FTP-003: Clarification: Default Values for Struct Members



A FIDL struct member may have a default value. Today, support of defaults is partially implemented (see section below), this proposal aims to clarify how defaults ought to behave.


  1. It provides regularity across the language binding, provides protections from inconsistent or unexpected uses, and
  2. Eliminates laborious manual, member-by-member initializations when a language demands explicit initializations, and

Non-motivation includes:

  • Saving bytes in wire format

It is not a motivation to save bytes in wire format or to save processing power in doing encoding or decoding.

Today's Implementation

Defaults can be expressed in the FIDL language on struct members:

  • (+) There is support for numerical literals, boolean literals, and string literals.
  • (-) No type checking is provided by fidlc of assignability of a literal to a struct member. It is possible to have a string literal “hello” assigned to a bool, a negative number assigned to a uint, or an out-of-bound number assigned to an int16.
  • (-) Language binding support is inconsistent, today it only exists for C++, and Dart bindings. There is no support for Go, C, or Rust.

For example (from //zircon/system/host/fidl/examples/types.test.fidl):

struct default_values {
    bool b1 = true;
    bool b2 = false;
    int8 i8 = -23;
    int16 i16 = 34;
    int32 i32 = -34595;
    int64 i64 = 3948038;
    uint8 u8 = 0;
    uint16 u16 = 348;
    uint32 u32 = 9038;
    uint64 u64 = 19835;
    float32 f32 = 1.30;
    float64 f64 = 0.0000054;
    string s = "hello";


Default values MAY be defined on struct members. Defaults appear at the end of a field definition with a C-like = {value} pattern.


// cat.fidl

enum CatAction : int8 {
    SIT = -10;
    WALK = 0;
    SNEAK = 2;

struct Location {
    uint8 pos_x = 10;  // Position X
    uint8 pos_y;       // Position Y. Default unspecified. Fall-back to 0
    float32 pos_z = 3.14;  // Position Z.
    float32 pos_t;         // Default unspecified. Fall-back to 0.0

struct Cat {
    string name;      // Automatic default to empty string
    CatAction action = CatAction::SNEAK;
    Location loc;



Please refer to FTP-006 which clarified the semantics of defaults, and requirements on bindings.

Supported Types

  • Primitive types:
    • bool, int8, int16, int32, int64, uint8, uint16, uint32, uint64, float32, float64
  • Non-nullable string, string:N
    • string:N shall zero out the memory that is reserved, and not used.

Unsupported Types

  • array<T>:N
    • Set to zero
  • Non-nullable types: vector<T>, vector<T>:N
    • Set to zero
  • Nullable types: string?, string:N?, vector<T>?, vector<T>:N?
    • Set to null
  • handle
  • struct
    • While each individual member in the struct may have a default, a struct itself does not have a default.
  • union
    • To avoid any conflict, any default value of a member of the union, or that of a substructure (in any depth) of the union shall be ignored.

Nuances of Defaults

The focus is on the value itself, and not on the manner of assigning the value. This implies two things at least:

  • There is no distinction - if a default value is used because the parameter of interest was explicitly assigned by another mechanism, or not.
  • There is no extra (transparent) layer of logic to assign values at the time of marshalling or unmarshalling.


Here are some example implemention ideas for C, Rust, and Go Bindings

// in FIDL "default.fidl"
struct Location {
    uint8 pos_x = 10;
    uint8 pos_y = 20;
    uint8 pos_x;       // Should be set to "zero" according to above.
// C binding "defaults/fidl.h"
typedef struct _Location_raw {
   uint8_t pos_x;
   uint8_t pos_y;
   uint8_t pos_z
} Location;

Location Location_default = { 10, 20, 0 }; // Or in the source file.
                                           // May be used for memcmp,
memcpy, etc.

#define Location(my_instance) Location my_instance = Location_default;
// C code "example.c"
#include <fidl.h>
void showme(Location loc) {
    printf("(%u, %u, %u)\n", loc.pos_x, loc.pos_y, loc.pos_z);

int main() {
    Location beta;
    Location gamma = Location_default;
    showme(alpha); showme(beta); showme(gamma);
    return 0;
// Rust binding
struct Location {
    pos_x: u8,
    pos_y: u8,
    pos_z: u8,
impl std::default::Default for Location {
    fn default() -> Self { Self { pos_x: 10, pos_y: 20, pos_z: 0 } }
// Go binding, using export control
type location struct {
    pos_x  uint8
    pos_y  uint8
    pos_z  uint8

Func NewLocation() location {
    loc := location{}
    loc.pos_x = 10
    loc.pos_y = 20
    // loc.pos_z = 0  Maybe ommited.
    return loc

Backwards compatibility

This change makes the FIDL file source backward-incompatible. No ABI or wire format change is needed.




Drawbacks, alternatives, and unknowns

It is not evaluated if implementation of this in all language bindings will be straightforward.

Prior art and references

Protocol buffer, Flat buffer provides default values. Golang has a concept of zero values where variables declared without an explicit initial values are explicitly initialized as zero.

An open source approach

// From https://github.com/creasty/defaults
type Sample struct {
        Name   string `default:"John Smith"`
        Age    int    `default:"27"`
        Gender Gender `default:"m"`

        Slice       []string       `default:"[]"`
        SliceByJSON []int          `default:"[1, 2, 3]"` // Supports JSON format
        Map         map[string]int `default:"{}"`
        MapByJSON   map[string]int `default:"{\"foo\": 123}"`

        Struct    OtherStruct  `default:"{}"`
        StructPtr *OtherStruct `default:"{\"Foo\": 123}"`

        NoTag  OtherStruct               // Recurses into a nested struct even without a tag
        OptOut OtherStruct `default:"-"` // Opt-out