How do I fix 'use of deleted function' errors in C++?

In C++, 'use of deleted function' errors typically occur when trying to use functions that have been explicitly deleted by the programmer. This often involves copy constructors, copy assignment operators, move constructors, or move assignment operators. Understanding why these functions have been deleted is crucial for resolving these errors.

Common Reasons for 'Use of Deleted Function' Errors

  • Deleted Copy Constructor/Assignment Operator: If a class has a member variable that does not allow copying, the compiler will delete the generated copy constructor and assignment operator.
  • Deleted Move Constructor/Assignment Operator: Similarly, if a class contains a member that is non-movable, the move operations may also be deleted.
  • Incorrect Object Lifecycle Management: Using pointers or references to classes with deleted copies can lead to attempts at illegal copies.

How to Fix 'Use of Deleted Function' Errors

Here are steps to resolve these errors:

  1. Define Necessary Constructors/Operators: If your class needs to support copying or moving and its members support it, define the necessary constructors and assignment operators.
  2. Use Smart Pointers: If your class holds dynamic memory or resources, consider using smart pointers (like std::shared_ptr or std::unique_ptr) to manage them.
  3. Restrict Copy/Move Operations: If copying or moving a class instance is not intended, declare the copy and move constructors and assignment operators as deleted.
  4. Check Member Types: Ensure that all member variables of your class allow copying/moving if you intend to copy/move the class itself.

Example

#include #include class NonCopyable { public: NonCopyable() = default; // Deleting copy constructor and copy assignment operator NonCopyable(const NonCopyable&) = delete; NonCopyable& operator=(const NonCopyable&) = delete; // Move constructor and move assignment operator NonCopyable(NonCopyable&&) = default; NonCopyable& operator=(NonCopyable&&) = default; void show() { std::cout << "NonCopyable instance" << std::endl; } }; int main() { NonCopyable obj1; // NonCopyable obj2 = obj1; // This line would cause a 'use of deleted function' error NonCopyable obj2 = std::move(obj1); // Correct usage with move semantics obj2.show(); return 0; }

C++ use of deleted function copy constructor move constructor C++ error handling object copying class design.

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