C++Builder Learn about Deleted Default Constructor in C++

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Learn about Deleted Default Constructor in C++
By Yilmaz Yoru June 18, 2021

The Constructor in C++ is a function, a method in the class, but it is a ‘special method’ that is automatically called when an object of a class is created. We don’t need to call this function. Whenever a new object of a class is created, the Constructor allows the class to initialize member variables or allocate storage. This is why the name Constructor is given to this special method.

class myclass
{         
  public:   
    myclass()
    {
       std::cout << "myclass is constructed!\n";
    };
}

There are different contractor types and the Default Constructor in classes is one of these. This method not only used in classes but also used with struct and union data types Do you want to learn what is default constructor or what kind of methods we have that we can declare and use default constructors? In this post, we will try to explain the Default Constructor with examples.

A Default Constructor is a constructor type in Classes that is called when class is defined with no arguments, or it is defined with an empty parameter list, or with default arguments provided for every parameter. A type with a public default constructor is Default Constructible.

In the definition of a default Constructor, class_name must name the current class or current instantiation of a class template. It must be a qualified class name when declared at namespace scope or in a friend declaration.

Deleted Default Constructor​

We can easily default the Default Constructor by using = default; after class name with (); We can do this outside of a Class Definition. The default statement means that you want to use the compiler-generated version of that function, so you don’t need to specify a body.

We can also use = delete to specify that you don’t want the compiler to generate that function automatically. In another way, = delete means that the compiler will not generate those constructors when declared and this is only allowed on copy constructor and assignment operator.

There is also = 0 usage; it means that a function is purely virtual and you cannot instantiate an object from this class. You need to derive from it and implement this method. Thus, the term = 0 is not the same as the term = delete.

Syntax,

class_name() = delete;

An example with a class;

class myclass
{
  public:
   myclass() = delete;
};

Here is a full example,

#include <iostream>
 
class myclass
{
  public:
   int x = 49;
   myclass() = delete;
};
 
 
int main()
{
 class myclass test;  //   Error: 'myclass' has been explicitly marked deleted here
 
 std::cout << test.x << '\n';
 
 getchar();
 
 return 0;
}