Inspecting result

Suppose we will be writing a function print_half that takes a std::string representing an integer and prints half the integer:

outcome::result<void> print_half(const std::string& text);

The type result<void> means that there is no value to be returned upon success, but that the operation might still fail, and we may be interested in inspecting the cause of the failure. The class template result<> is declared with the attribute [[nodiscard]], which means the compiler will warn you if you forget to inspect the returned object (in C++ 17 or later).

The implementation will do the following: if the integral number can be represented by an int, we will convert to int and use its arithmetical operations. If the number is too large, we will fall back to using a custom BigInt implementation that needs to allocate memory. In the implementation we will use the function convert defined in the previous section.

outcome::result<void> print_half(const std::string& text)
{
  if (outcome::result<int> r = convert(text))     // #1
  {
    std::cout << (r.value() / 2) << std::endl;    // #2
  }
  else
  {
    if (r.error() == ConversionErrc::TooLong)     // #3
    {
      BOOST_OUTCOME_TRY (i, BigInt::fromString(text));  // #4
      std::cout << i.half() << std::endl;
    }
    else
    {
      return r.as_failure();                      // #5
    }
  }
  return outcome::success();                      // #6
}

#1. You test if result<> object represents a successful operation with contextual conversion to bool.

#2. The function .value() extracts the successfully returned int.

#3. The function .error() allows you to inspect the error sub-object, representing information about the reason for failure.

#4. Macro BOOST_OUTCOME_TRY represents a control statement. It implies that the expression in the second argument returns a result<>. The function is defined as:

/*static*/ outcome::result<BigInt> BigInt::fromString(const std::string& s)

Our control statement means: if fromString returned failure, this same error information should be returned from print_half, even though the type of result<> is different. If fromString returned success, we create variable i of type BigInt with the value returned from fromString. If control goes to subsequent line, it means fromString succeeded and variable of type BigInt is in scope.

#5. In the return statement we extract the error information and use it to initialize the return value from print_half. We could have written return r.error(); instead, and it would have the same effect, but r.as_failure() will work when implicit construction from E has been disabled due to T and E having a constructibility relationship.

#6. Function success() returns an object of type success<void> representing success. This is implicitly converted by all result and outcome types into a successful return, default constructing any T if necessary.