Re: [boost] [review] Review of Outcome v2 (Fri-19-Jan to Sun-28-Jan, 2018)

Subject: Re: [boost] [review] Review of Outcome v2 (Fri-19-Jan to Sun-28-Jan, 2018)
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2018-02-05 08:43:49

2018-02-03 23:32 GMT+01:00 Rob Stewart via Boost <boost_at_[hidden]>:

>
> That result<T,EC> is declared with [[nodiscard]] can be helpful. It means
> the return value of a function returning a result cannot be ignored.
> Unfortunately, the compiler doesn't require that the programmer do anything
> with the result except save it. (A warning might alert the programmer to do
> something with such a variable, but there's no enforcement.)

Let me expand on this a bit. I have the following program where I save the
result, but not inspect it:

```
#include "outcome.hpp"
namespace out = OUTCOME_V2_NAMESPACE;

out::result<void> fun() { return out::success(); }

int main()
{
  auto r = fun();
}
```

When I compile with clang, with -Wall -Wextra -Werror, I get an error
saying, "unused variable 'r'". See online examle:
https://wandbox.org/permlink/WE7dK6r5XCIxJ83f

When I compile with GCC, I do not get this warning. But in my work, w
compile programs with GCC, but still use clang for the static analysis
pass, and nonetheless this bug would be detected.

Of course, you could still overcome all the safety precautions and trigger
an UB:

```
#include "outcome.hpp"
namespace out = OUTCOME_V2_NAMESPACE;

out::result<void> fun() { return out::success(); }

int main()
{
  auto r = fun();
  (void)r;
}
```

But I would not expect protection from such conscious attempts of
overcoming safety measures.

> The same is not documented for outcome, however. Thus, when using outcome,
> because one might need to convey an exception to the caller, there is no
> compiler help to ensure the programmer doesn't ignore the return value.

When I test the following program, it does tell me that I am not inspecting
the returned outcome:

```
#include "outcome.hpp"
namespace out = OUTCOME_V2_NAMESPACE;

out::outcome<int> fun() { return out::success(1); }

int main()
{
  fun();
}
```

So, you probably mean something else than I think. Could you illustrate
your concerns with an example?

Regards,
&rzej;