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Data Structures

yyjson consists of two types of data structures: immutable and mutable.

| Immutable | Mutable
--- | --- | ---
Document | yyjson_doc | yyjson_mut_doc
Value | yyjson_val | yyjson_mut_val

- Immutable data structures are returned when reading a JSON document. They cannot be modified.
- Mutable data structures are created when building a JSON document. They can be modified.
- yyjson also provides some functions to convert between these two types of data structures.

Please note that the data structures described in this document are considered private, and it is recommended to use the public API to access them.

# Immutable Value

Each JSON value is stored in an immutable yyjson_val struct:

struct yyjson_val {
uint64_t tag;
union {
uint64_t u64;
int64_t i64;
double f64;
const char *str;
void *ptr;
size_t ofs;
} uni;
}
yyjson_val::uni
yyjson_val_uni uni
Definition yyjson.h:4767
yyjson_val::tag
uint64_t tag
Definition yyjson.h:4766
yyjson_val
Definition yyjson.h:4765

The type of the value is stored in the lower 8 bits of the tag.
The size of the value, such as string length, object size, or array size, is stored in the higher 56 bits of the tag.

Modern 64-bit processors are typically limited to supporting fewer than 64 bits for RAM addresses (Wikipedia). For example, Intel64, AMD64, and ARMv8 have a 52-bit (4PB) physical address limit. Therefore, it is safe to store the type and size information within the 64-bit tag.

# Immutable Document

A JSON document stores all strings in a contiguous memory area.
Each string is unescaped in-place and ended with a null-terminator.
For example:

yyjson_doc

A JSON document stores all values in another contiguous memory area.
The object and array containers store their own memory usage, allowing easy traversal of the child values.
For example:

yyjson_doc

# Mutable Value

Each mutable JSON value is stored in an yyjson_mut_val struct:

struct yyjson_mut_val {
uint64_t tag;
union {
uint64_t u64;
int64_t i64;
double f64;
const char *str;
void *ptr;
size_t ofs;
} uni;
yyjson_mut_val *next;
}
yyjson_mut_val::next
yyjson_mut_val * next
Definition yyjson.h:5593
yyjson_mut_val::uni
yyjson_val_uni uni
Definition yyjson.h:5592
yyjson_mut_val::tag
uint64_t tag
Definition yyjson.h:5591
yyjson_mut_val
Definition yyjson.h:5590

The tag and uni fields are the same as the immutable value, and the next field is used to build a linked list.

# Mutable Document

A mutable JSON document is composed of multiple yyjson_mut_val.

The child values of an object or array are linked as a cycle,
the parent holds the tail of the circular linked list, enabling yyjson to perform operations append, prepend and remove_first in constant time.

For example:

yyjson_mut_doc

# Memory Management

A JSON document (yyjson_doc, yyjson_mut_doc) is responsible for managing the memory of all its JSON values and strings. When a document it is no longer needed, it is important for the user to call yyjson_doc_free() or yyjson_mut_doc_free() to free the memory associated with it.

A JSON value (yyjson_val, yyjson_mut_val) has the same lifetime as its document. The memory is managed by its document and and cannot be freed independently.

For more information, refer to the API documentation.