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150 lines
6.1 KiB
Markdown
150 lines
6.1 KiB
Markdown
# <small>nlohmann::basic_json_document::</small>read
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```cpp
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template<typename InputType>
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void read(InputType&& input,
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const bool allow_exceptions = true,
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const bool ignore_comments = false,
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const bool ignore_trailing_commas = false);
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```
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(Re-)parses `input` into `#!cpp *this`, discarding the document's previous value and reusing its memory (the node
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index, the decoded-string buffer, and, if applicable, the owned copy of the text) rather than allocating a fresh
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document. [`parse()`](parse.md) is implemented in terms of this function, applied to a default-constructed document.
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## Template parameters
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`InputType`
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: A compatible input; see [`parse`](parse.md#template-parameters).
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## Parameters
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`input` (in)
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: Input to parse from.
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`allow_exceptions` (in)
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: whether to throw exceptions in case of a parse error (optional, `#!cpp true` by default)
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`ignore_comments` (in)
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: whether comments should be ignored and treated like whitespace (`#!cpp true`) or yield a parse error
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(`#!cpp false`); (optional, `#!cpp false` by default)
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`ignore_trailing_commas` (in)
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: whether trailing commas in arrays or objects should be ignored and treated like whitespace (`#!cpp true`) or
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yield a parse error (`#!cpp false`); (optional, `#!cpp false` by default)
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## Exceptions
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Same as [`parse`](parse.md#exceptions).
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## Complexity
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Linear in the length of the input.
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## Notes
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Every view taken from `#!cpp *this` before the call — including the previous [`root()`](root.md) — is invalidated,
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whether or not the new parse succeeds; take fresh views from [`root()`](root.md) afterward.
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!!! warning "`input` must not point into the document itself"
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`read()` releases the document's previous text, its decoded strings, and (for an editable document) its edits
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*before* it parses `input`. An `input` that is, or points into, any of them violates a precondition: it may seem to
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work, parse empty or garbled text, be reported by a sanitizer, or read freed memory (undefined behavior).
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This applies to every `string_view` obtained from the document whose characters live in memory that `read()`
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releases:
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- [`source()`](source.md) of a document that owns its text;
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- [`get_string()`](../basic_json_view/get_string.md) (also as `get<string_view_t>()` or a `string_view` member of a
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decoded struct) of a string with escapes, of a string of a document that owns its text, and of an edited string;
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- [`number_token()`](../basic_json_view/number_token.md) of a number of a document that owns its text, and of a
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number written by an edit;
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- the key of an object member (`key()`) if it has escapes, was written by an edit, or the document owns its text.
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It does not apply to a text that the caller owns and the document borrows, nor to the strings, keys, and tokens that
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are read from such a text unchanged, nor to the decoded strings of a borrowed [image](load.md), which live in the
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caller's image: `read()` never touches them. To parse JSON that a document holds in a string of its own (for
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example, JSON embedded in a string value), copy the string first, for example with
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`#!cpp std::string inner(doc.root()["payload"].get_string()); doc.read(inner);`.
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A debug build (one without `NDEBUG`) checks this with `assert()`: an `input` in the document's text, decoded
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strings, or image, or in the most recent buffer of strings written by edits, ends the program. (Strings written
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by earlier edits are not covered.) A release build does not check, to keep the cost of reusing a document at a
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minimum. The check uses `assert()` directly, not [`JSON_ASSERT`](../macros/json_assert.md), which `json.hpp`
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undefines at its end.
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`input` is borrowed or owned by the same rules as [`parse()`](parse.md#notes); a document can borrow on one call and
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own on the next, since ownership is decided freshly each time. An input that is copied (see the table in
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[`parse()`](parse.md#notes)) is written into the document's previous owned copy, reusing its memory; a moved-in
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`#!cpp std::string` replaces it.
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Reusing a document matters most for large inputs: the operating system provides the memory of a fresh node index one
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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
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a 55 MB document into a reused document took about 40 % less time than parsing it into a fresh one. Programs that parse
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many documents of similar size should therefore keep one document and call `read()`.
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An integer argument that is not a `#!cpp bool` where the flags are expected, such as `#!cpp read(ptr, len)`, does not
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compile; see [`parse`](parse.md#notes).
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## Examples
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??? example
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The example below parses a sequence of messages into the same document, reusing its memory instead of allocating
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a new document for each one.
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```cpp
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#include <iostream>
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#include <string>
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#include <vector>
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#include <nlohmann/json_view.hpp>
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using nlohmann::json_document;
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int main()
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{
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std::cout << std::boolalpha;
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std::vector<std::string> messages =
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{
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R"({"id": 1})", R"({"id": 2, "tag": "x"})", R"({"id": 3})"
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};
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// parse into the same document over and over: its node index and decode
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// buffer are reused instead of being freed and reallocated for each message
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json_document doc;
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std::size_t total = 0;
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for (const auto& msg : messages)
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{
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doc.read(msg);
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total += doc.root().size();
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}
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std::cout << total << '\n';
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// read() can also change what kind of input is owned/borrowed between calls
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doc.read(std::string(R"({"owned": true})"));
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std::cout << doc.owns_source() << '\n';
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}
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```
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Output:
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```json
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4
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true
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```
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## See also
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- [`parse`](parse.md) - deserialize from a compatible input
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- [`root`](root.md) - the view of the root value
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- [`load`](load.md) - read a document from an image instead of parsing JSON text
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- [Zero-copy JSON views](../../features/json_view/lifetime.md) - reusing a document and what it invalidates
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## Version history
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- Added in version 3.13.0.
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