Niels Lohmann
9cb9f7fbad
An image is a document stored so that loading it needs no
parsing: save() writes the node index, the text and the decoded
strings; the static load() reads an image written by save().
load() takes a pointer and size, a borrowed vector, or an owned
rvalue vector; the nodes are copied so they are aligned and can
be edited, while the text and decoded strings stay in the image.
image_check controls how much load() trusts the input: full
checks structure, bounds, strings and numbers, the parser's own
guarantees; bounds checks structure and bounds only; none skips
all checks, for images from a trusted source.
Layout is little-endian only ("NJVI" header, nodes, text, decoded
strings), following the idea of zero-copy formats such as
FlatBuffers and YaFF; the check follows FlatBuffers' Verifier.
New errors: parse_error.116 for a malformed image or a failed
check, type_error.320 for a discarded document or a big-endian
target.
A dedicated fuzzer and 6,000 seeded corruptions, checked under
ASan/UBSan, found and fixed two gaps: unchecked reserved header
fields, and unbounded null/boolean offsets that could make
dump() throw std::length_error.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.8 KiB
nlohmann::basic_json_document::read
template<typename InputType>
void read(InputType&& input,
const bool allow_exceptions = true,
const bool ignore_comments = false,
const bool ignore_trailing_commas = false);
(Re-)parses input into #!cpp *this, discarding the document's previous value and reusing its memory (the node
index, the decoded-string buffer, and, if applicable, the owned copy of the text) rather than allocating a fresh
document. parse() is implemented in terms of this function, applied to a default-constructed document.
Template parameters
InputType- A compatible input; see
parse.
Parameters
input(in)- Input to parse from.
allow_exceptions(in)- whether to throw exceptions in case of a parse error (optional,
#!cpp trueby default) ignore_comments(in)- whether comments should be ignored and treated like whitespace (
#!cpp true) or yield a parse error (#!cpp false); (optional,#!cpp falseby default) ignore_trailing_commas(in)- whether trailing commas in arrays or objects should be ignored and treated like whitespace (
#!cpp true) or yield a parse error (#!cpp false); (optional,#!cpp falseby default)
Exceptions
Same as parse.
Complexity
Linear in the length of the input.
Notes
Every view taken from #!cpp *this before the call -- including the previous root() -- is invalidated,
whether or not the new parse succeeds; take fresh views from root() afterward.
input is borrowed or owned by the same rules as parse(); a document can borrow on one call and
own on the next, since ownership is decided freshly each time.
Reusing a document matters most for large inputs: the operating system provides the memory of a fresh node index one
page at a time, and every page costs a page fault the first time it is written. On x86-64 Linux (4 KiB pages), parsing
a 55 MB document into a reused document took about 40 % less time than parsing it into a fresh one. Programs that parse
many documents of similar size should therefore keep one document and call read().
Examples
??? example
The example below parses a sequence of messages into the same document, reusing its memory instead of allocating
a new document for each one.
```cpp
--8<-- "examples/basic_json_document__read.cpp"
```
Output:
```json
--8<-- "examples/basic_json_document__read.output"
```
See also
- parse - deserialize from a compatible input
- root - the view of the root value
- load - read a document from an image instead of parsing JSON text
Version history
- Added in version 3.13.0.