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|  How to Fix Error: use of deleted function 'ClassName(const ClassName&)'

How to Fix Error: use of deleted function 'ClassName(const ClassName&)'

October 14, 2024

Solve the C++ error with this step-by-step guide for firmware developers. Understand causes and learn effective solutions to fix 'use of deleted function' issues.

How to Fix Error: use of deleted function 'ClassName(const ClassName&)'

 

Understand the Error

  • The error "use of deleted function 'ClassName(const ClassName&)'" implies that you are trying to use a copy constructor (copy semantics) for a class where this operation has been explicitly declared as deleted.
  • This situation typically arises when a class's resources cannot be safely or sensibly copied, such as when the class manages a raw resource like a file handle or raw memory pointer.

 

Identify the Deleted Function

  • Check the class definition for a declaration of the deleted copy constructor. It will appear as 'ClassName(const ClassName&) = delete;'
  • Alternatively, the class might inherit from another class that has a deleted copy constructor, so inspect any base classes as well.

 

Consider Why the Deletion Exists

  • The deletion serves to prevent shallow copying or to avoid the potential for double deletion of a managed resource.
  • If the class is managing resources that should not be duplicated, such as unique handles or singleton patterns, then copying is inherently undesirable.

 

Solutions

  • Use Move Semantics: Implement move constructor and move assignment if you need transfer ownership of resources and copying is not feasible. This involves adding 'ClassName(ClassName&& other)' and 'ClassName& operator=(ClassName&& other)' to the class.
class ClassName {
public:
    // Constructor, destructor, etc.
    
    // Implement move constructor
    ClassName(ClassName&& other) noexcept {
        // Transfer ownership of resources
    }

    // Implement move assignment operator
    ClassName& operator=(ClassName&& other) noexcept {
        // Guard self-assignment
        if (this != &other) {
            // Release current resources, if any
            // Transfer ownership of resources
        }
        return *this;
    }

    // Deleted copy constructor to prevent copying
    ClassName(const ClassName&) = delete;
    ClassName& operator=(const ClassName&) = delete;
};
  • Redesign Resource Management: Consider using smart pointers (e.g., `std::unique_ptr`, `std::shared_ptr`) to handle resource management, which can provide safe copying or sharing semantics suitable for your needs.

 

Implementing an Alternative Strategy

  • Explicit Cloning: If a copy must be made, implement a method that performs a deep copy operation, e.g., `clone()` method.
class ClassName {
public:
    ClassName* clone() const {
        auto cloned = new ClassName();
        // Perform deep copy of resources
        return cloned;
    }

    ClassName(const ClassName&) = delete;
    ClassName& operator=(const ClassName&) = delete;
};
  • Avoid Copying Altogether: Sometimes, it's viable to rethink the logic to avoid the need for copying, perhaps by using references or pointers instead.
  • Use References Instead: Where feasible, pass and store references or pointers instead of copying the whole object.

 

Testing the Solution

  • After making modifications, thoroughly test to ensure that the solution correctly manages resources without unintended side effects such as resource leaks or double frees.
  • Run static analysis and utilize tools like Valgrind to check for memory management issues.

 

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