Niels Lohmann
f64464daf1
The single features/json_view.md page grew to ~600 lines covering eight
topics. Split it into features/json_view/ with an overview page and
pages for ownership and lifetime, differences from basic_json, reading
values, generic code, editing, and images.
The new differences page collects what was spread over several sections
("What is the same", "What is different", "Duplicate keys", and notes in
the conversion and serialization sections) under one summary table, and
adds the parts of the basic_json interface a view does not have. The
other pages now link to it instead of restating the rules.
Internal links across the documentation point to the new pages.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
3.2 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. An input that is copied (see the table in
parse()) is written into the document's previous owned copy, reusing its memory; a moved-in
#!cpp std::string replaces it.
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().
An integer argument that is not a #!cpp bool where the flags are expected, such as #!cpp read(ptr, len), does not
compile; see parse.
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 inputroot- the view of the root valueload- read a document from an image instead of parsing JSON text- Zero-copy JSON views - reusing a document and what it invalidates
Version history
- Added in version 3.13.0.