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.
