how to do heavy initial processing on object creation without having it in the constructor?

Let's say i have and object of type List that needs a lot of parsing and processing just on creation,and then some other processing will be done by the class's user using member functions,but there always will be some heavy work/processing needed to be done just on creation based on certain constructor's parameters. so my question is,being told that it is very bad practice to do all this heavy work in the constructor and the constructor should only be doing light to mid work to initialize the object ,how should i tackle this?