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zircon/system/ulib/zxc/README.md

Zircon Common Library (ZXC)

Introduction

ZXC is a library of C++ vocabulary types, providing fundamental C++ primitives that promote ergonomics, safety, and consistency.

Vocabulary Types

ZXC provides the following vocabulary types:

  • fitx::result: An efficient value-or-error result type.
  • zx::status: A specialization of fitx::result for zx_status_t errors.

fitx::result<E, Ts...>

fitx::result is an efficient implementation of the general value-or-error result type pattern. The type supports returning either an error, or zero or one values from a function or method.

This type is designed to address the following goals:

  • Improve error handling and propagation ergonomics.
  • Avoid the safety hazards and inefficiency of out parameters.
  • Support effective software composition patterns.

Basic Usage

fitx::result<E, T?> may be used as the return type of a function or method. The first template parameter E is the type to represent the error value. The optional template parameter T is zero or one types to represent the value to return on success. The value type may be empty, however, an error type is required.

#include <lib/fitx/result.h>

// Define an error type and set of distinct error values. A success value is not
// necessary as the error and value spaces of fitx::result are separate.
enum class Error {
  InvalidArgs,
  BufferNotAvailable,
  RequestTooLarge,
};

// Returns a pointer the buffer and its size. Returns Error::BufferNotAvailable
// when the buffer is not available.
struct BufferResult {
  uint8_t* const buffer;
  const size_t buffer_size;
};
fitx::result<Error, BufferResult> GetBuffer();

fitx::result<Error> FillBuffer(uint8_t* data, size_t size) {
  if (data == nullptr || size == 0) {
    return fitx::as_error(Error::InvalidArgs);
  }
  
  auto result = GetBuffer()
  if (result.is_ok()) {
    auto [buffer, buffer_size] = result.value();
    if (size > buffer_size) {
      return fitx::as_error(Error::RequestTooLarge);
    }
    std::memcpy(buffer, data, size);
    return fitx::ok();
  } else {
    return result.take_error();
  }
}

Returning Values

fitx::result emphasizes ease of use when returning values or otherwise signaling success. The result type provides a number of constructors, most of which are implicit, to make returning values simple and terse.

The following constructors are supported. Error and value constructors are listed for completeness.

Copy/Move Construction and Assignment

The result type generally has the same trivial/non-trivial and copy/move constructibility as the least common denominator of the error type E and the value type T, if any. That is, it is only trivially constructible when all of the supplied types are trivially constructible and it is only copy constructible when all of the supplied types are copy constructible.

The result type is only copy/move assignable when the types E and T are trivially copy/move assignable, otherwise the result is not assignable.

fitx::success

fitx::result<E, T?> is implicitly constructible from fitx::success<U?>, when T is constructible from U or both are emtpy.

fitx::ok(U?) is a utility function that deduces U from the given argument, if any.

fitx::success is not permitted as the error type of fitx::result.

fitx::result<Error> CheckBounds(size_t size) {
  if (size > kSizeLimit) {
    return fitx::as_error(Error::TooBig);
  }
  return fitx::ok();
}
fitx::failed

The special sentinel type fitx::failed may be used as the error type when an elaborated (enumerated) error is not necessary. When fitx::failed is used as the error type, the result type is implicitly constructible from fitx::failed.

fitx::failed is not permitted as a value type of fitx::result.

fitx::result<fitx::failed> CheckBounds(size_t size) {
  if (size > kSizeLimit) {
    return fitx::failed();
  }
  return fitx::ok();
}
fitx::error

The result type is implicitly constructible from any instance of error space of the result, regardless of which types are used.

fitx::result<std::string, size_t> StringLength(const char* string) {
  if (string == nullptr) {
    // Uses the deduction guide to deduce fitx::error<const char*>. The
    // fitx::result error constructor is permitted because std::string is
    // constructible from const char*.
    return fitx::error("String may not be nullptr!");
  }
  return fitx::ok(strlen(string));
}
fitx::result<F, U?> with Compatible Error and Value

The result fitx::result<E, T?> type is implicitly constructible from any other fitx::result<F, U?>, where the error type E is constructible from the error type F and T is constructible from U, if any.

fitx::result<const char*, const char*> GetMessageString();

fitx::result<std::string, std::string> GetMessage() {
  return GetMessageString();
}

Discriminating Errors from Values

fitx::result has two predicate methods, is_ok() and is_error(), that determine whether a result represents success and contains zero or one values, or represents an error and contains an error value, respectively.

fitx::result<const char*, size_t> GetSize();

void Example() {
  auto result = GetSize();
  if (result.is_ok()) {
    printf("size=%zu\n", result.value());
  }
  if (result.is_error()) {
    printf("error=%s\n", result.error_value());
  }
}

Accessing Values

fitx::result supports several methods to access the value from a successful result.

fitx:result::value() Accessor Methods

The value of a successful result may be accessed using the value() methods of fitx::result.

fitx::result<Error, A> GetValues();

void Example() {
  auto result = GetValues();
  if (result.is_ok()) {
    A a = result.value();
  }
}
fitx::result::take_value() Accessor Method

The value of a successful result may be propagated to another result using the take_value() method of fitx::result.

fitx::result<Error, A> GetValues();

fitx::result<Error, A> Example() {
  auto result = GetValues();
  if (result.is_ok()) {
    return result.take_value();
  } else {
    ConsumeError(result.take_error());
    return fitx::ok();
  }
}
fitx::result::operator->() Accessor Method

The members of the underlying value of a successful result may be accessed using the operator->() overloads of fitx::result.

struct FooBarResult {
  Foo foo;
  Bar bar;
};
fitx::result<Error, FooBarResult> GetFooBar();

void Example() {
  auto result = GetFooBar();
  if (result.is_ok()) {
    ConsumeFoo(std::move(result->foo));
    ConsumeBar(std::move(result->bar));
  }
}

fitx::result forwards to the underlying value's operator->() overload when one is defined.

struct FooBarResult {
  Foo foo;
  Bar bar;
};
fitx::result<Error, std::unique_ptr<FooBarResult>> GetFooBar();

void Example() {
  auto result = GetFooBar();
  if (result.is_ok()) {
    ConsumeFoo(std::move(result->foo));
    ConsumeBar(std::move(result->bar));
  }
}

Returning Errors

Returning errors with fitx::result<E, T?> always involves wrapping the error value in an instance of fitx::error<F>, where E is constructible from F. This ensures that error values are never ambiguous, even when E and T are compatible types.

There are a variety ways to return errors:

Direct fitx::error

The most direct way to return an error is to use fitx::error directly.

fitx::error has a single argument deduction guide fitx::error(T) -> fitx::error<T> when compiling for C++17 and above to simplify basic error return values.

fitx::result<std::string, size_t> StringLength(const char* string) {
  if (string == nullptr) {
    return fitx::error<std::string>("String is nullptr!");
  }
  return fitx::ok(strlen(string));
}

fitx::result<std::string, size_t> StringLength(const char* string) {
  if (string == nullptr) {
    return fitx::error("String is nullptr!");
  }
  return fitx::ok(strlen(string));
}

// Error with multiple values.
using Error = std::pair<std::string, Backtrace>;

fitx::result<Error, size_t> StringLength(const char* string) {
  if (string == nullptr) {
    return fitx::error<Error>("String is nullptr!", Backtrace::Get());
  }
  return fitx::ok(strlen(nullptr));
}
fitx::as_error Utility Function

The single-argument utility function fitx::as_error may be used to simplify returning a fitx::error<F> by deducting F from the argument type.

This function is a C++14 compatible alternative to the deduction guide.

fitx::result<std::string, size_t> StringLength(const char* string) {
  if (string == nullptr) {
    return fitx::as_error("String is nullptr!"); // Deduces fitx::error<const char*>.
  }
  return fitx::ok(strlen(string));
}

Handling Errors

fitx::result supports ergonomic error handling and propagation.

error_value() and take_error() Access Methods

The error value of a result may be accessed by reference using the error_value() methods.

The error may be propagated using the take_error() method, which returns the error value as an instance of fitx::error<E>, as required to pass the error to fitx::result.

fitx::result<const char*> Example() {
  if (auto result = GetValues()) {
    // Use values ...
    return fitx::ok();
  } else {
    LOG_ERROR("Failed to get values: %s\n", result.error_value());
    return result.take_error();
  }
}

Relational Operators

fitx::result supports a variety of relational operator variants.

fitx::success and fitx::failure

fitx::result is always equal/not equal comparable to instances of fitx::success<> and fitx::failed.

Comparing to fitx::success<> is equivalent to comparing to is_ok(). Comparing to fitx::failed is equivalent to comparing to is_error().

fitx::result<Error, A> GetValues();

fitx::result<Error> Example() {
  auto result = GetValues();
  if (result == fitx::ok()) {
    return fitx::ok();
  }
  return result.take_error();
}
Any fitx::result with Compatible Value Types

fitx::result<E, T?> and fitx::result<F, U?> are comparable when T is comparable to U, if any. The error types are not compared, only the is_ok() predicate and values are compared.

Comparing two result types has the same empty and lexicographic ordering as comparing std::optional<T>.

fitx::result<Error, int> GetMin();
fitx::result<Error, int> GetMax();

bool TestEqual() {
  // Returns true when both results have values and the values are the same.
  return GetMin() == GetMax();
}
Any Type U Comparable to T

When fitx::result<E, T> has a single value type T, the result is comparable to any type U that is comparable to T.

fitx::result<Error, std::string> GetMessage();

bool TestMessage() {
  // Returns true if there is a message and it matches the string literal.
  return GetMessage() == "Expected message";
}

zx::status<T?>

zx::status<T?> is a specialization for Zircon zx_status_t errors, based on fitx::result<zx_status_t, T?>, to make inter-op safer and more natural.

The namespace zx has aliases of the support types and functions in fitx:

  • zx::ok
  • zx::error
  • zx::failed
  • zx::success
  • zx::as_error

Returning and Using Values

Returning values with zx::status is the same as returning values with the base fitx::result. The status type supports the same value constructors, conversions, and accessors as the base result type.

Returning Errors

zx::status enforces the convention that errors are distinct from values by disallowing the value ZX_OK as an error value with a runtime assertion.

Instead of using ZX_OK to signal success, simply return a value or values. When the value set is empty, return zx::ok() to signal success, just as with the base result type.

zx::error_status Utility

zx::error_status is a simple alias of zx::error<zx_status_t> for convenience.

Handling Errors

Handling errors with zx::status is the same as handling errors with fitx::result. All of the same constructs and accessors are available.

The status_value() accessor returns ZX_OK when the status is in the value state. It is still invalid to call error_value() or take_error() in the value state.

The status_string() accessor returns a string constant representing the value returned by status_value().

zx::status<> CheckValues(const foo&);
zx::status<foo> GetValues();

// Simple pass through.
zx_status_t check_foo_and_bar(const foo& foo_in) {
  // Returns a consistent value, regardless of error/value state.
  return CheckValues(foo_in).status_value();
}

// Normal error handling with control flow scoping.
zx_status_t get_foo_and_bar(foo* foo_out) {
  if (foo_out == nullptr) {
    return ZX_ERR_INVALID_ARGS;
  }

  auto result = GetValues();
  if (result.is_ok()) {
    *foo_out = result.value();
    return ZX_OK; // Could return result.status_value() but this is more explicit.
  } else {
    LOG(ERROR, "Call to GetValues failed: %s\n", result.status_string());
    return result.error_value();
  }
}

Propagating Errors with zx::make_status(zx_status_t)

zx::make_status is a utility function to simplify forwarding zx_status_t values through functions returning zx::status<> with an empty value set.

This is primarily to simplify interfacing with FFIs.

zx_status_t check_foo_and_bar(const foo&, const &bar);

// Without using zx::forward_status.
zx::status<> CheckValues(const foo& foo_in, const bar& bar_in) {
  const zx_status_t status = check_foo_and_bar(foo_in, bar_in);
  if (status == ZX_OK) {
    return zx::ok();
  } else {
    return zx::error_status(status);
  }
}

// With using zx::forward_status.
zx::status<> CheckValues(const foo& foo_in, const bar& bar_in) {
  return zx::make_status(check_foo_and_bar(foo_in, bar_in));
}

Guidelines

Use the following guidelines to make the most of the result type's ergonomic and safety features.

Return Multiple Values Using Aggregates

Define an aggregate structure to return multiple values. Use meaningful names for each aggregate member to improve readability.

struct CreateFooBarResult {
  Foo foo;
  Bar bar;
};
fitx::<Error, CreateFooBarResult> CreateFooBar(Baz baz);

Use Named Constructors for Complex Initialization

For types that require complex initialization that could fail, use a static method (i.e. a named constructor) to perform the initialization. Make the constructor private and only perform member initialization using values passed in from the named constructor.

class Foo {
 public:
  fitx::result<Error, Foo> Create(size_t size) {
    auto buffer_result = AllocateBuffer(size);
    if (buffer_result.is_error()) {
      return buffer_result.take_error();
    }
    auto bar_result = Bar::Create(size);
    if (bar_result.is_error()) {
      return bar_result.take_error());
    }
    return fitx::ok(Foo{std::move(buffer_result.value()), size, std::move(bar_result.value())});
  }

 private:
  Foo(std::unique_ptr<uint8_t[]> buffer, size_t size, Bar bar)
    : buffer_{std::move(buffer)}, size_{size}, bar_{std::move(bar)} {}
  
  std::unique_ptr<uint8_t[]> buffer_;
  size_t size_;
  Bar bar_;
};

Prefer Result Types to Output Parameters

Output parameters are often used when an operation might fail. Typically, the return value is used to indicate success or (possibly enumerated) failure, while other values are returned using the output parameters.

Output parameters introduce ambiguities that should be avoided:

  • Are parameters pure outputs or mutable inputs?
  • Is nullptr permitted or will it cause a CHECK-fail?
  • What are the pre-conditions of the output states?
  • What happens to the pre-existing states of the outputs on success?
  • What states are the outputs left in on failure?
  • What are the lifetime requirements of the output variables?

These ambiguities are often the source of subtle bugs. The result pattern avoids ambiguity by construction: returning a value or an error is mutally exclusive.

Consider the following example using output parameters. It is difficult to infer the answers to the questions above without referring to documentation. Even with documentation, there is non-trivial cognitive load to check that the implementation is correct, that callers follow the rules, and that the documentation is consistent.

enum class Status {
  Ok,
  InvalidArgs,
  NoMemory,
  Clamped,
};

Status AllocateBuffer(size_t size, std::unique_ptr<uint8_t[]>* buffer_out, size_t* size_out) {
  if (size == 0) {
    buffer_out->reset(nullptr);
    size_out = 0; // Forgot to dereference size_out!
  }
  if (buffer_out == nullptr) {
    return Status::InvalidArgs;
  }

  // What about size_out == nullptr?

  if (size > kMaxSize) {
    size = kMaxSize;
  }

  fbl::AllocChecker checker;
  std::unique_ptr<uint8_t[]> buffer{new (&checker) uint8_t[size]};
  if (!checker.check()) {
    *size_out = 0; // What state should buffer_out be left in?
    return Status::NoMemory;
  }

  *buffer_out = std::move(buffer);
  *size_out = size;

  // Was size really clamped or just concidently the max?
  return size == kMaxSize ? Status::Clamped : Status::Ok;
}

Compare the previous example with the following example using the result type. The Status type only enumerates the two possible reasons for failure, there is no need to enumerate success states. The output states are well-defined and there is very little cognitive load to validate the implementation and callers.

enum class Status {
  InvalidArgs,
  NoMemory,
};

struct AllocateResult {
  std::unique_ptr<uint8_t[]> buffer;
  size_t size;
};

fitx::result<Status, AllocateResult> AllocateBuffer(size_t size) {
  if (size == 0) {
    return fitx::error(Status::InvalidArgs);
  }
  if (size > kMaxSize) {
    size = kMaxSize;
  }

  fbl::AllocChecker checker;
  std::unique_ptr<uint8_t[]> buffer{new (&checker) uint8_t[size]};
  if (!checker.check()) {
    return fitx::error(Status::NoMemory);
  }

  return fitx::ok(AllocateResult{std::move(buffer), size});
}