1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
|
template accept(x) =
static: assert(compiles(x))
template reject(x) =
static: assert(not compiles(x))
type
BaseObj = object of RootObj
DerivedObj = object of BaseObj
NonDerivedObj = object
Container[T] = object
var base: BaseObj
var derived: DerivedObj
var nonDerived: NonDerivedObj
var baseContainer: Container[BaseObj]
var derivedContainer: Container[DerivedObj]
var nonDerivedContainer: Container[NonDerivedObj]
# We can fake covariance by listing some specific derived types that
# will be allowed with our overload. This is not a real covariance,
# because there will be multiple instantiations of the proc, but for
# many purposes, it will suffice:
proc wantsSpecificContainers(c: Container[BaseObj or DerivedObj]) = discard
accept wantsSpecificContainers(baseContainer)
accept wantsSpecificContainers(derivedContainer)
reject wantsSpecificContainers(nonDerivedContainer)
reject wantsSpecificContainers(derived)
# Now, let's make a more general solution able to catch all derived types:
type
DerivedFrom[T] = concept type D
var derived: ref D
var base: ref T = derived
proc wantsDerived(x: DerivedFrom[BaseObj]) = discard
accept wantsDerived(base)
accept wantsDerived(derived)
reject wantsDerived(nonDerived)
reject wantsDerived(baseContainer)
proc wantsDerivedContainer(c: Container[DerivedFrom[BaseObj]]) = discard
accept wantsDerivedContainer(baseContainer)
accept wantsDerivedContainer(derivedContainer)
reject wantsDerivedContainer(nonDerivedContainer)
# The previous solutions were solving the problem for a single overload.
# Let's solve it for multiple overloads by introducing a converter:
type
OtherContainer[T] = object
proc wantsBaseContainer1(c: OtherContainer[BaseObj]) = discard
proc wantsBaseContainer2(c: OtherContainer[BaseObj]) = discard
converter derivedToBase(c: OtherContainer[DerivedFrom[BaseObj]]): OtherContainer[BaseObj] = discard
block:
var baseContainer: OtherContainer[BaseObj]
var derivedContainer: OtherContainer[DerivedObj]
var nonDerivedContainer: OtherContainer[NonDerivedObj]
accept wantsBaseContainer1(derivedContainer)
reject wantsBaseContainer1(nonDerivedContainer)
accept wantsBaseContainer2(derivedContainer)
reject wantsBaseContainer2(nonDerivedContainer)
|
