What is default constructor in C++ with examples?

In this chapter of C++ interview questions, we will learn about:

• what is default constructor in c++?
• syntax of default constructor in c++
• example of default constructor in c++

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What is default constructor in c++?

A constructor that does not take any argument and has no parameters is known as a default constructor in C++. 

 

For the initialization of an object, default constructor is a must need. If a constructor is not defined explicitly by programmar. No problem :)

The compiler will provide one default constructor internally if it is needed. 

Syntax of default constructor in c++

class class_name(parameter 1, parameter 2, parameter 3, ....)
{
//define a constructor, or 
//default constructor will be provided by compiler
}

In above syntax since we have not defined constructor ourself so it will be provided implicitly by compiler.

 

Next, in below example, we will see example of constructor in c++ where no parameter constructor will be created by user.

 

Example of default constructor in C++

Default constructor in c++ example program is shown below:

class Line
{
 public:
 int size;

//default constructor
Line()
{
 size=15;
}
};

int main()
{
 //default constructor called when object is created
 Line l;
 cout<<"Line size is"<<" "<<l.size<<" "<<"cm";

return 0;
}

Output

Line size is 15 cm

 

For the Above code as soon as the object is created, a constructor is called for initializing its data members.

In above case, we have provided the no parameter default constructor Line() { } but if we don't provide one then it will be provided by compiler in such case to help initialize the data members.