basic_json.md

jsoncons::basic_json

#include <jsoncons/basic_json.hpp>

template< 
    class CharT,
    class Policy = sorted_policy,
    class Allocator = std::allocator<char>
> class basic_json;

namespace pmr {
    template <typename CharT,typename Policy>
    using basic_json = jsoncons::basic_json<CharT, Policy, std::pmr::polymorphic_allocator<char>>;
    using json = basic_json<char,sorted_policy>;                                                     (since 0.171.0)
    using wjson = basic_json<wchar_t,sorted_policy>;
    using ojson = basic_json<char, order_preserving_policy>;
    using wojson = basic_json<wchar_t, order_preserving_policy>;
}

A basic_json is a union type that can hold one of a number of possible data members, some that require an allocator (a long string, byte string, array, or object), and other trivially copyable ones that do not (an empty object, short string, number, boolean, or null). The data member may be tagged with a semantic_tag that provides additional information about its value. The sizeof a basic_json regardless of its template parameters is normally 16 bytes.

A basic_json is allocator-aware, and supports allocator propagation to allocator-aware arrays or objects. Every constructor has a version that accepts an allocator argument. A long string, byte string, array or object contains a pointer to underlying storage, the allocator is used to allocate that storage, and it is retained in that storage. For other data members the allocator argument is ignored. For more about allocators, see Allocators.

When assigned a new basic_json value, the old value is overwritten. The member data type of the new value may be different from the old value.

A basic_json can support multiple readers concurrently, as long as it is not being modified. If it is being modified, it must be by one writer with no concurrent readers.

Several aliases for common character types and policies for ordering an object's name/value pairs are provided:

Type	Definition
jsoncons::json	jsoncons::basic_json<char,jsoncons::sorted_policy,std::allocator<char>>
jsoncons::ojson	jsoncons::basic_json<char,jsoncons::order_preserving_policy,std::allocator<char>>
jsoncons::wjson	jsoncons::basic_json<wchar_t,jsoncons::jsoncons::sorted_policy,std::allocator<char>>
jsoncons::wojson	jsoncons::basic_json<wchar_t,jsoncons::order_preserving_policy,std::allocator<char>>
jsoncons::pmr::json (0.171.0)	jsoncons::pmr::basic_json<char,jsoncons::sorted_policy>
jsoncons::pmr::ojson (0.171.0)	jsoncons::pmr::basic_json<char,jsoncons::order_preserving_policy>
jsoncons::pmr::wjson (0.171.0)	jsoncons::pmr::basic_json<wchar_t,jsoncons::sorted_policy>
jsoncons::pmr::wojson (0.171.0)	jsoncons::pmr::basic_json<wchar_t,jsoncons::order_preserving_policy>

Template parameters

CharT	Character type of text string
Policy	Implementation policy for arrays and objects
Allocator	Allocator type for allocating internal storage for long strings, byte strings, arrays and objects. The allocator type may be a stateless allocator, a std::pmr::polymorphic_allocator, or a std::scoped_allocator_adaptor. Non-propagating stateful allocators, such as the Boost.Interprocess allocators, must be wrapped by a std::scoped_allocator_adaptor.

Member types

Member type	Definition
char_type	CharT
policy_type	Policy
allocator_type (until 0.171.0)	A stateless allocator, or a non-propagating stateful allocator
allocator_type (after 0.171.0)	A stateless allocator, std::pmr::polymorphic_allocator, or std::scoped_allocator_adaptor
char_traits_type	std::char_traits<char_type>
char_allocator_type	allocator_type rebound to char_type
reference	basic_json&
const_reference	const basic_json&
pointer	basic_json*
const_pointer	const basic_json*
string_view_type	basic_string_view<char_type>
key_type	A ContiguousContainer to char_type
key_value_type	key_value<key_type,basic_json>
object_iterator	A RandomAccessIterator to key_value_type
const_object_iterator	A const RandomAccessIterator to const key_value_type
array_iterator	A RandomAccessIterator to basic_json
const_array_iterator	A const RandomAccessIterator to const basic_json
object_range_type	range<object_iterator,const_array_iterator> (since 0.173.3)
const_object_range_type	range<const_object_iterator,const_array_iterator> (since 0.173.3)
array_range_type	range<array_iterator,const_array_iterator> (since 0.173.3)
const_array_range_type	range<const_array_iterator,const_array_iterator> (since 0.173.3)
proxy_type	proxy<basic_json>. The proxy_type class supports conversion to basic_json&.

Static member functions

parse	Parses JSON.
make_array	Makes a multidimensional basic_json array.
const basic_json& null()	Returns a null value

Member functions

(constructor)	constructs the basic_json value
(destructor)	destructs the basic_json value
operator=	assigns values

allocator_type get_allocator() const

For a long string, byte string, array, or object, returns the retained allocator used to allocate memory for their storage, otherwise attempts to return a default constructed allocator.

Ranges and Iterators

array_range	Returns a range that supports a range-based for loop over the elements of a basic_json array.
obect_range	Returns a range that supports a range-based for loop over the key-value pairs of a basic_json object.

Capacity

size_t size() const noexcept	Returns the number of elements in a basic_json array, or the number of members in a object, or zero
bool empty() const noexcept	Returns true if a basic_json string, object or array has no elements, otherwise false
size_t capacity() const	Returns the size of the storage space currently allocated for a basic_json object or array
void reserve(std::size_t n)	Increases the capacity of a basic_json object or array to allow at least n members or elements
void resize(std::size_t n)	Resizes a basic_json array so that it contains n elements
void resize(std::size_t n, const basic_json& val)	Resizes a basic_json array so that it contains n elements that are initialized to val
void shrink_to_fit()	Requests the removal of unused capacity

Accessors

bool contains(const string_view_type& key) const noexcept	Returns true if an object has a member with the given key , otherwise false
is	Checks if a basic_json value matches a type.
as	Attempts to convert a basic_json value to a value of a type.
operator[]	Access or insert specified element.
at at_or_null	Return the specified value.
get_value_or	Return the specified value if available, otherwise a default value.

semantic_tag tag() const

Returns the semantic_tag associated with this value

uint64_t ext_tag() const

If tag() == semantic_tag::ext, returns a format specific tag associated with a byte string value, otherwise return 0. An example is a MessagePack type in the range 0-127 associated with the MessagePack ext format family, or a CBOR tag preceeding a byte string.

json_type type() const

Returns the json type associated with this value

object_iterator find(const string_view_type& name)
const_object_iterator find(const string_view_type& name) const

Returns an object iterator to a member whose name compares equal to name. If there is no such member, returns object_range.end(). Throws std::domain_error if not an object.

Modifiers

void clear()	Remove all elements from an array or members from an object, otherwise do nothing
erase	Erases array elements and object members
push_back	Adds a value to the end of a basic_json array
insert	Inserts elements
emplace_back	Constructs a value in place at the end of a basic_json array
emplace	Constructs a value in place before a specified position in a basic_json array
try_emplace	Constructs a key-value pair in place in a basic_json object if the key does not exist, does nothing if the key exists
insert_or_assign	Inserts a key-value pair in a basic_json object if the key does not exist, or assigns a new value if the key already exists
merge	Inserts another basic_json object's key-value pairs into a basic_json object, if they don't already exist.
merge_or_update	Inserts another basic_json object's key-value pairs into a basic_json object, or assigns them if they already exist.
void swap(basic_json& val) noexcept	Exchanges the content of the basic_json value with the content of val, which is another basic_json value

Serialization

dump	Serializes basic_json value to a string, stream, or basic_json_visitor.

Non member functions

bool operator==(const basic_json& lhs, const basic_json& rhs)

Returns true if two basic_json objects compare equal, false otherwise.

bool operator!=(const basic_json& lhs, const basic_json& rhs)

Returns true if two basic_json objects do not compare equal, false otherwise.

bool operator<(const basic_json& lhs, const basic_json& rhs)

Compares the contents of lhs and rhs lexicographically.

bool operator<=(const basic_json& lhs, const basic_json& rhs)

Compares the contents of lhs and rhs lexicographically.

bool operator>(const basic_json& lhs, const basic_json& rhs)

Compares the contents of lhs and rhs lexicographically.

bool operator>=(const basic_json& lhs, const basic_json& rhs)

Compares the contents of lhs and rhs lexicographically.

std::basic_istream<char_type>& operator>>(std::basic_istream<char_type>& is, basic_json& o)

Reads a basic_json value from a stream.

std::basic_ostream<char_type>& operator<<(std::basic_ostream<char_type>& os, const basic_json& o)

Inserts basic_json value into stream.

std::basic_ostream<char_type>& print(const basic_json& val)  
std::basic_ostream<char_type>& print(const basic_json& val, const basic_json_options<CharT>& options)  

Inserts basic_json value into stream using the specified basic_json_options if supplied.

std::basic_ostream<char_type>& pretty_print(const basic_json& val)  
std::basic_ostream<char_type>& pretty_print(const basic_json& val, const basic_json_options<CharT>& options)  

Inserts basic_json value into stream using the specified basic_json_options if supplied.

void swap(basic_json& a, basic_json& b) noexcept

Exchanges the values of a and b