pw_result#

pw::Result<T> is a class template for use in returning either a pw::Status error or an object of type T.

pw::Result<T>’s implementation is closely based on Abseil’s StatusOr<T> class. There are a few differences:

  • pw::Result<T> uses pw::Status, which is much less sophisticated than absl::Status.

  • pw::Result<T>’s functions are constexpr and pw::Result<T> may be used in constexpr statements if T is trivially destructible.

Usage#

Usage of pw::Result<T> is identical to Abseil’s absl::StatusOr<T>. See Abseil’s documentation and usage tips for guidance.

pw::Result<T> is returned from a function that may return pw::OkStatus() and a value or an error status and no value. If ok() is true, the pw::Result<T> contains a valid T. Otherwise, it does not contain a T and attempting to access the value is an error.

pw::Result<T> can be used to directly access the contained type:

#include "pw_result/result.h"

if (pw::Result<Foo> foo = TryCreateFoo(); foo.ok()) {
  foo->DoBar();
}

pw::Result is compatible with PW_TRY and PW_TRY_ASSIGN, for example:

#include "pw_status/try.h"
#include "pw_result/result.h"

pw::Result<int> GetAnswer();  // Example function.

pw::Status UseAnswer() {
  const pw::Result<int> answer = GetAnswer();
  if (!answer.ok()) {
    return answer.status();
  }
  if (answer.value() == 42) {
    WhatWasTheUltimateQuestion();
  }
  return pw::OkStatus();
}

pw::Status UseAnswerWithTry() {
  const pw::Result<int> answer = GetAnswer();
  PW_TRY(answer.status());
  if (answer.value() == 42) {
    WhatWasTheUltimateQuestion();
  }
  return pw::OkStatus();
}

pw::Status UseAnswerWithTryAssign() {
  PW_TRY_ASSIGN(const int answer, GetAnswer());
  if (answer == 42) {
    WhatWasTheUltimateQuestion();
  }
  return pw::OkStatus();
}

Warning

Be careful not to use larger types by value as this can quickly consume unnecessary stack.

Warning

This module is experimental. Its impact on code size and stack usage has not yet been profiled. Use at your own risk.

Monadic Operations#

pw::Result<T> also supports monadic operations, similar to the additions made to std::optional<T> in C++23. These operations allow functions to be applied to a pw::Result<T> that would perform additional computation.

These operations do not incur any additional FLASH or RAM cost compared to a traditional if/else ladder, as can be seen in the Size report.

// Without monads
pw::Result<Image> GetCuteCat(const Image& img) {
   pw::Result<Image> cropped = CropToCat(img);
   if (!cropped.ok()) {
     return cropped.status();
   }
   pw::Result<Image> with_tie = AddBowTie(*cropped);
   if (!with_tie.ok()) {
     return with_tie.status();
   }
   pw::Result<Image> with_sparkles = MakeEyesSparkle(*with_tie);
   if (!with_sparkles.ok()) {
     return with_parkes.status();
   }
   return AddRainbow(MakeSmaller(*with_sparkles));
}

// With monads
pw::Result<Image> GetCuteCat(const Image& img) {
  return CropToCat(img)
         .and_then(AddBoeTie)
         .and_then(MakeEyesSparkle)
         .transform(MakeSmaller)
         .transform(AddRainbow);
}

pw::Result<T>::and_then#

The pw::Result<T>::and_then member function will return the result of the invocation of the provided function on the contained value if it exists. Otherwise, returns the contained status in a pw::Result<U>, which is the return type of provided function.

// Expositional prototype of and_then:
template <typename T>
class Result {
  template <typename U>
  Result<U> and_then(Function<Result<U>(T)> func);
};

Result<Foo> CreateFoo();
Result<Bar> CreateBarFromFoo(const Foo& foo);

Result<Bar> bar = CreateFoo().and_then(CreateBarFromFoo);

pw::Result<T>::or_else#

The pw::Result<T>::or_else member function will return *this if it contains a value. Otherwise, it will return the result of the provided function. The function must return a type convertible to a pw::Result<T> or void. This is particularly useful for handling errors.

// Expositional prototype of or_else:
template <typename T>
class Result {
  template <typename U>
    requires std::is_convertible_v<U, Result<T>>
  Result<T> or_else(Function<U(Status)> func);

  Result<T> or_else(Function<void(Status)> func);
};

// Without or_else:
Result<Image> GetCuteCat(const Image& image) {
  Result<Image> cropped = CropToCat(image);
  if (!cropped.ok()) {
    PW_LOG_ERROR("Failed to crop cat: %d", cropped.status().code());
    return cropped.status();
  }
  return cropped;
}

// With or_else:
Result<Image> GetCuteCat(const Image& image) {
  return CropToCat(image).or_else(
      [](Status s) { PW_LOG_ERROR("Failed to crop cat: %d", s.code()); });
}

Another useful scenario for pw::Result<T>::or_else is providing a default value that is expensive to compute. Typically, default values are provided by using pw::Result<T>::value_or, but that requires the default value to be constructed regardless of whether we actually need it.

// With value_or:
Image GetCuteCat(const Image& image) {
  // GenerateCuteCat() must execute regardless of the success of CropToCat
  return CropToCat(image).value_or(GenerateCuteCat());
}

// With or_else:
Image GetCuteCat(const Image& image) {
  // GenerateCuteCat() only executes if CropToCat fails.
  return *CropToCat(image).or_else([](Status) { return GenerateCuteCat(); });
}

pw::Result<T>::transform#

The pw::Result<T>::transform member method will return a pw::Result<U> which contains the result of the invocation of the given function if *this contains a value. Otherwise, it returns a pw::Result<U> with the same pw::Status value as *this.

The monadic methods for and_then and transform are fairly similar. The primary difference is that and_then requires the provided function to return a pw::Result, whereas transform functions can return any type. Users should be aware that if they provide a function that returns a pw::Result to transform, this will return a pw::Result<pw::Result<U>>.

// Expositional prototype of transform:
template <typename T>
class Result {
  template <typename U>
  Result<U> transform(Function<U(T)> func);
};

Result<int> ConvertStringToInteger(std::string_view);
int MultiplyByTwo(int x);

Result<int> x = ConvertStringToInteger("42")
                  .transform(MultiplyByTwo);

Size report#

The table below showcases the difference in size between functions returning a Status with an output pointer, and functions returning a Result, in various situations.

Note that these are simplified examples which do not necessarily reflect the usage of Result in real code. Make sure to always run your own size reports to check if Result is suitable for you.

Label

Segment

Before

Delta

After

Simple function

(all)
(same)
0
(same)

Simple function without inlining

(all)
(same)
0
(same)

Returning a larger object (std::span)

(all)
(same)
0
(same)

Using and_then instead of if ladder

(all)
(same)
0
(same)

Using or_else instead of if ladder

(all)
(same)
0
(same)

Using transform instead of if ladder

(all)
(same)
0
(same)

Zephyr#

To enable pw_result for Zephyr add CONFIG_PIGWEED_RESULT=y to the project’s configuration.