c assignment operator overloading derived class

Overloading assignment operator in a derived class

Class Derived inherits class Base in below class hierarchy. Class Base has some private dynamically allocated data (not shown) that is deep copied in its overloaded assignment operator. Assume that the implementation of overloaded assignment operator is correct in Base . You have to select the correct implementation of assignment operator in class Derived from given choices.

Note that class Derived 's data member str points to a dynamically allocated std::string object that must be deep copied inside assignment operator. Select the most appropriate implementation of assignment operator in Derived from given choices:

21.12 — Overloading the assignment operator

Understanding C++ Template Assignment Operator Overloading: Why "using" Isn't Working

Introduction.

When working with C++ templates and inheritance, certain nuances can arise that may lead to unexpected behavior or compile-time errors. A common issue arises when trying to delegate functions, such as the assignment operator, from a base class to a derived class using the using directive. In this post, we will explore why the using IParameterBase<TYPE>::operator=; directive in a templated class structure results in issues when assigning objects, and how to resolve them.

The Problem Context

Consider a template-based class hierarchy where IParameterBase<TYPE> is derived from IParameter , and ParameterTx<TYPE> extends IParameterBase<TYPE> . The intent is to use the base class assignment operator for assignments in the derived class.

Here is a snippet of the relevant part of the code structure:

Understanding the Issue

The issue arises during the assignment param1 = param2; in the main() function. The expectation is that the assignment operator from IParameterBase will be used. However, this operator is not automatically used because it handles TYPE values, and we are dealing with ParameterTx<TYPE> types.

In C++, when you write:

The type of param2 is ParameterTx<uint16_t> . Consequently, the compiler looks for an assignment operator that can handle ParameterTx objects. The using directive in ParameterTx intended to expose operator= from IParameterBase doesn't apply effectively here because it expects a TYPE (i.e., uint16_t , not ParameterTx<uint16_t> ).

Deleted Copy Assignment Operator

Complicating the matter, a copy assignment operator is automatically generated by the compiler for IParameterBase . However, it gets implicitly deleted because the base class IParameter has a constant member ( const size_t size_ ), making it non-copy-assignable.

1. Explicitly Redefine the Assignment Operator

One straightforward resolution involves explicitly undeleting the assignment operator and making it perform as intended:

This code tells the compiler how to handle assignment operations between IParameterBase types.

2. Use an Explicit Cast

Another approach is to explicitly cast before assignment, ensuring type compatibility:

This requires casting param2 to a uint16_t , though it adds verbosity and potential for manual errors.

Understanding the implications of class templates, inheritance, and operator overloading is crucial in C++. Using the using keyword to inherit operators can lead to confusion when type mismatches occur. It emphasizes the need to define clear operator overloads for classes involved in template-based hierarchies. The solutions provided help mitigate these issues, aligning operator functionalities with expected use cases.

For more intricate programming discussions and solutions, explore the Godbolt Compiler Explorer .

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The assignment operator ( = ) is, strictly speaking, a binary operator. Its declaration is identical to any other binary operator, with the following exceptions:

• It must be a nonstatic member function. No operator= can be declared as a nonmember function.
• It is not inherited by derived classes.
• A default operator= function can be generated by the compiler for class types, if none exists.

The following example illustrates how to declare an assignment operator:

The supplied argument is the right side of the expression. The operator returns the object to preserve the behavior of the assignment operator, which returns the value of the left side after the assignment is complete. This allows chaining of assignments, such as:

The copy assignment operator is not to be confused with the copy constructor. The latter is called during the construction of a new object from an existing one:

It is advisable to follow the rule of three that a class which defines a copy assignment operator should also explicitly define copy constructor, destructor, and, starting with C++11, move constructor and move assignment operator.

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• Copy Constructors and Copy Assignment Operators (C++)

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When should we write our own assignment operator in C++?

The answer is same as Copy Constructor. If a class doesn’t contain pointers, then there is no need to write assignment operator and copy constructor. The compiler creates a default copy constructor and assignment operators for every class. The compiler created copy constructor and assignment operator may not be sufficient when we have pointers or any run time allocation of resource like file handle, a network connection..etc. For example, consider the following program.

Output of above program is “10”. If we take a look at main(), we modified ‘t1’ using setValue() function, but the changes are also reflected in object ‘t2’. This type of unexpected changes cause problems. Since there is no user defined assignment operator in the above program, compiler creates a default assignment operator, which copies ‘ptr’ of right hand side to left hand side. So both ‘ptr’s start pointing to the same location.

We can handle the above problem in two ways.

1) Do not allow assignment of one object to other object. We can create our own dummy assignment operator and make it private.

2) Write your own assignment operator that does deep copy.

Same is true for Copy Constructor.

Following is an example of overloading assignment operator for the above class.

We should also add a copy constructor to the above class, so that the statements like “Test t3 = t4;” also don’t cause any problem.

Note the if condition in assignment operator. While overloading assignment operator, we must check for self assignment. Otherwise assigning an object to itself may lead to unexpected results (See this ). Self assignment check is not necessary for the above ‘Test’ class, because ‘ptr’ always points to one integer and we may reuse the same memory. But in general, it is a recommended practice to do self-assignment check.

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