If a C class defines a destructor, copy constructor, or copy assignment operator, it should also define the other two (Rule of Three); with C 11, add move constructor and move assignment operator (Rule of Five) to properly manage resources and avoid undefined behavior from shallow copies or double frees.
The Rule of Three and Rule of Five in C are guidelines that help manage resource handling in classes, especially when dealing with manual memory management or system resources like file handles or network connections.
Rule of Three
The Rule of Three states that if a class defines any of the following three special member functions, it likely needs to define all three:
- Destructor – to release resources (e.g., free memory, close files)
- Copy constructor – to properly copy an object’s resources
- Copy assignment operator – to assign one object to another with correct resource handling
This rule exists because the default versions generated by the compiler perform shallow copies. If your class owns a pointer to dynamically allocated memory, a shallow copy leads to multiple objects pointing to the same memory. When one is destroyed, the memory is freed, leaving others with dangling pointers—causing crashes or undefined behavior.
Example:If you write a destructor to delete[] an array, you should also write a copy constructor and copy assignment operator to deep-copy the data, avoiding double-free errors.
Rule of Five
With C 11, move semantics were introduced. The Rule of Five extends the Rule of Three by adding two more functions related to moving objects efficiently:
- Move constructor – transfers ownership of resources from a temporary (rvalue) object
- Move assignment operator – assigns resources from a temporary object
Now, if your class manages resources and you need to customize copying or destruction, you should consider defining all five functions for correctness and performance.
When to use Rule of Five:Classes that directly manage resources (like raw pointers, file handles, etc.) often need all five. However, modern C encourages using smart pointers (std::unique_ptr, std::shared_ptr) and standard containers (std::vector), which handle their own cleanup. In such cases, the default versions work fine, and you don’t need to define any of the five—you can rely on automatic generation.
Key Takeaway
If your class needs a destructor to clean up resources, seriously consider whether you also need custom copy/move constructors and assignment operators. Alternatively, use RAII types (like smart pointers) so you don’t have to define any of them—the compiler-generated defaults will be safe and efficient.
Basically, follow the Rule of Five when managing resources manually, but prefer letting standard library types do the work for you.
The above is the detailed content of What is the Rule of Three/Five in C. For more information, please follow other related articles on the PHP Chinese website!

Hot AI Tools

Undress AI Tool
Undress images for free

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

ArtGPT
AI image generator for creative art from text prompts.

Stock Market GPT
AI powered investment research for smarter decisions

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

InstallaC compilerlikeg usingpackagemanagersordevelopmenttoolsdependingontheOS.2.WriteaC programandsaveitwitha.cppextension.3.Compiletheprogramusingg hello.cpp-ohellotogenerateanexecutable.4.Runtheexecutablewith./helloonLinux/macOSorhello.exeonWi

Custom allocator can be used to control the memory allocation behavior of C containers. 1. The LoggingAllocator in the example implements memory operation logging by overloading allocate, deallocate, construct and destroy methods; 2. The allocator needs to define value_type and rebind templates to meet the STL container type conversion requirements; 3. The allocator triggers log output during construction and copying, which is convenient for tracking the life cycle; 4. Actual applications include memory pools, shared memory, debugging tools and embedded systems; 5. Since C 17, construct and destroy can be processed by std::allocator_traits by default

Use the std::system() function to execute system commands, which need to include header files and pass in C-style string commands, such as std::system("ls-l"), and the return value is -1, which means that the command processor is not available.

The answer is to define a class that contains the necessary type alias and operations. First, set value_type, reference, pointer, difference_type and iterator_category, then implement dereference, increment and comparison operations. Finally, provide begin() and end() methods in the container to return the iterator instance, making it compatible with STL algorithms and range for loops.

AstaticvariableinC retainsitsvaluebetweenfunctioncallsandisinitializedonce.2.Insideafunction,itpreservesstateacrosscalls,suchascountingiterations.3.Inaclass,itissharedamongallinstancesandmustbedefinedoutsidetheclasstoavoidlinkingerrors.4.Staticvaria

Real-time systems require deterministic responses, because correctness depends on the result delivery time; hard real-time systems require strict deadlines, missed will lead to disasters, while soft real-time allows occasional delays; non-deterministic factors such as scheduling, interrupts, caches, memory management, etc. affect timing; the construction plan includes the selection of RTOS, WCET analysis, resource management, hardware optimization and rigorous testing.

Use std::ifstream and std::istreambuf_iterator to efficiently read the entire contents of the file to strings, including spaces and line breaks, and is suitable for medium-sized text files.

std::array is a container that encapsulates fixed-sized arrays in C, with both security and performance. It needs to include header files, and supports secure access methods such as operator[], at(), etc., and provides member functions such as size(), fill(), and can use range for loop traversal to avoid pointer degradation and improve code clarity and security.
