nlohmann::basic_json_view::get_string¶

string_view_t get_string() const;

Returns the string value as a string_view_t, without copying it.

Return value¶

The string, as a string_view_t that points either into the document's source() text (a string with no escape sequences), or into the document's own buffer of decoded strings (a string that contains escape sequences, such as "\n" or "\u00e9", which had to be decoded once when the document was parsed).

Exception safety¶

Strong guarantee: there are no changes to the view or the document it refers to.

Exceptions¶

Throws type_error.302 if the value is not a string; example: "type must be string, but is array".

This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

Complexity¶

Constant.

Notes¶

basic_json_view has no BasicJsonType value stored anywhere, so unlike BasicJsonType, it has no get_ref() to hand out a reference to a stored string_t. get_string() (equivalently, get<string_view_t>()) is the zero-copy alternative: BasicJsonType::get_ref<const string_t&>() is its closest counterpart, except that it returns a view instead of a reference to a value that must already exist.

get_string() borrows the string: nothing is copied, whereas get<std::string>() copies it. For a borrowed document, the returned string_view_t is valid as long as the caller's buffer; for an owned one, as long as the document.

The returned string_view_t is valid exactly as long as the view that produced it -- see the validity rules of basic_json_view -- and, for a string with no escapes, for as long as the document's source text, which for a borrowed document must remain valid and unmodified for as long as the document exists, since floating-point values and unescaped strings are read from the source lazily.

Examples¶

Example

The example below pulls one field out of a JSON text that stands in for a large API response, and shows that no std::string was allocated for it: the returned view still points inside the original buffer. A field that contains an escape sequence cannot point into the original text -- it was decoded once into the document's own buffer instead -- but still avoids a per-field allocation.

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

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

int main()
{
    // a "large response" stand-in: only the "id" field is ever read out of it
    const std::string text =
        R"({"id": "8f14e45f-ceea-467e-bb92-963f5e3c7a08", "note": "created via API\n", "payload": "..."})";
    const json_document doc = json_document::parse(text);
    const json_view response = doc.root();

    const json_view::string_view_t id = response["id"].get_string();
    std::cout << id << '\n';

    // no std::string was allocated for "id": its bytes still live inside
    // the original buffer, so id's data lies inside [text.data(),
    // text.data() + text.size())
    const bool id_in_source = id.data() >= text.data() && id.data() + id.size() <= text.data() + text.size();
    std::cout << std::boolalpha << id_in_source << '\n';

    // "note" contains an escape sequence ('\n'), so it was decoded once
    // into the document's own buffer -- get_string() still avoids a copy
    // into a new std::string, but the bytes no longer live inside "text"
    const json_view::string_view_t note = response["note"].get_string();
    const bool note_in_source = note.data() >= text.data() && note.data() + note.size() <= text.data() + text.size();
    std::cout << std::boolalpha << note_in_source << '\n';
}

Output:

8f14e45f-ceea-467e-bb92-963f5e3c7a08
true
false

Version history¶

Added in version 3.13.0 unreleased.