value::at

## `value::at`

Access an element, with bounds checking.

### Synopsis

    value&
    at(
        string_view key,
        boost::source_location const& loc = BOOST_CURRENT_LOCATION) &; (1)

    value&&
    at(
        string_view key,
        boost::source_location const& loc = BOOST_CURRENT_LOCATION) &&; (2)

    value const&
    at(
        string_view key,
        boost::source_location const& loc = BOOST_CURRENT_LOCATION) const&; (3)

    value&
    at(
        std::size_t pos,
        boost::source_location const& loc = BOOST_CURRENT_LOCATION) &; (4)

    value&&
    at(
        std::size_t pos,
        boost::source_location const& loc = BOOST_CURRENT_LOCATION) &&; (5)

    value const&
    at(
        std::size_t pos,
        boost::source_location const& loc = BOOST_CURRENT_LOCATION) const&; (6)

### Description

This function is used to access elements of the underlying container, or throw an exception if that could not be done.

- (1)–(3) is equivalent to `this->as_object(loc).at(key, loc)`.

- (4)–(6) is equivalent to `this->as_array(loc).at(pos, loc)`.

### Complexity

Constant.

### Exception Safety

Strong guarantee.

### Parameters

Name | Description
--- | ---
`key` | The key of the element to find.
`loc` | `boost::source_location` to use in thrown exception; the source location of the call site by default.
`pos` | A zero-based array index.

### Exceptions

Type | Thrown On
--- | ---
`boost::system::system_error` | The underlying type of value is not the container type corresponding to the first argument (i.e. using an index with an `object`).
`boost::system::system_error` | An element corresponding to the first argument was not found.

### See Also

`as_array`, `as_object`.

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