How to call parameterized constructor of member object variable in a class' default constructor in C++?

I want to initialize member object variables in the default constructor of the class.

Let's consider the following,

class ABC {    
    ABC(int A, int B) {
        a = A;
        b = B;
    }

    int a;
    int b;
};


class Foo {
    Foo();

    ABC obj1;
};

From the above example, I would like to initialize "obj1" in "Foo::Foo()". One of the restrictions I have is that I cannot do so in the initializer list, as I need to do some computation before I could initialize the member. So the option available (ASFAIK) is to do so in the body of the default constructor only.

Any inputs, how could I do this?