Example is bad

[qknight]

I try to understand how to add proper error handling by looking at: https://github.com/ned14/status-code/blob/b70d070c02fe74f85d1a05f91b1d8c1dbf7168b0/example/file_io_error.cpp#L172-L190

But that example only handles the error case, can you extend the code so it also does something useful?

Expected behavior: the open_resource should return a file handle so I can read/write something. Instead it has a LINE and FILE, what is that used for?

[ned14]

You're right that that example isn't much use. It was intended for a WG21 audience only.

I'll delete it shortly, is the example in the Readme.md okay for you?

[qknight]

For some time I was under the impression that the error would contain the actual return value of the function but could not see how this would be implemented. I guess this was never the plan, right? The concept is always what is shown in the Readme.md:

system_error2::system_code sc;  // default constructs to empty
native_handle_type h = open_file(path, sc);
// Is the code a failure?
if(sc.failure())
{
  // Do semantic comparison to test if this was a file not found failure
  // This will match any system-specific error codes meaning a file not found
  if(sc != system_error2::errc::no_such_file_or_directory)
  {
    std::cerr << "FATAL: " << sc.message().c_str() << std::endl;
    std::terminate();
  }
}

Where the sc is passed in as an additional value and processed before using the return value.

[ned14]

The WG21 example was for exposition, showing how lightweight exception throws of std::error would work.

You're right that for current C++, either you pass in a lvalue ref as a parameter, or use Experimental.Outcome to return an outcome::experimental::result<T> whose alternate E value is an error. So the Result would be T if successful, E if an error.

[qknight]

Thanks for your insights. I guess that using outcome::experimental::result<T> is not straight forward so we bail and use the former method you mentioned.

[ned14]

Thanks for your insights. I guess that using outcome::experimental::result is not straight forward so we bail and use the former method you mentioned.

Would you be able to clarify how result<T> is not straightforward? It would be useful to learn what the obstacles would be.

I'll reopen this if it's okay, to remind me to go delete that bad example file.

[qknight]

I tried to put together a minimal example and failed since I don't understand your outcome implementation, especially the experimental part of it. Also I failed to get the template magic together.

Here is some pseudo-code I want to illustrate the concept I was trying:

// probably include the generated header from here i suppose...
// https://github.com/ned14/outcome
#include "outcome.hpp"

outcome::experimental::result<T> moveMountain(bool input) {
  if (input == true) {
    return "worked";
  } else {
   // system_error2::system_code sc;  // default constructs to empty
   return system_error2::errc::no_such_file_or_directory
  };
};

outcome::experimental::result<T> ret = moveMountain(true);
... check returned type and print the result ...
... probably with logic from here: https://stackoverflow.com/questions/13636540/how-to-check-for-the-type-of-a-template-parameter ...
if (ret is of type E)
  std::cerr << "error happened" << ret..message().c_str() << std::endl;
else
  std::cout << ret;

outcome::experimental::result<T> ret = moveMountain(false);
if (ret is of type E)
  std::cerr << "error happened" << ret..message().c_str() << std::endl;
else
  std::cout << ret;

Can we create a minimal example from that? Thanks for your help, btw!

[qknight]

It would also help if the two versions of the documentation, both links below, would actually server the same content:

• https://ned14.github.io/status-code/index.html
• https://github.com/ned14/status-code#quick-synthesis-of-a-custom-status-code-domain-for-any-arbitrary-enumeration-type

And if the github pages webpage (second link) could container a link to the source code (first link).

[ned14]

How about this https://godbolt.org/z/Tbr5h7:

#include "outcome-experimental.hpp"

#include <iostream>
#include <string>

namespace outcome_e = OUTCOME_V2_NAMESPACE::experimental;

template<class T> using result = outcome_e::status_result<T>;

result<std::string> moveMountain(bool input) {
  if (input == true) {
    return "worked";
  } else {
   // system_error2::system_code sc;  // default constructs to empty
   return outcome_e::errc::no_such_file_or_directory;
  };
};

void test1()
{
    if(auto r = moveMountain(true))
    {
        std::cout << r.value() << std::endl;;
    }
    else
    {
        // BUG: message() is supposed to print without c_str()!
        std::cerr << "error happened " << r.error().message().c_str() << std::endl;
    }
}

NOTE TO SELF: message() isn't serialising to ostream, and I could have sworn I fixed that six months ago!

Re: those two pages getting out of sync, apologies, the website is manually regenerated using tools on my laptop which I keep forgetting to do. I'll leave this issue open to remind me.

[qknight]

Wow, that works! Thanks!

Can you please add this as a comment to the source code as well:

// Use https://github.com/ned14/outcome#usage-as-a-single-header-file and go to the download page, there is the file outcome-experimental.hpp also.
#include "outcome-experimental.hpp"

[ned14]

Ok, can you check what I just pushed in that commit there to see if it addresses all your issues?

The only remaining issue is the docs which need to be regenerated. For that I need my personal laptop, so I'll keep this issue open.

[ned14]

Regened the docs, so closing.