You can't do this because in a theory class is a set of objects. Assigning attributes (chosing a condition) equals to chosing one of the elements of set. If you want to initialize attribute in the declaration it means, you don't actualy need this variable. And if you want to make an attribute with an initial value you should use constructor. Because class it's an instrumet of solving problems of the same type. Try to read theory of programming.
You can't do that because you have to let the person using your class choose which constructor they want to use - the default one, the copy constructor, one of your other constructors, etc.