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 *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, true by default)

ignore_comments (in)

whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default)

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)

Exceptions¶

Same as parse.

Complexity¶

Linear in the length of the input.

Notes¶

Every view taken from *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 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 bool where the flags are expected, such as read(ptr, len), does not compile; see parse.

Examples¶

#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';
}

4
true

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 unreleased.