C++ Tutorial - Copy Constructors

In this lesson, I'll be looking in depth at constructors, default constructors, and copy constructors plus related topics. This gets a bit more complex than stuff we've seen before. Lets remind ourselves about constructors.

In C++ a constructor is a function with the same name as the class. Its purpose is to

• Create a new instance of the class
• Initialize the member data of the instance

Default Constructors

If you have written a constructor for your class then when you create an instance of it, the constructor is automatically called. If you haven't supplied a constructor, then the compiler will create a default one for you. This default constructor takes no parameters, or has default values for all parameters. A default parameter is one where the parameter value is supplied in the definition. For example in the class aclassname

aclassname(int param=5) {
  value = param
}

The 5 is the default value parameter. It means that the constructor can be called as

aclassname fred

and the value 5 is passed in, or 7 below

aclassname fred(7) // overrides the default 5

Here's a longer example.

class phonerec {
    public:
    string name;
    string phonenumber[16];
    phonerec(string myname ="Me",string myphone="555-1000") ; // defaults
};

phonerec p; // calls the constructor a default parameter "Me"
phonerec p("David","0207 384") ;

You can of course write your own default constructor. You only get a default constructor supplied for free if you haven't written any kind of constructor. If you create one with non defaulted parameters and the compiler needs a default constructor, it will generate an error. It also doesn't like multiple default constructors as it has no way to tell which to use.

On the next page : Initializing Default Constructors