How do you use the non-default constructor for a member?

I have two classes

class a {
    public:
        a(int i);
};

class b {
    public:
        b(); //Gives me an error here, because it tries to find constructor a::a()
        a aInstance;
}

How can I get it so that aInstance is instantiated with a(int i) instead of trying to search for a default constructor? Basically, I want to control the calling of a's constructor from within b's constructor.