Re: Enforcing checks for return code
On Thursday, 18 February 2016 at 10:46:03 UTC, Marc Schütz wrote: On Thursday, 18 February 2016 at 07:21:05 UTC, Chris Katko wrote: [...] As Jonathan said, there's no such built-in feature, and exception are preferred over return codes. However, you can implement such a check at run time: [...] Thank you all for your insight. I think I have all the information I need.
Re: Enforcing checks for return code
On Thursday, 18 February 2016 at 07:21:05 UTC, Chris Katko wrote:
Hello. I'm almost brand-new to the D language and still
absorbing things.
I'm wondering if it's possible to fire off a compile-time (or
worst case, a run-time) warning or error if a function is
called, but the return value is not checked.
I'm not trying to enforce whether someone actually deciphers
the value's meaning correctly. I just want to enforce that
somewhere, a variable or expression is receiving the return
value of a particular function.
Any ideas?
As Jonathan said, there's no such built-in feature, and exception
are preferred over return codes. However, you can implement such
a check at run time:
struct ForceCheck(T) {
private T payload;
private bool checked = false;
@disable this();
@disable this(this);
this()(auto ref T payload) { this.payload = payload; }
ref T get() {
this.checked = true;
return payload;
}
alias get this;
void ignore() {
this.checked = true;
}
~this() {
assert(this.checked, "you forgot to check the return
value");
}
}
auto forceCheck(T)(auto ref T value) {
return ForceCheck!T(value);
}
auto foo() {
return forceCheck(42);
}
void main() {
{
auto a = foo();
if(a != 42) { } // stored values
}
{
if(foo() != 42) { } // direct access
}
{
foo().ignore(); // explicitly ignore return value
}
{
auto b = foo(); // this one asserts
foo(); // as does this one
}
}
I guess it's reasonably performant; it could be optimized further
by only adding the `checked` member if assertions are enabled
(using `version(assert)`).
Re: Enforcing checks for return code
On Thursday, 18 February 2016 at 07:21:05 UTC, Chris Katko wrote:
Hello. I'm almost brand-new to the D language and still
absorbing things.
I'm wondering if it's possible to fire off a compile-time (or
worst case, a run-time) warning or error if a function is
called, but the return value is not checked.
I'm not trying to enforce whether someone actually deciphers
the value's meaning correctly. I just want to enforce that
somewhere, a variable or expression is receiving the return
value of a particular function.
Any ideas?
I imagine I could use a compiler flag to warn, but that's a
global setting. I'm looking more for a specified subset of
functions.
Off hand, I see two reasonable ways to do this:
1) Instead of actually returning the value, require the caller to
pass the destination variable as a `ref` or `out` parameter.
Disadvantages: This could be a bit slower than using actual
return values, and it would change the syntax.
2) Wrap the return value in a struct like this:
struct MustUse(T) {
pure: nothrow:
private:
T payload;
bool used;
public:
@property ref T use() @nogc {
used = true;
return payload;
}
alias use this;
this(T payload) @nogc {
this.payload = payload;
this.used = false;
}
~this() {
if(!used)
throw new Error(T.stringof ~ " return value was not
used.");
}
}
// For convenience, use this function to infer T from the type of
paylaod.
MustUse!T mustUse(T)(T payload) {
return MustUse!T(payload); }
Example on DPaste: http://dpaste.dzfl.pl/8ba6ebf05f32
With inlining and optimizations, even the above generic
implementation might be faster than passing by ref/out. A
specialization for non-null pointer values could almost certainly
be faster.
Disadvantages: On the other hand, it might be slower. The only
way to know for sure, is to write some non-trivial examples and
benchmark. Regardless, the fact that it signals the error at
runtime instead of compile time could be annoying.
Re: Enforcing checks for return code
On Thursday, February 18, 2016 07:21:05 Chris Katko via Digitalmars-d-learn wrote: > Hello. I'm almost brand-new to the D language and still absorbing > things. > > I'm wondering if it's possible to fire off a compile-time (or > worst case, a run-time) warning or error if a function is called, > but the return value is not checked. > > I'm not trying to enforce whether someone actually deciphers the > value's meaning correctly. I just want to enforce that somewhere, > a variable or expression is receiving the return value of a > particular function. > > Any ideas? > > I imagine I could use a compiler flag to warn, but that's a > global setting. I'm looking more for a specified subset of > functions. D has no such feature, and very little D code is going to use return codes. D has exceptions, and throwing an exception is the normal way to indicate that there was a problem. You certainly _can_ choose to use error codes if you'd like, but it's not normal practice, and the language does nothing extra to support it. - Jonathan M Davis
Enforcing checks for return code
Hello. I'm almost brand-new to the D language and still absorbing things. I'm wondering if it's possible to fire off a compile-time (or worst case, a run-time) warning or error if a function is called, but the return value is not checked. I'm not trying to enforce whether someone actually deciphers the value's meaning correctly. I just want to enforce that somewhere, a variable or expression is receiving the return value of a particular function. Any ideas? I imagine I could use a compiler flag to warn, but that's a global setting. I'm looking more for a specified subset of functions.
