errors

layout: true
<div class="footer"><div class="flexcontainer"><span>.body[[https://kirkshoop.github.io/norawthread/errors.html](https://kirkshoop.github.io/norawthread/errors.html)]</span><span>.center[C++Now 2017]</span><span>.body[© 2017 Kirk Shoop ([github](http://github.com/kirkshoop) [twitter](http://twitter.com/kirkshoop))]</span></div></div>

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class: middle

.center[

# errors

.accent[
### forgotten, but not gone]

]

???

> *

---

# some apis that return errors

```
std::error_code okApiCall(){
    return std::error_code{};
}
std::error_code failApiCall(){
    return std::errc::io_error;
}
```

???

> *

---

# two wrongs don't make a right

```
auto e = failApiCall();
e = failApiCall();
```

???

> * errors are ignored by default
> * exceptions are not ingored by default

---

# two rights are still wrong

```
auto e = okApiCall();
e = okApiCall();
```

???

> *

---

# Is it a runtime error?

.center[
.image-60[
![](content/semiavoid.gif)]]

???

> * unexpected events happen in systems

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# Is it a code bug?

.center[
.image-60[
![](content/traincrossing.gif)]]

???

> * flaws in systems should result in a stop and a report
> * this is intended to ensure that code bugs do not ship

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class: middle

.center[
.accent[
# what should happen if an error is forgotten?]]

???

> *

---
class: middle

.center[
.accent[
# how would a forgotten error be detected?]]

???

> *

---

# errors need to get a life .. time

```
// begin scope
auto e = failApiCall();
// end scope `boom`
// begin scope
e = failApiCall();
// end scope `boom`
```

???

> *

---

# go BOOM

```
unique_error_code ge;
// default is safe

ge.reset(okApiCall());
// unssafe after calling api

// `boom`
ge.reset(okApiCall());
```

???

> *

---

# go BOOM

```
unique_error_code ge;
// default is safe

ge.reset(okApiCall());
// unssafe after calling api

// `boom`
```

???

> *

---

# no BOOM

```
unique_error_code ge;
// default is safe

ge.reset(okApiCall());
// unssafe after calling api

if (!ge.ok()) {
    return ge.release();
}
// safe once it is checked
```

```
ge.reset(okApiCall());
// unssafe after calling api

return ge.release();
// safe once it is released
```

???

> *

---

# example implementation for a Win32 winerror type

```
enum class winerror_error : DWORD {};

inline winerror_error winerror_cast(DWORD raw) {
	return static_cast<winerror_error>(raw);
}

template<>
struct error_traits<winerror_error>
{
	using value_type = winerror_error;

static inline winerror_error initiate() {
		return winerror_cast(ERROR_SUCCESS);
	}

static inline bool ok(winerror_error winerror) {
		return winerror == winerror_cast(ERROR_SUCCESS);
	}
};

```

---

# example implementation for a Win32 winerror type

```
using unique_winerror = gsl::unique_error<winerror_error>;

inline unique_winerror make_winerror_if(BOOL is_last_error) {
	unique_winerror result;
	if (is_last_error) {
		return result.reset(GetLastError());
	}
	return result;
}
```

---

# example implementation for a Win32 winerror type

```
unique_winerror winerror;

PCCERT_CONTEXT certcontext = CertCreateCertificateContext(
    X509_ASN_ENCODING | PKCS_7_ASN_ENCODING,
    &key.cert[0],
    key.cert.size()
);
winerror = make_winerror_if(!certcontext);

if (!winerror) {
    std::wcout << L"could not get cert context" << std::endl; 
}
```

[https://github.com/kirkshoop/authentication](https://github.com/kirkshoop/authentication)

---

# where is the code?

[github](https://github.com/kirkshoop/GSL/commit/a8c46e58d318158aec5d9c53236428d0ae39703a)

???

> *