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title: Field Configuration sidebar_position: 5 id: cpp_field_configuration license: | Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

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This page explains how to configure field-level metadata for serialization.

Overview

Apache Fory™ provides two ways to specify field-level metadata at compile time:

1. fory::field<> template - Inline metadata in struct definition
2. FORY_FIELD_TAGS macro - Non-invasive metadata added separately

These enable:

• Tag IDs: Assign compact numeric IDs for schema evolution
• Nullability: Mark pointer fields as nullable
• Reference Tracking: Enable reference tracking for shared pointers

The fory::field Template

template <typename T, int16_t Id, typename... Options>
class field;

Template Parameters

Parameter	Description
T	The underlying field type
Id	Field tag ID (int16_t) for compact serialization
Options	Optional tags: fory::nullable, fory::ref

Basic Usage

#include "fory/serialization/fory.h"

using namespace fory::serialization;

struct Person {
  fory::field<std::string, 0> name;
  fory::field<int32_t, 1> age;
  fory::field<std::optional<std::string>, 2> nickname;
};
FORY_STRUCT(Person, name, age, nickname);

The fory::field<> wrapper is transparent - you can use it like the underlying type:

Person person;
person.name = "Alice";           // Direct assignment
person.age = 30;
std::string n = person.name;     // Implicit conversion
int a = person.age.get();        // Explicit get()

Tag Types

fory::nullable

Marks a smart pointer field as nullable (can be nullptr):

struct Node {
  fory::field<std::string, 0> name;
  fory::field<std::shared_ptr<Node>, 1, fory::nullable> next;  // Can be nullptr
};
FORY_STRUCT(Node, name, next);

Valid for: std::shared_ptr<T>, std::unique_ptr<T>

Note: For nullable primitives or strings, use std::optional<T> instead:

// Correct: use std::optional for nullable primitives
fory::field<std::optional<int32_t>, 0> optional_value;

// Wrong: nullable is not allowed for primitives
// fory::field<int32_t, 0, fory::nullable> value;  // Compile error!

fory::not_null

Explicitly marks a pointer field as non-nullable. This is the default for smart pointers, but can be used for documentation:

fory::field<std::shared_ptr<Data>, 0, fory::not_null> data;  // Must not be nullptr

Valid for: std::shared_ptr<T>, std::unique_ptr<T>

fory::ref

Enables reference tracking for shared pointer fields. When multiple fields reference the same object, it will be serialized once and shared:

struct Graph {
  fory::field<std::string, 0> name;
  fory::field<std::shared_ptr<Graph>, 1, fory::ref> left;    // Ref tracked
  fory::field<std::shared_ptr<Graph>, 2, fory::ref> right;   // Ref tracked
};
FORY_STRUCT(Graph, name, left, right);

Valid for: std::shared_ptr<T> only (requires shared ownership)

Combining Tags

Multiple tags can be combined for shared pointers:

// Nullable + ref tracking
fory::field<std::shared_ptr<Node>, 0, fory::nullable, fory::ref> link;

Type Rules

Type	Allowed Options	Nullability
Primitives, strings	None	Use std::optional<T> if nullable
std::optional<T>	None	Inherently nullable
std::shared_ptr<T>	nullable, ref	Non-null by default
std::unique_ptr<T>	nullable	Non-null by default

Complete Example

#include "fory/serialization/fory.h"

using namespace fory::serialization;

// Define a struct with various field configurations
struct Document {
  // Required fields (non-nullable)
  fory::field<std::string, 0> title;
  fory::field<int32_t, 1> version;

  // Optional primitive using std::optional
  fory::field<std::optional<std::string>, 2> description;

  // Nullable pointer
  fory::field<std::unique_ptr<std::string>, 3, fory::nullable> metadata;

  // Reference-tracked shared pointer
  fory::field<std::shared_ptr<Document>, 4, fory::ref> parent;

  // Nullable + reference-tracked
  fory::field<std::shared_ptr<Document>, 5, fory::nullable, fory::ref> related;
};
FORY_STRUCT(Document, title, version, description, metadata, parent, related);

int main() {
  auto fory = Fory::builder().xlang(true).build();
  fory.register_struct<Document>(100);

  Document doc;
  doc.title = "My Document";
  doc.version = 1;
  doc.description = "A sample document";
  doc.metadata = nullptr;  // Allowed because nullable
  doc.parent = std::make_shared<Document>();
  doc.parent->title = "Parent Doc";
  doc.related = nullptr;  // Allowed because nullable

  auto bytes = fory.serialize(doc).value();
  auto decoded = fory.deserialize<Document>(bytes).value();
}

Compile-Time Validation

Invalid configurations are caught at compile time:

// Error: nullable and not_null are mutually exclusive
fory::field<std::shared_ptr<int>, 0, fory::nullable, fory::not_null> bad1;

// Error: nullable only valid for smart pointers
fory::field<int32_t, 0, fory::nullable> bad2;

// Error: ref only valid for shared_ptr
fory::field<std::unique_ptr<int>, 0, fory::ref> bad3;

// Error: options not allowed for std::optional (inherently nullable)
fory::field<std::optional<int>, 0, fory::nullable> bad4;

Backwards Compatibility

Existing structs without fory::field<> wrappers continue to work:

// Old style - still works
struct LegacyPerson {
  std::string name;
  int32_t age;
};
FORY_STRUCT(LegacyPerson, name, age);

// New style with field metadata
struct ModernPerson {
  fory::field<std::string, 0> name;
  fory::field<int32_t, 1> age;
};
FORY_STRUCT(ModernPerson, name, age);

FORY_FIELD_TAGS Macro

The FORY_FIELD_TAGS macro provides a non-invasive way to add field metadata without modifying struct definitions. This is useful for:

• Third-party types: Add metadata to types you don't own
• Clean structs: Keep struct definitions as pure C++
• Isolated dependencies: Confine Fory headers to serialization config files

Usage

// user_types.h - NO fory headers needed!
struct Document {
  std::string title;
  int32_t version;
  std::optional<std::string> description;
  std::shared_ptr<User> author;
  std::shared_ptr<User> reviewer;
  std::shared_ptr<Document> parent;
  std::unique_ptr<Data> data;
};

// serialization_config.cpp - fory config isolated here
#include "fory/serialization/fory.h"
#include "user_types.h"

FORY_STRUCT(Document, title, version, description, author, reviewer, parent, data)

FORY_FIELD_TAGS(Document,
  (title, 0),                      // string: non-nullable
  (version, 1),                    // int: non-nullable
  (description, 2),                // optional: inherently nullable
  (author, 3),                     // shared_ptr: non-nullable (default)
  (reviewer, 4, nullable),         // shared_ptr: nullable
  (parent, 5, ref),                // shared_ptr: non-nullable, with ref tracking
  (data, 6, nullable)              // unique_ptr: nullable
)

FORY_FIELD_TAGS Options

Field Type	Valid Combinations
Primitives, strings	(field, id) only
std::optional<T>	(field, id) only
std::shared_ptr<T>	(field, id), (field, id, nullable), (field, id, ref), (field, id, nullable, ref)
std::unique_ptr<T>	(field, id), (field, id, nullable)

API Comparison

Aspect	fory::field<> Wrapper	FORY_FIELD_TAGS Macro
Struct definition	Modified (wrapped types)	Unchanged (pure C++)
IDE support	Template noise	Excellent (clean types)
Third-party classes	Not supported	Supported
Header dependencies	Required everywhere	Isolated to config
Migration effort	High (change all fields)	Low (add one macro)

Related Topics

• Type Registration - Registering types with FORY_STRUCT
• Schema Evolution - Using tag IDs for schema evolution
• Configuration - Enabling reference tracking globally