value

## `value`

JSON documents are represented in memory as instances of `value`: a Regular type which satisfies DefaultConstructible, CopyConstructible, CopyAssignable, MoveConstructible, MoveAssignable, and many of the requirements of allocator-aware containers. It is implemented as a variant internally, and can dynamically store any of the six defined JSON value types:

- null: A monostate value, equivalent to `nullptr`.

- boolean: A boolean: either `true` or `false`.

- number: An integral or floating point value.

- string: A sequence of zero or more Unicode characters, similar to `std::string`.

- array: An ordered list of values, like `std::vector`.

- object: A collection of name/value pairs, also known as an associative array.

## Working With Values

A `value` constructed from `nullptr` or default constructed represents a null JSON element:

    value jv1;
    value jv2( nullptr );

    assert( jv1.is_null() );
    assert( jv2.is_null() );

The member function `value::kind` may be used to query the kind stored in the value. Alternatively, member functions like `value::is_object` `value::is_number` may be used to check whether or not the value is a particular kind:

    value jv( object_kind );

    assert( jv.kind() == kind::object );
    assert( jv.is_object() );
    assert( ! jv.is_number() );

Functions like `value::if_object` actually return a pointer to the object if the value is an object, otherwise they return null. This allows them to be used both in boolean contexts as above, and in assignments or conditional expressions to capture the value of the pointer:

    value jv( object_kind );

    if( auto p = jv.if_object() )
        return p->size();

After a `value` is constructed, its kind can change depending on what is assigned to it, or by calling functions such as `value::emplace_array` or `value::emplace_bool`. If the assignment is successful, in other words it completes without any exceptions then the value is replaced. Otherwise, the value is unchanged. All operations which can fail to modify a value offer the strong exception safety guarantee.

    value jv;
    jv = value( array_kind );

    assert( jv.is_array() );

    jv.emplace_string();

    assert( jv.is_string() );

The following table shows all of the ways to determine and access the contents of a `value`:

Table 2. `value` Accessors
Kind | Representation | Emplacement | Kind Test | Pointer Access | `result` Access | Checked Access | Unchecked Access
`kind::array` | `array` | `value::emplace_array` | `value::is_array` | `value::if_array` | `value::try_as_array` | `value::as_array` | `value::get_array`
`kind::object` | `object` | `value::emplace_object` | `value::is_object` | `value::if_object` | `value::try_as_object` | `value::as_object` | `value::get_object`
`kind::string` | `string` | `value::emplace_string` | `value::is_string` | `value::if_string` | `value::try_as_string` | `value::as_string` | `value::get_string`
`kind::int64` | `std::int64_t` | `value::emplace_int64` | `value::is_int64` | `value::if_int64` | `value::try_as_int64` | `value::as_int64` | `value::get_int64`
`kind::uint64` | `std::uint64_t` | `value::emplace_uint64` | `value::is_uint64` | `value::if_uint64` | `value::try_as_uint64` | `value::as_uint64` | `value::get_uint64`
`kind::double_` | `double` | `value::emplace_double` | `value::is_double` | `value::if_double` | `value::try_as_double` | `value::as_double` | `value::get_double`
`kind::bool_` | `bool` | `value::emplace_bool` | `value::is_bool` | `value::if_bool` | `value::try_as_bool` | `value::as_bool` | `value::get_bool`
`kind::null` | `std::nullptr_t` | `value::emplace_null` | `value::is_null` | — | `value::try_as_null` | — | —

The emplace members of `value` return a typed reference to the underlying representation. For example, the call to `value::emplace_string` in the previous example returns a `string&`. This table shows the underlying type for each kind:

Kind | Type | Description
--- | --- | ---
`kind::object` | `object` | An associative array of string keys mapping to `value` elements with an interface similar to `std::unordered_map`, that remembers insertion order.
`kind::array` | `array` | An ordered list of `value` elements with an interface similar to `std::vector`.
`kind::string` | `string` | A UTF-8 encoded Unicode string of characters with an interface similar to `std::string`.
`kind::int64` | `std::int64_t` | A 64 bit signed integer.
`kind::uint64` | `std::uint64_t` | A 64 bit unsigned integer.
`kind::double_` | `double` | A `double` holding a floating-point value.
`kind::bool_` | `bool` | A `bool` holding `true` or `false`.
`kind::null` | — | A monostate value representing null.

The return value from emplace can be used to perform assignment or to capture a reference to the underlying element for later inspection or modification:

    value jv;
    jv.emplace_string() = "Hello, world!";

    int64_t& num = jv.emplace_int64();
    num = 1;

    assert( jv.is_int64() );

If the `kind` of a `value` is known, functions such as `value::as_bool` or `value::as_string` may be used to obtain a reference to the underlying representation without changing the existing value:

    value jv( true );
    jv.as_bool() = true;

    jv.as_string() = "Hello, world!"; // throws an exception

However, as shown above these functions throw an exception if the kind in the `value` does not match the kind denoted by the function signature. This can be used as a concise form of validation: access values as if they were the right type, but handle the resulting exception indicating if the schema of the JSON is not valid.

We can query a value for its underlying representation of a particular kind in a way that does not throw exceptions by requesting a pointer which may be null, instead of a reference. Here we use `value::if_string` to conditionally perform an assignment without using exceptions:

    value jv( string_kind );
    if( string* str = jv.if_string() )
        *str = "Hello, world!";

| If the value’s kind is known statically, a reference to the underlying representation may be obtained by dereferencing the pointer without checking. This avoids the code overhead of the possible exception when using, for example `value::as_string`:
    value jv( string_kind );

    // The compiler's static analysis can see that
    // a null pointer is never dereferenced.
    *jv.if_string() = "Hello, world!";

Functions returning `result` allow you to use both approaches:

    value jv( string_kind );
    if( boost::system::result<string&> str = jv.try_as_string() )
        *str = "Hello, world!";

    try
    {
        jv.try_as_bool().value() = true;
    }
    catch(...)
    {
    }

## Formatted Output

When a `value` is formatted to a `std::ostream`, the result is serialized JSON as if by calling `serialize`.

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