Array of unknown length in constructor initializer list

I have a class with a member array. The length is a constant, but this constant is not known until compile time (In my actual code, this constant is defined differently for different compilation targets). The type of the array is a class with no default constructor.

#define CONSTANT 2

class Data {
public:
    Data(int number){}
    Data(){}
};

class DemoClass {
private:
    Data _member[CONSTANT];
public:
    DemoClass():
        _member{
            Data(0),
            Data(0)
        }
    {
        // stuff
    }
};

In this example, I can set _member using the initializer list. However, if the value of COSNTANT changes, I have to change that initializer list.

In theory, changing DemoClass to have a default constructor that calls the other constructor with an argument of 0 would work for my case, because I will always call the Data constructor with 0. However, I cannot change DemoClass because it is in an external library.

One solution I've considered is creating the following class:

class CustomData : public Data {
public:
    CustomData() : Data(0){}
};

This works, but it seems a bit complicated. Is there a simpler way to initialize this array?