11.4. Constructors and Destructors in Derived Classes

As we explained in the preceding section, instantiating a derived-class object begins a chain of constructor calls in which the derived-class constructor, before performing its own tasks, invokes its direct base class’s constructor either explicitly (via a base-class member initializer) or implicitly (calling the base class’s default constructor). Similarly, if the base class is derived from another class, the base-class constructor is required to invoke the constructor of the next class up in the hierarchy, and so on. The last constructor called in this chain is the one of the class at the base of the hierarchy, whose body actually finishes executing first. The most derived-class constructor’s body finishes executing last. Each base-class constructor initializes the base-class data members that the derived-class object inherits. In the CommissionEmployee/BasePlusCommissionEmployee hierarchy that we’ve been studying, when a program creates a BasePlusCommissionEmployee object, the CommissionEmployee constructor is called. Since class CommissionEmployee is at the base of the hierarchy, its constructor executes, initializing the private CommissionEmployee data members that are part of the BasePlusCommissionEmployee object. When CommissionEmployee’s constructor completes execution, it returns control to BasePlusCommissionEmployee’s constructor, which initializes the BasePlusCommissionEmployee object’s baseSalary.

When a derived-class object is destroyed, the program calls that object’s destructor. This begins a chain (or cascade) of destructor calls in which the derived-class destructor and the destructors of the direct and indirect base classes and the classes’ members execute in reverse of the order in which the constructors executed. When a derived-class object’s destructor is called, the destructor performs its task, then invokes the destructor of the next base class up the hierarchy. This process repeats until the destructor of the final base class at the top of the hierarchy is called. Then the object is removed from memory.

Base-class constructors, destructors and overloaded assignment operators (Chapter 10) are not inherited by derived classes. Derived-class constructors, destructors and overloaded assignment operators, however, can call base-class versions.