import std.math;

class Test
{
    double a = 0.4;
    double b;

    this(int b)
    {
        this.b = b;
    }

    this() {}
}

unittest
{
    auto t = new Test(5);
    auto z = new Test;

    assert(t.b == 5);
    assert(isNaN(z.b));
}