Soia's C++ Code Generator

Official plugin for generating C++ code from .soia files.

Targets C++17 and higher.

Installation

From your project's root directory, run npm i --save-dev soia-cc-gen.

In your soia.yml file, add the following snippet under generators:

  - mod: soia-cc-gen
    config:
      writeGoogleTestHeaders: true

The npm run soiac command will now generate C++ code within the soiagen directory.

For more information, see this C++ project example.

C++ generated code guide

The examples below are for the code generated from this .soia file.

Referring to generated symbols

Every generated symbol lives in a namespace called soiagen_${path}, where ${path} is the path to the .soia file relative from the root of the soia source directory, with the ".soia" extension removed, and slashes replaced with underscores.

#include "soiagen/user.h"

using ::soiagen_user::User;
using ::soiagen_user::UserRegistry;

Constructing structs

// You can construct a struct like this:
User john;
john.user_id = 42;
john.name = "John Doe";

// Or you can use the designated initialized syntax:
User jane = {
    // Keep fields in alphabetical order
    .name = "Jane Doe",
    .pets = {{
                  .name = "Fluffy",
                  .picture = "cat",
              },
              {
                  .name = "Rex",
                  .picture = "dog",
              }},
    .subscription_status = soiagen::kPremium,
    .user_id = 43,
};

// ${StructName}::whole forces you to initialize all the fields of the struct.
// You will get a compile-time error if you miss one.
User lyla = User::whole{
    .name = "Lyla Doe",
    .pets =
        {
            User::Pet::whole{
                .height_in_meters = 0.05f,
                .name = "Tiny",
                .picture = "🐁",
            },
        },
    .quote = "This is Lyla's world, you just live in it",
    .subscription_status = soiagen::kFree,
    .user_id = 44,
};

Constructing enums

// Use soiagen::${kFieldName} for constant variants.
User::SubscriptionStatus john_status = soiagen::kFree;
User::SubscriptionStatus jane_status = soiagen::kPremium;

// Compilation error: MONDAY is not a field of the SubscriptionStatus enum.
// User::SubscriptionStatus sara_status = soiagen::kMonday;

// Use soiagen::wrap_${field_name} for data variants.
User::SubscriptionStatus jade_status =
    soiagen::wrap_trial_start_time(absl::FromUnixMillis(1743682787000));

// The ${kFieldName} and wrap_${field_name} symbols are also defined in the
// generated class.
User::SubscriptionStatus lara_status = User::SubscriptionStatus::kFree;

Conditions on enums

if (john_status == soiagen::kFree) {
  std::cout << "John, would you like to upgrade to premium?\n";
}

// Call is_${field_name}() to check if the enum holds a data variant.
if (jade_status.is_trial_start_time()) {
  // as_${field_name}() returns the value held by the enum
  const absl::Time trial_start_time = jade_status.as_trial_start_time();
  std::cout << "Jade's trial started on " << trial_start_time << "\n";
}

// One way to do an exhaustive switch on an enum.
switch (lara_status.kind()) {
  case User::SubscriptionStatus::kind_type::kConstUnknown:
    // UNKNOWN is the default value for an uninitialized SubscriptionStatus.
    // ...
    break;
  case User::SubscriptionStatus::kind_type::kConstFree:
    // ...
    break;
  case User::SubscriptionStatus::kind_type::kConstPremium:
    // ...
    break;
  case User::SubscriptionStatus::kind_type::kValTrialStartTime: {
    const absl::Time& trial_start_time = lara_status.as_trial_start_time();
    std::cout << "Lara's trial started on " << trial_start_time << "\n";
  }
}

// Another way to do an exhaustive switch using the visitor pattern.
struct Visitor {
  void operator()(soiagen::k_unknown) const {
    std::cout << "Lara's subscription status is UNKNOWN\n";
  }
  void operator()(soiagen::k_free) const {
    std::cout << "Lara's subscription status is FREE\n";
  }
  void operator()(soiagen::k_premium) const {
    std::cout << "Lara's subscription status is PREMIUM\n";
  }
  void operator()(
      User::SubscriptionStatus::wrap_trial_start_time_type& w) const {
    const absl::Time& trial_start_time = w.value;
    std::cout << "Lara's trial started on " << trial_start_time << "\n";
  }
};
lara_status.visit(Visitor());

Serialization

Use ToDenseJson, ToReadableJson or ToBytes to serialize a soia value.

// Serialize a soia value to JSON with ToDenseJson or ToReadableJson.
std::cout << soia::ToDenseJson(john) << "\n";
// [42,"John Doe"]

std::cout << soia::ToReadableJson(john) << "\n";
// {
//   "user_id": 42,
//   "name": "John Doe"
// }

// The dense flavor is the flavor you should pick if you intend to
// deserialize the value in the future. Soia allows fields to be renamed, and
// because fields names are not part of the dense JSON, renaming a field does
// not prevent you from deserializing the value.
// You should pick the readable flavor mostly for debugging purposes.

// The binary format is not human readable, but it is a bit more compact than
// JSON, and serialization/deserialization can be a bit faster.
// Only use it when this small performance gain is likely to matter, which
// should be rare.
std::cout << soia::ToBytes(john).as_string() << "\n";
// soia�+Jane Doe����Fluffy�cat��Rex�dog

Deserialization

Use Parse to deserialize a soia value from JSON or binary format.

absl::StatusOr<User> maybe_john = soia::Parse<User>(soia::ToDenseJson(john));
assert(maybe_john.ok() && *maybe_john == john);

Keyed arrays

A keyed_items<T, get_key> is a container that stores items of type T and allows for fast lookups by key using a hash table.

UserRegistry user_registry;
soia::keyed_items<User, soiagen::get_user_id<>>& users = user_registry.users;
users.push_back(john);
users.push_back(jane);
users.push_back(lyla);

assert(users.size() == 3);
assert(users[0] == john);

const User* maybe_jane = users.find_or_null(43);
assert(maybe_jane != nullptr && *maybe_jane == jane);

assert(users.find_or_default(44).name == "Lyla Doe");
assert(users.find_or_default(45).name == "");

Equality and hashing

Soia structs and enums are equality comparable and hashable.

absl::flat_hash_set<User> user_set;
user_set.insert(john);
user_set.insert(jane);
user_set.insert(jane);
user_set.insert(lyla);

assert(user_set.size() == 3);

Constants

const User& tarzan = soiagen_user::k_tarzan();
assert(tarzan.name == "Tarzan");

Soia services

Starting a soia service on an HTTP server

Full example here.

Sending RPCs to a soia service

Full example here.

Dynamic reflection

using ::soia::reflection::GetTypeDescriptor;
using ::soia::reflection::TypeDescriptor;

// A TypeDescriptor describes a soia type. It contains the definition of all
// the structs and enums referenced from the type.
const TypeDescriptor& user_descriptor = GetTypeDescriptor<User>();

// TypeDescriptor can be serialized/deserialized to/from JSON.

absl::StatusOr<TypeDescriptor> reserialized_type_descriptor =
    TypeDescriptor::FromJson(user_descriptor.AsJson());
assert(reserialized_type_descriptor.ok());

Static reflection

Static reflection allows you to inspect and modify values of generated soia types in a typesafe maneer.

See string_capitalizer.h.

User tarzan_copy = soiagen_user::k_tarzan();
// CapitalizeStrings recursively capitalizes all the strings found within a
// soia value.
CapitalizeStrings(tarzan_copy);

std::cout << tarzan_copy << "\n";
// {
//   .user_id: 123,
//   .name: "TARZAN",
//   .quote: "AAAAAAAAAAYAAAAAAAAAAYAAAAAAAAAA",
//   .pets: {
//     {
//       .name: "CHEETA",
//       .height_in_meters: 1.67,
//       .picture: "🐒",
//     },
//   },
//   .subscription_status: ::soiagen::wrap_trial_start_time(absl::FromUnixMillis(1743592409000 /* 2025-04-02T11:13:29+00:00 */)),
// }

// ...

Writing unit tests with GoogleTest

Full example here.

Struct matchers

const User john = {
    .name = "John Doe",
    .pets =
        {
            {.height_in_meters = 1.67, .name = "Cheeta", .picture = "🐒"},
        },
    .quote = "Life is like a box of chocolates.",
    .user_id = 42,
};

EXPECT_THAT(john, (StructIs<User>{
                      // Only the specified fields are tested
                      .pets = testing::ElementsAre(StructIs<User::Pet>{
                          .height_in_meters = testing::FloatNear(1.7, 0.1),
                      }),
                      .quote = testing::StartsWith("Life is"),
                      .user_id = 42,
                  }));

Enum matchers

User::SubscriptionStatus john_status = soiagen::kFree;

EXPECT_THAT(john_status, testing::Eq(soiagen::kFree));

User::SubscriptionStatus jade_status =
    soiagen::wrap_trial_start_time(absl::FromUnixMillis(1743682787000));

EXPECT_THAT(jade_status, IsTrialStartTime());
EXPECT_THAT(jade_status, IsTrialStartTime(testing::Gt(absl::UnixEpoch())));