nlohmann::basic_json_view::front

basic_json_view front() const;

Returns the first element of an array, the first member value of an object, or the value itself if it is primitive (as for BasicJsonType::front(), a primitive value is a range of one element).

Return value

The first element or member value. For a primitive value (number, string, boolean), the value itself.

Exception safety

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

Exceptions

Throws invalid_iterator.214 if the view is null or discarded, or if it is an empty array or object.

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

Unlike BasicJsonType::front(), which has undefined behavior for an empty array or object, front() throws invalid_iterator.214 for an empty array, an empty object, a null value, or a discarded view.

Examples

??? example

The example below reads only the earliest entry of a build log with `front()`, without materializing the rest
of the (possibly long) log.

```cpp
#include <iostream>
#include <nlohmann/json_view.hpp>

using json_document = nlohmann::json_document;

int main()
{
    // the build log of a running job; front() reads just the earliest event
    // without materializing the (possibly long) rest of the log
    json_document log = json_document::parse(R"(["queued", "started", "compiling", "linking", "done"])");
    std::cout << log.root().front().materialize().dump() << '\n';

    // an empty log -- front() throws instead of the undefined behavior
    // basic_json::front() has for an empty array
    json_document empty_log = json_document::parse("[]");
    try
    {
        static_cast<void>(empty_log.root().front());
    }
    catch (const nlohmann::json::invalid_iterator& e)
    {
        std::cout << e.what() << '\n';
    }
}

```

Output:

```json
"queued"
[json.exception.invalid_iterator.214] cannot get value

```

Version history

Added in version 3.13.0.

2.6 KiB Raw Blame History