C API in C++ with RAII, two alternatives to implement error handling (Exceptions)

Question

I have an API written in C, which produces a result by returning a pointer to allocated memory.
For using it with C++ (C++11) I've wrapped the function calls in objects, which keep the result in a std::shared_ptr. So far so good.

However, the C library features two functions for every operation. One produces possibly an error, the other never. Let's call them
some_pod * do_it_with_error(Parameter ..., Error **)
and
some_pod * do_it_without_error(Parameter ...)

I can pass in the address of an Error * pointer to the first function and if there's an error it won't be NULL afterwards.

For solving this I thought of two different implementations.
First, I could SFINAE for choosing between the with_error and without_error functions, like so:

template<typename NoThrow = void>
struct do_it {
  public:
    void operator()(...)
    {
      Error * error = NULL;
      m_result = std::shared_ptr<some_pod>(do_it_with_error(..., &error));
      if (error) {
        throw(MyExceptionWithError(error));
      }
    }
  private:
    std::shared_ptr<some_pod> m_result;
};

template<>
class do_it<std::nothrow_t> {
  public:
    void operator()(...) noexcept
    {
      if (! m_result) {
        m_result = std::shared_ptr<some_pod>(do_it_without_error(...));
      }
    }
  private:
    std::shared_ptr<some_pod> m_result;
};

Usage:

do_it<> di_error;
try {
  di_error(...); // might throw
} catch (...) {}

do_it<std::nothrow_t> di_no_error;
di_no_error(...); // won't throw for sure

However, since the result is wrapped in a std::shared_ptr there will also be a get() method:

const std::shared_ptr<some_pod> & get(void) { return m_result; }

For error checking I could just implement a another method check.
The default implementation would now look like this:

struct do_it {
  public:
    // will never throw
    const std::shared_ptr<some_pod> &
    get(void) noexcept
    {
      if (! m_result) {
        m_result = std::shared_ptr<some_pod>(do_it_without_error(...));
      }
      return m_result;
    }

    // might throw
    do_it &
    check(void)
    {
      if (! m_result) {
        Error * error = NULL;
        m_result = std::shared_ptr<some_pod>(do_it_with_error(..., &error));
        if (error) {
          throw ...;
        }
      }
      return *this;
    }

  private:
    std::shared_ptr<some_pod> m_result;
};

Usage:

do_it di_error;
try {
  auto result = di_error.check().get();
} catch (...) {}
do_it di_no_error;
di_no_error.get();

So, both implementations seem to be equally good (or bad) to me.
How could I decide which one to use.
What are the Pros and Cons?

score 1 · Accepted Answer · answered Feb 05 '14 at 11:08

Too complicated - wrap them both as if either could throw an error, that the 2nd fn will never throw only means the compiler will never have to call the error handling routines (and might even optimise it out entirely) - either way your code will perform just as well thanks to C++s design.

Your code will also be much easier to understand, even with this redundant error handling (that one day might be used for other, unexpected errors)

score 1 · Answer 2 · answered Feb 05 '14 at 12:23

In C the error objects are somewhat pain to handle, so there is the other overload that stores the errors somewhere. But in C++ exceptions make it all much easier to handle. So I would only wrap the variant that reports errors and converted the errors to exceptions and ignore that the other API even exists.

score 0 · Answer 3 · answered Feb 05 '14 at 23:34

For each operation, you have three distinct subjects to take care about, say, workload, result and error handling. Let's see them all in isolation:

Workload: shouldn't be duplicated. Always call the C function possibly producing error information, as it's the most complete of each pair.
Result: PODs can be kept as they are in the C API and hold/retrieved through shared_ptr objects, but this is only one of many possible options -if, certainly, a quite straightforward one. Maybe some of this PODs are worth to be wrapped into, or converted to, copy-constructible/move-constructible classes and directly returned from the operator() call.
Error handling: the way your C++ function objects do (or do not) communicate error-related information is seconday and orthogonal to the main workload. Ideally, you want to be able to change that error handling mechanism depending on the context, without affecting the workload itself. So view it as a dependency that you inject on the function object at your will, either at construction time or at calling time. This way, you can add additional error handling mechanisms in the future, other than current do-nothing and exception-throwing ones (what if you do end wanting to notify errors thorugh console messages, for example?).

I'll end providing a simple implementation that could serve as example:

class DoSomeStuff
{
public:
    class Result
    {
    public:
        Result() : _result() {}
        Result(const Result &rhs) : _result(rhs) {}
       ~Result() {}

        Result &operator=(const Result &rhs)
        {
            if(&rhs != this)
                _result = rhs._result;

            return *this;
        }

        int GetSomeData() const { return _result->_some_int_value; }

    private:
        shared_ptr<some_pod> _result;
    };

    struct IErrorHandler
    {
        virtual void NotifyError(Error &) = 0;
    };

    Result operator()(/*params*/, IErrorHandler &errorHandler)
    {
        Error *error = { nullptr };

        shared_ptr<some_pod_type> result = { do_some_stuff_with_error(/*params*/, &error) };

        if(error != nullptr)
            errorHandler.NotifyError(*error);

        return Result(result);
    }
};

Note that Result and IErrorHandler don't have to be included inside the function class; I've done this way because they're both dependencies, but can be extracted in order to avoid duplication among several function classes.

C API in C++ with RAII, two alternatives to implement error handling (Exceptions)

3 Answers3