nlohmann::basic_json::start_pos¶

#if JSON_DIAGNOSTIC_POSITIONS
constexpr std::size_t start_pos() const noexcept;
#endif

Returns the position of the first character in the JSON string from which the value was parsed from.

JSON type	return value
object	position of the opening `{`
array	position of the opening `[`
string	position of the opening `"`
number	position of the first character
boolean	position of `t` for `true` and `f` for `false`
null	position of `n`

Return value¶

the position of the first character of the value in the parsed JSON string, if the value was created by the parse function, or std::string::npos if the value was constructed otherwise

Exception safety¶

No-throw guarantee: this member function never throws exceptions.

Complexity¶

Constant.

Notes¶

Note

The function is only available if macro JSON_DIAGNOSTIC_POSITIONS has been defined to 1 before including the library header.

Invalidation

The returned positions are only valid as long as the JSON value is not changed. The positions are not updated when the JSON value is changed.

Examples¶

Example

#include <iostream>

#define JSON_DIAGNOSTIC_POSITIONS 1
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    std::string json_string = R"(
    {
        "address": {
            "street": "Fake Street",
            "housenumber": 1
        }
    }
    )";
    json j = json::parse(json_string);

    std::cout << "Root diagnostic positions: \n";
    std::cout << "\tstart_pos: " << j.start_pos() << '\n';
    std::cout << "\tend_pos:" << j.end_pos() << "\n";
    std::cout << "Original string: \n";
    std::cout << "{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }" << "\n";
    std::cout << "Parsed string: \n";
    std::cout << json_string.substr(j.start_pos(), j.end_pos() - j.start_pos()) << "\n\n";

    std::cout << "address diagnostic positions: \n";
    std::cout << "\tstart_pos:" << j["address"].start_pos() << '\n';
    std::cout << "\tend_pos:" << j["address"].end_pos() << "\n\n";
    std::cout << "Original string: \n";
    std::cout << "{            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }" << "\n";
    std::cout << "Parsed string: \n";
    std::cout << json_string.substr(j["address"].start_pos(), j["address"].end_pos() - j["address"].start_pos()) << "\n\n";

    std::cout << "street diagnostic positions: \n";
    std::cout << "\tstart_pos:" << j["address"]["street"].start_pos() << '\n';
    std::cout << "\tend_pos:" << j["address"]["street"].end_pos() << "\n\n";
    std::cout << "Original string: \n";
    std::cout << "\"Fake Street\"" << "\n";
    std::cout << "Parsed string: \n";
    std::cout << json_string.substr(j["address"]["street"].start_pos(), j["address"]["street"].end_pos() - j["address"]["street"].start_pos()) << "\n\n";

    std::cout << "housenumber diagnostic positions: \n";
    std::cout << "\tstart_pos:" << j["address"]["housenumber"].start_pos() << '\n';
    std::cout << "\tend_pos:" << j["address"]["housenumber"].end_pos() << "\n\n";
    std::cout << "Original string: \n";
    std::cout << "1" << "\n";
    std::cout << "Parsed string: \n";
    std::cout << json_string.substr(j["address"]["housenumber"].start_pos(), j["address"]["housenumber"].end_pos() - j["address"]["housenumber"].start_pos()) << "\n\n";
}

Output:

Root diagnostic positions: 
    start_pos: 5
    end_pos:109
Original string: 
{
        "address": {
            "street": "Fake Street",
            "housenumber": 1
        }
    }
Parsed string: 
{
        "address": {
            "street": "Fake Street",
            "housenumber": 1
        }
    }

address diagnostic positions: 
    start_pos:26
    end_pos:103

Original string: 
{            "street": "Fake Street",
            "housenumber": 1
        }
Parsed string: 
{
            "street": "Fake Street",
            "housenumber": 1
        }

street diagnostic positions: 
    start_pos:50
    end_pos:63

Original string: 
"Fake Street"
Parsed string: 
"Fake Street"

housenumber diagnostic positions: 
    start_pos:92
    end_pos:93

Original string: 
1
Parsed string: 
1

The output shows the start/end positions of all the objects and fields in the JSON string.

Version history¶

Added in version 3.12.0.