What is the role of constructor using default argument?

Answer 1

A default constructor is any constructor that has no arguments or where all required arguments are assigned default values. The latter provides a useful means of overloading a default constructor without incurring otherwise unnecessary code duplication.

Consider the following class definition:

class complex {

double re, im;

public:

complex (): re {0}, im {0} {}

complex (double real): re {real}, im {0} {}

complex (double real, double imaginary): re {real}, im {imaginary} {}

// ...

}

Although the constructors are relatively trivial, the class definition is much more verbose than it needs to be. Essentially we have three constructors all doing exactly the same job. We could use in-class initialisation to eliminate the unnecessary default initialisations in the constructors:

class complex { double re {0}, im {0};

public:

complex () {}

complex (double real): re {real} {}

complex (double real, double imaginary): re {real}, im {imaginary} {}

// ...

}

However, because all three constructors are essentially doing the same thing, it makes sense to combine them into a single default constructor with default arguments:

class complex {

double re {0}, im {0};

public:

complex (double real=0, double imaginary=0): re {real}, im {imaginary} {}

// ...

}