Friday, July 27, 2007

Inadequacies of __traits

It is good that the __traits syntax is extensible, since it is missing a few important things. Some of these did not occur to me until I actually started writing code with it.

Take this class:

class Foo {
    int i;
    this() { i = 10; }
    this(int i) { this.i = i; }
    void foo() { writefln("Foo.i is %s", this.i); }
    real foo(double) { return 2.0; }
    static int bar(int j) { return j*2; }
    static real bar() { return 5.0; }
__traits(allMembers, Foo) gives us this:


One of these in particular jumps out, which is "_ctor". This obviously has something to do with the constructor. If the class doesn't define a constructor, then it is not present. If the class defines a destructor, then a "_dtor" member will be present. It turns out that "_ctor" is only very narrowly useful.

All of the following give false:

writefln(__traits(isVirtualFunction, __traits(getMember, Foo, "_ctor")));
writefln(__traits(isAbstractFunction, __traits(getMember, Foo, "_ctor")));
writefln(__traits(isFinalFunction, __traits(getMember, Foo, "_ctor")));

The same results are given if we just say Foo._ctor. This can only mean _ctor is a static function... except it isn't:

auto f = new Foo; // okay
auto g = Foo._ctor(50); // Error: need 'this' to access member this
auto h = f._ctor(100); // okay

What the heck? What is _ctor supposed to be, anyway? Interestingly, by calling f._ctor(100), it modifies f, and f.i becomes 100; f and h refer to the same object.

Next, the allMembers trait doesn't include static members of the class. I suggest either adding an isStaticMember trait and including static members in allMembers and derivedMembers; or adding a pair of allStaticMembers and derivedStaticMembers traits.

However, my primary complaint with traits is that you can only determine the signatures of the overloads of virtual functions. This most notably means you cannot determine the overloads of a global function, and being unable to do it for static and final functions is unfortunate as well. Additionally, you cannot determine the signatures of the class's constructor (which I might expect the _ctor member to be used for).

Here are some more minor issues:

  • There is no isStaticFunction trait. Though it is not strictly necessary, since you can determine it by process of elimination using the other isFooFunction traits, it would be convenient.
  • There is no isDynamicArray trait. This isn't much of an issue, since you can determine whether something is a dynamic array type using regular templates, but it would be consistent with the isStaticArray and isAssociativeArray traits.

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