Destructors

A destructor, sometimes abbreviated as dtor, is a special member function that automates clean-up actions just before an object is destroyed.

When is an object destroyed?

• When a local (automatic) object goes out of scope (e.g., when a function call returns)
• When an object created by the new operator is specifically destroyed by the use of the operator delete
• Just before the program terminates, all objects with static storage are destroyed

The destructor's name is the classname preceded by the tilde (~) character. It has no return type and no parameters, so it cannot be overloaded. If the class definition contains no destructor definition, the compiler will supply one that looks like this:

ClassName::~ClassName() { }

We will look at a less trivial example of a destructor in the next section.

When do we need to write a destructor?

In general, a class that directly manages or shares an external resource (opens a file, opens a network connection, creates a process, etc.) needs to free the resource at some appropriate time. Such classes are usually wrappers that are responsible for object cleanup.

Qt's container classes make it easy for us to avoid writing code that directly manages dynamic arrays.

You do not need a destructor if your class:

• Has simple type members that are not pointers
• Has class members with properly defined destructors themselves

The default compiler-generated destructor calls the destructors on each of its class members in the order that they are listed in the class definition before the object is destroyed. It does nothing with pointers or simple types.