nlohmann::basic_json_view::operator<<¶

std::ostream& operator<<(std::ostream& o, const basic_json_view& v);

Not available when JSON_NO_IO is defined.

Serializes the given view v to the output stream o, using dump -- exactly as operator<<(std::ostream&, const basic_json&) does for a basic_json value.

The indentation of the output can be controlled with the member variable width of the output stream o. For instance, using the manipulator std::setw(4) on o sets the indentation level to 4, and the serialization result is the same as calling v.dump(4). A width of 0 or less (the default) selects the most compact representation, as v.dump(-1) does.
The indentation character can be controlled with the member variable fill of the output stream o. For instance, the manipulator std::setfill('\t') sets indentation to use a tab character rather than the default space character.
As for basic_json, o's width is reset to 0 after this call, whether or not it was greater than 0 before.

Numbers are always written as v.dump() writes them by default, i.e. as with number_format::shortest; there is no way to select number_format::source through the stream.

Parameters¶

o (in, out): stream to write to
v (in): view to serialize

Return value¶

the stream o

Exceptions¶

May throw std::bad_alloc, propagated from dump. Unlike operator<<(std::ostream&, const basic_json&), there is no UTF-8 decoding step that could throw type_error.316, and no error_handler to choose between -- see the Exceptions of dump.

Complexity¶

Linear, as dump.

Examples¶

Example

The example below writes one record out of a larger batch straight to a log stream -- compact for a one-line entry, and pretty-printed with std::setw/std::setfill for a readable dump -- without ever building a BasicJsonType value for the record, or for the rest of the batch.

#include <iostream>
#include <iomanip>
#include <nlohmann/json_view.hpp>

using json_document = nlohmann::json_document;
using json_view = nlohmann::json_view;

int main()
{
    // one order out of a large incoming batch -- write it straight to a log
    // stream without ever building a basic_json value for it, or for the
    // rest of the batch
    const json_document doc = json_document::parse(R"(
      [{"id": 1, "item": "cable"}, {"id": 2, "item": "adapter"}]
    )");
    const json_view orders = doc.root();

    // compact, for a one-line log entry
    std::cout << orders[1] << '\n';

    // std::setw sets the indentation level, exactly as for basic_json
    std::cout << std::setw(2) << orders[1] << "\n\n";

    // std::setfill changes the indentation character
    std::cout << std::setw(1) << std::setfill('\t') << orders[1] << '\n';
}

Output:

{"id":2,"item":"adapter"}
{
  "id": 2,
  "item": "adapter"
}

{
    "id": 2,
    "item": "adapter"
}

Version history¶

Added in version 3.13.0 unreleased.