In a modern C++ IDE, one of the features of its modern is the copy assignment operator that is used with “operator=” to create a new object from an existing one. In this post, we explain an eligible copy assignment operator in C++.
Table of Contents
What are classes and objects in C++?
Classes are defined in C++ using keyword class followed by the name of the class. Classes are the blueprint for the objects and they are user-defined data types that we can use in our program, and they work as an object constructor. Objects are an instantiation of a class, In another term. In C++ programming, most of the commands are associated with classes and objects, along with their attributes and methods. Here is a simple class example below,
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class Tmyclass { public: std::string str; }; |
then we can create our objects with this Type of myclass as below,
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Tmyclass o1, o2; |
What is copy assignment operator in C++?
The Copy Assignment Operator in a class is a non-template non-static member function that is declared with the “operator=“. When you create a class or a type that is copy assignable (that you can copy with the = operator symbol), it must have a public copy assignment operator. Here is a simple syntax for the forced (defaulted) copy assignment operator with default option;
Syntax (Since C++11),
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class_name & class_name :: operator= ( const class_name& ) = default; |
here is an example in a class:
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Tmyclass& operator=( const Tmyclass& other) = default; // Default Copy Assignment Operator |
This default copy assignment operator is declared automatically in a new class declaration, it is implicitly-defined or defaulted copy assignment operator and also a trivial copy assignment operator.
What is an eligible copy assignment operator in C++?
Before the C++11 standard, a copy assignment operator was ‘eligible’ when the copy assignment operator is either user-declared or both implicitly declared and definable. Since C++11 (and until C++20), copy assignment operator is generated automatically and the Eligible Copy Assignment Operator is a copy assignment operator that is eligible if this default operator or user defined copy assignment operator is not deleted.
Since C++20, a copy assignment operator in C++ is eligible:
- if it is not deleted
- if it has associated constraints, they are satisfied
- if there is no more constrained than this operator with the same first parameter type and the same cv/ref-qualifiers
Is there a simple example of an eligible copy assignment operator in C++?
Let’s give an example of an eligible copy assignment operator. Let’s assume that we have TmyclassA as a base class and we have a new TmyclassB class using TmyclassA as a base class.
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class TmyclassA { public: std::string str; }; class TmyclassB : public TmyclassA { // This class has Eligible Copy Assignment Operator from base }; |
Here, In C++11, C++14, C++17 standards, TmyclassB has eligible copy assignment operator from TmyclassA base. Because it is not deleted from the base. Now, let’s assume that we have TmyclassC as a base class and has deleted copy assignment operator, and we have a new TmyclassD class using TmyclassC as a base class.
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class TmyclassC { public: std::string str; TmyclassC& operator=( const TmyclassC& other) = delete; }; class TmyclassD : public TmyclassC { // This class has no Eligible Copy Assignment Operator from base, because it is deleted public: TmyclassD& operator=( const TmyclassD& other) // User Defined Copy Assignment Operator { str = other.str; } }; |
Here, TmyclassD has no eligible copy assignment operator from TmyclassC base, because default copy assignment operator is deleted in TmyclassC definition. Thus, user defined his own Copy Assignment Operator in TmyclassD.
Is there a full example of an eligible copy assignment operator in C++?
If we combine both examples above, here is a full example of an eligible copy assignment operator in C++.
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#include <iostream> #include <string> class TmyclassA { public: std::string str; }; class TmyclassB : public TmyclassA { // This class has Eligible Copy Assignment Operator from base }; class TmyclassC { public: std::string str; TmyclassC& operator=( const TmyclassC& other) = delete; }; class TmyclassD : public TmyclassC { // This class has no Eligible Copy Assignment Operator from base, because it is deleted public: TmyclassD& operator=( const TmyclassD& other) // User Defined Copy Assignment Operator { str = other.str; } }; int main() { TmyclassB o1, o2; o1.str = "LearnCplusplus.org"; o2 = o1; // Using Eligible Copy Assignment Operator std::cout << o1.str << std::endl; std::cout << o2.str << std::endl; TmyclassD o3, o4; o3.str = "I Love C++"; o4 = o3; // Using User Defined Copy Assignment Operator std::cout << o3.str << std::endl; std::cout << o4.str << std::endl; system("pause"); return 0; } |
As you see, in Modern C++, each new class has copy assignment operator and they are eligible in other class declarations when they have this base.
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