101 lines
4.2 KiB
Markdown
101 lines
4.2 KiB
Markdown
# nlohmann::basic_json_document::read
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```
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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 `*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()`](https://json.nlohmann.me/api/basic_json_document/parse/index.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` : A compatible input; see [`parse`](https://json.nlohmann.me/api/basic_json_document/parse/#template-parameters).
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## Parameters
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`input` (in) : Input to parse from.
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`allow_exceptions` (in) : whether to throw exceptions in case of a parse error (optional, `true` by default)
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`ignore_comments` (in) : whether comments should be ignored and treated like whitespace (`true`) or yield a parse error (`false`); (optional, `false` by default)
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`ignore_trailing_commas` (in) : whether trailing commas in arrays or objects should be ignored and treated like whitespace (`true`) or yield a parse error (`false`); (optional, `false` by default)
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## Exceptions
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Same as [`parse`](https://json.nlohmann.me/api/basic_json_document/parse/#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 `*this` before the call -- including the previous [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) -- is invalidated, whether or not the new parse succeeds; take fresh views from [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) afterward.
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`input` is borrowed or owned by the same rules as [`parse()`](https://json.nlohmann.me/api/basic_json_document/parse/#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()`](https://json.nlohmann.me/api/basic_json_document/parse/#notes)) is written into the document's previous owned copy, reusing its memory; a moved-in `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 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()`.
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An integer argument that is not a `bool` where the flags are expected, such as `read(ptr, len)`, does not compile; see [`parse`](https://json.nlohmann.me/api/basic_json_document/parse/#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 a new document for each one.
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```
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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 json_document = 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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```
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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](https://json.nlohmann.me/api/basic_json_document/parse/index.md) - deserialize from a compatible input
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- [root](https://json.nlohmann.me/api/basic_json_document/root/index.md) - the view of the root value
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- [load](https://json.nlohmann.me/api/basic_json_document/load/index.md) - read a document from an image instead of parsing JSON text
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## Version history
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- Added in version 3.13.0 unreleased.
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