Using constructors with arrays in D

Question

How do you call constructors when allocating an array with new?

For example, in the following code how would I call the constructor for each instantiation of A, initialising b to 5 for all 10 elements?

void main() {
    A[] a = new A[10];
}

class A {
    int b;
    this(int init) {
        b = init;
    }
}

I'm guessing it's not possible, but I can hope...

Answer 1

a simple loop should do (and it's the most readable)

foreach(ref el;a){
    el=new A(5);
}

or you can use the array initializer:

A[] a=[new A(5),new A(5),new A(5),new A(5),new A(5),
       new A(5),new A(5),new A(5),new A(5),new A(5)];

Answer 2

If you're dealing with a value type, you can use std.array.replicate.

auto a = replicate([5], 50);

would create an int[] of length 50 where each element is 5. You can do the same with a reference type, but all of the elements are going to refer to the same object.

auto a = replicate([new A(5)], 50);

will only call A 's constructor once, and you'll end up with an A[] where all of the elements refer to the same object. If you want them to refer to separate objects, you're either going to have to set each element individually

auto a = new A[](50);
foreach(ref e; a)
    e = new A(5);

or initialize the whole array with a literal

auto a = [new A(5), new A(5), new A(5)];

But that clearly will only work for relatively small arrays.

Answer 3

If you really want to do it in one line, you could write a macro to do it for you. I've borrowed code for the actual initialisation from the other answers.

template allocate(T) {
    T[] allocate(int size, int arg) {
        T[] result = new T[size];
        foreach(ref el; result)
            el=new T(arg);
        return result;
    }
}

Then you can allocate an entire array of 10 elements at once with:

A[] a = allocate!(A)(10, 5);

Of course this has fixed constructor arguments, but you could probably do something with variadic arguments to the template and some mixins to generate the correct constructor call.