Struct

Learn C structs with Struct.

Struct

Struct is the dart:ffi class that represents C structs on the Dart side. A Struct is declared with extends Struct and declares the C-side fields with annotations like @Int32() on the Dart side.

For example, class Point extends Struct { @Int32() external int x; @Int32() external int y; } is the equivalent of struct Point { int x; int y; } in C. Struct fields are external; this indicates the memory is managed on the C side.

Struct memory layout

The order and type of struct fields must match the C struct's memory layout exactly. Otherwise data corruption occurs. @Int32() takes 4 bytes, @Double() 8 bytes, @Int8() 1 byte.

Structs are referenced via Pointer<Point> and accessed with Pointer.ref. Struct memory is either allocated on the C side or from Dart with calloc/malloc. If memory is not freed, a leak occurs.

Exercise A StructInfo class

Define a StructInfo class with final String name; final int fieldCount; StructInfo(this.name, this.fieldCount); field and constructor. Add a String describe() => '$name: $fieldCount fields'; method. In main create final si = StructInfo('Point', 2);; print with print(si.describe());.

Show example solution

class StructInfo {
  final String name;
  final int fieldCount;
  StructInfo(this.name, this.fieldCount);
  String describe() => '$name: $fieldCount fields';
}

void main() {
  final si = StructInfo('Point', 2);
  print(si.describe());
}

Test yourself

How is a Struct declared?
- With extends Struct
- With extends Class
- With implements Struct
- With with Struct
Show answer

With extends Struct — a Struct is declared with extends Struct.
Match the Struct parts

extends Struct

Declaration form
@Int32()

Field annotation
external

Memory on C side
Pointer<T>

Struct reference

A Struct annotation

class Point extends Struct {
  @____() external int x;
}

Show answer

class Point extends Struct {
  @Int32() external int x;
}

Next lesson: Pointer

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