_UNSUPPORTED_COMPILER_CHECK': api/macros/json_skip_unsupported_compiler_check.md
|
||||
- 'JSON_STRICT_NUL_HANDLING': api/macros/json_strict_nul_handling.md
|
||||
|
||||
Reference in New Issue
Block a user