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Learn about Deleted Implicitly-Declared Copy Constructor in C++

Do you want to know what is Deleted Implicitly Declared Copy Constructor ? Implicitly-Declared Copy Constructor helps you to do this, here is the full post;

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. Here is a simple constructor class example below,

There are different constructor types in classes and the Copy Constructor is one of these. Copy Constructors not only used in classes but also used with struct and union data types

The Copy Constructor in classes (class_name) is a non-template constructor whose first parameter is class_name&‍, const class_name&‍, volatile class_name&‍, or const volatile class_name&‍ . It can be used with no other parameters or with the rest of the parameters all have default values.

The Copy Constructor is a constructor type for classes that class_name must name the current class, or it should be a qualified class name when it is declared at namespace scope or in a friend declaration.

Deleted Implicitly-Declared Copy Constructor

In C++, we can implicitly declare a copy constructor while it is deleted in previous class. The implicitly-declared or defaulted copy constructor for a class is defined as deleted if,

  • class has non-static data members that cannot be copied
  • class has direct or virtual base class that cannot be copied
  • class has direct or virtual base class with a deleted or inaccessible destructor
  • class is a union-like class and has a variant member with non-trivial copy constructor
  • class has a data member of rvalue reference type;
  • class has a user-defined copy constructor or copy assignment operator

The compiler declares a Copy Constructor as a non-explicit inline public member of its class If a copy constructor is not defined for a class type (struct, class, or union), We can Implicitly declare a copy constructor when defining a new class.

Remember that copy constructor has this syntax,

and this Copy Constructor will be declared implicitly when declaring a new class as below,

If we combine both with a Deleted copy Constructor, here is an Deleted Implicitly-Declared Copy Constructor example,

Here is the full Deleted Implicitly-Declared Copy Constructor example that causes errors as below,

To avoid this error, we can declare a Copy Constructor in a new class (my_otherclass) as below,

output will be like this.

As in this example, Deleted Implicit declared copy constructor copied values by the declared copy constructor.

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About author

33+ years of coding with more than 30+ programming languages, mostly C++ on Windows, Android, Mac-OS, iOS, Linux and some other operating systems. Dr. Yilmaz Yoru was born in 1974, Eskisehir-Turkey. He graduated from the department of Mechanical Engineering of Eskisehir Osmangazi University in 1997. One year later he started to work in the same university as an assistant. He received his MSc and PhD degrees from the same department of the same university. He has married and he is a father of a son. Some of his interests are Programming, Thermodynamics, Fluid Mechanics and Artificial Intelligence. He also likes the graphical 2D & 3D design and high-end innovations.
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