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 true by default)
ignore_comments (in): whether comments should be ignored and treated like whitespace (#!cpp true) or yield a parse error (#!cpp false); (optional, #!cpp false by 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 false by 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
#include <iostream>
#include <string>
#include <vector>
#include <nlohmann/json_view.hpp>

using json_document = nlohmann::json_document;

int main()
{
    std::cout << std::boolalpha;

    std::vector<std::string> messages =
    {
        R"({"id": 1})", R"({"id": 2, "tag": "x"})", R"({"id": 3})"
    };

    // parse into the same document over and over: its node index and decode
    // buffer are reused instead of being freed and reallocated for each message
    json_document doc;
    std::size_t total = 0;
    for (const auto& msg : messages)
    {
        doc.read(msg);
        total += doc.root().size();
    }
    std::cout << total << '\n';

    // read() can also change what kind of input is owned/borrowed between calls
    doc.read(std::string(R"({"owned": true})"));
    std::cout << doc.owns_source() << '\n';
}

```

Output:

```json
4
true

```

Version history

Added in version 3.13.0.

4.0 KiB Raw Blame History