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# nlohmann::basic_json::front

    reference front();
    const_reference front() const;

Returns a reference to the first element in the container. For a JSON container `c`, the expression `c.front()` is equivalent to `*c.begin()`.

## Return value

In case of a structured type (array or object), a reference to the first element is returned. In case of number, string, boolean, or binary values, a reference to the value is returned.

## Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

## Exceptions

If the JSON value is `null`, exception `invalid_iterator.214` is thrown.

## Complexity

Constant.

## Notes

Precondition

The array or object must not be empty. Calling `front` on an empty array or object yields undefined behavior.

## Examples

Example

The following code shows an example for `front()`.

    #include <iostream>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        // create JSON values
        json j_null;
        json j_boolean = true;
        json j_number_integer = 17;
        json j_number_float = 23.42;
        json j_object = {{"one", 1}, {"two", 2}};
        json j_object_empty(json::value_t::object);
        json j_array = {1, 2, 4, 8, 16};
        json j_array_empty(json::value_t::array);
        json j_string = "Hello, world";

        // call front()
        //std::cout << j_null.front() << '\n';          // would throw
        std::cout << j_boolean.front() << '\n';
        std::cout << j_number_integer.front() << '\n';
        std::cout << j_number_float.front() << '\n';
        std::cout << j_object.front() << '\n';
        //std::cout << j_object_empty.front() << '\n';  // undefined behavior
        std::cout << j_array.front() << '\n';
        //std::cout << j_array_empty.front() << '\n';   // undefined behavior
        std::cout << j_string.front() << '\n';
    }

Output:

    true
    17
    23.42
    1
    1
    "Hello, world"

## See also

- back to access the last element

## Version history

- Added in version 1.0.0.
- Adjusted code to return reference to binary values in version 3.8.0.

Last update: May 1, 2022
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