Boost C++ Libraries Home Libraries People FAQ More

PrevUpHomeNext

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:

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 or 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::is_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 1.2. value Accessors


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!";
[Tip] Tip

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!";
Formatted Output

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


PrevUpHomeNext