The basic idea is that in D, any statically identifiable information
(known at compile-time), can be used to assign class members as they are
declared.
Any time a new object is created, it will take those default values
specified for its members.
This is a small example demonstrating default initialization for both
structs and classes. In fact, compared to the version of C++ I used to
use (I can't speak for C++11), the syntax is far more consistent and less
verbose than initializer lists.
struct Dummy {
int field1 = 10;
int field2 = 11;
}
class MyClass {
int a = 0;
int[] b = [1, 2, 3];
Dummy c = Dummy(4, 5);
int d = 6;
this() {
}
this(int val) {
d = val;
}
}
void main() {
MyClass first = new MyClass();
MyClass second = new MyClass(7);
assert(first.a == 0);
assert(first.b == [1, 2, 3]);
assert(first.c.field1 == 4);
assert(first.d == 6);
assert(second.c.field1 == 4);
assert(second.d == 7);
}
You are correct that in the case of the second constructor, two
assignments effectively take place, d = 6, then d = 7. However, I do not
think the compiler can know what you intend to do in the constructor, or
even that you will not use the default value of d before reassigning it.
In short, I think the optimization would tend to become more of a source
of problems than a gain in performance in any meaningful way.
- Vijay
On Tue, 31 Jan 2012, Zachary Lund wrote:
In C++, they provide a mechanism to initialize class variables to a passed
value.
class Test
{
int bob;
public:
Test(int jessica) : bob(jessica) { }
};
The above basically says "int this.bob = jessica;" as opposed to this:
class Test
{
int bob;
public:
Test(int jessica) { bob = jessica; }
};
which basically says "int this.bob = void; bob = jessica;". Now, I'm not a
speed freak but this is a quick and should be a painless optimization. D
allows defaults set by the class but cannot seem to find anything to allow me
variable initialization values. Not that it's that big of a deal but am I
missing something?
