How Does the Colon (:) Syntax Work in C Constructors for Member Initialization and Base Class Calls?

Understanding the Colon Syntax in Constructors

In C , constructors are functions that initialize objects upon creation. While constructor names typically match the class name, they can have unique initialization syntax. One notable aspect of constructor syntax is the use of a colon (:) followed by an argument list.

This syntax, known as a member initializer list, serves two primary purposes:

• Calling Base Class Constructors:
  When a derived class is created, its base class(es) must also be initialized. The member initializer list allows you to call the constructor of the base class, passing arguments as needed.
• Initializing Data Members:
  The member initializer list can also be used to initialize non-static data members of the class. This is done by specifying the data member name, followed by an assignment operator and an argument.

For instance, consider the following code:

class demo 
{
private:
    unsigned char len, *dat;

public:
    demo(unsigned char le = 5, unsigned char default) : len(le) 
    { 
        dat = new char[len];                                      
        for (int i = 0; i <= le; i++)                             
            dat[i] = default;
    }

    void ~demo(void) 
    {                                            
        delete [] *dat;                                           
    }
};

In this example, the constructor has two parameters, le and default. The member initializer list : len(le) assigns the value of le to the len data member.

Additionally, in the derived class newdemo:

class newdemo : public demo 
{
private:
    int *dat1;

public:
    newdemo(void) : demo(0, 0)
    {
     *dat1 = 0;                                                   
     return 0;                                                    
    }
};

The member initializer list : demo(0, 0) calls the base class constructor demo with the arguments 0 and 0, initializing the len and default data members of the base class.

Member initializer lists are a convenient and efficient way to initialize data members and call base class constructors, enhancing the safety and clarity of your code.

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