28E. Constructors

Constructors are a feature that make initialization of structures convenient. After defining a constructor for type Cell appropriately, you can write one line,

  Cell* p = new Cell(22, "fixed");

  Cell* p = new Cell;
  p->item = 22;
  p->name = "fixed";

Defining constructors

---

A constructor definition looks like a function definition, with parameters, but it has no return-type. The constructor's name must be the same as the name of the type. Within the constructor definition, initialize the fields. For example,

  struct Cell
  {
    int         item;
    const char* name;

    Cell(int it, const char* nm)
    {
      item = it;
      name = nm;
    }
  };

It is good policy to initialize every field of a structure in a constructor. That does not necessarily mean that the constructor has a parameter for each field, but many constructors do.

Using constructors

---

You can use a constructor in three ways, which we illustrate for type Cell. For our purposes, the first one is most important.

Cell* p = new Cell(20, "a cell");

Expression new Cell(it,nm) creates a new cell in the heap and runs the constructor on that new cell, with the given parameters. The expression's value is a pointer to the new cell, and new Cell(it, nm) has type Cell*.

Cell c(20, "a cell");

This is equivalent to Cell c; c.item = 20; c.name = "a cell";

Cell(20, "a cell")

Expression Cell(it, nm) creates a cell in the frame of the current function and initializes that cell. Expression Cell(it, nm) has type Cell. For example, Cell c = Cell(20, "a cell"); is equivalent to Cell c(20, "a cell");