The rule of three/five/zero

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function declaration
inline specifier
Exception specifications (deprecated)
noexcept specifier (C++11)
Exceptions
throw-expression	try-catch block
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Compound types Union types
Specifiers
decltype (C++11) auto (C++11) alignas (C++11)	const/volatile constexpr (C++11)
Storage duration specifiers
Initialization
Default initialization Value initialization Zero initialization Copy initialization Direct initialization	Aggregate initialization List initialization (C++11) Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Initializer lists this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Rule of three

If a class requires a user-defined destructor, a user-defined copy constructor, or a user-defined copy assignment operator, it almost certainly requires all three.

Because C++ copies and copy-assigns objects of user-defined types in various situations (passing/returning by value, manipulating a container, etc), these special member functions will be called, if accessible, and if they are not user-defined, they are implicitly-defined by the compiler.

The implicitly-defined special member functions are typically incorrect if the class is managing a resource whose handle is an object of non-class type (raw pointer, POSIX file descriptor, etc), whose destructor does nothing and copy constructor/assignment operator performs a "shallow copy" (copy the value of the handle, without duplicating the underlying resource).

class rule_of_three
{
    char* cstring; // raw pointer used as a handle to a dynamically-allocated memory block
 public:
    rule_of_three(const char* arg)
    : cstring(new char[std::strlen(arg)+1]) // allocate
    {
        std::strcpy(cstring, arg); // populate
    }
    ~rule_of_three()
    {
        delete[] cstring;  // deallocate
    }
    rule_of_three(const rule_of_three& other) // copy constructor
    {
        cstring = new char[std::strlen(other.cstring) + 1];
        std::strcpy(cstring, other.cstring);
    }
    rule_of_three& operator=(const rule_of_three& other) // copy assignment
    {
        char* tmp_cstring = new char[std::strlen(other.cstring) + 1];
        std::strcpy(tmp_cstring, other.cstring);
        delete[] cstring;
        cstring = tmp_cstring;
        return *this;
    }
// alternatively, reuse destructor and copy ctor
//  rule_of_three& operator=(rule_of_three other)
//  {
//      std::swap(cstring, other.cstring);
//      return *this;
//  }
};

Classes that manage non-copyable resources through copyable handles may have to declare copy assignment and copy constructor private and not provide their definitions or define them as deleted. This is another application of the rule of three: deleting one and leaving the other to be implicitly-defined will most likely result in errors.

Rule of five

Because the presence of a user-defined destructor, copy-constructor, or copy-assignment operator prevents implicit definition of the move constructor and the move assignment operator, any class for which move semantics are desirable, has to declare all five special member functions:

class rule_of_five
{
    char* cstring; // raw pointer used as a handle to a dynamically-allocated memory block
 public:
    rule_of_five(const char* arg)
    : cstring(new char[std::strlen(arg)+1]) // allocate
    {
        std::strcpy(cstring, arg); // populate
    }
    ~rule_of_five()
    {
        delete[] cstring;  // deallocate
    }
    rule_of_five(const rule_of_five& other) // copy constructor
    {
        cstring = new char[std::strlen(other.cstring) + 1];
        std::strcpy(cstring, other.cstring);
    }
    rule_of_five(rule_of_five&& other) : cstring(other.cstring) // move constructor
    {
        other.cstring = nullptr;
    }
    rule_of_five& operator=(const rule_of_five& other) // copy assignment
    {
        char* tmp_cstring = new char[std::strlen(other.cstring) + 1];
        std::strcpy(tmp_cstring, other.cstring);
        delete[] cstring;
        cstring = tmp_cstring;
        return *this;
    }
    rule_of_five& operator=(rule_of_five&& other) // move assignment
    {
        delete[] cstring;
        cstring = other.cstring;
        other.cstring = nullptr;
        return *this;
    }
// alternatively, replace both assignment operators with 
//  rule_of_five& operator=(rule_of_five other)
//  {
//      std::swap(cstring, other.cstring);
//      return *this;
//  }
};

Unlike Rule of Three, failing to provide move constructor and move assignment is usually not an error, but a missed optimization opportunity.

Rule of zero

Classes that have custom destructors, copy/move constructors or copy/move assignment operators should deal exclusively with ownership (which follows from the Single Responsibility Principle). Other classes should not have custom destructors, copy/move constructors or copy/move assignment operators.

class rule_of_zero
{
    std::string cppstring;
 public:
    rule_of_zero(const std::string& arg) : cppstring(arg) {}
};

When a base class is intended for polymorphic use, its destructor may have to be declared public and virtual. This blocks implicit moves (and deprecates implicit copies), and so the special member functions have to be declared as defaulted

class base_of_five_defaults
{
 public:
    base_of_five_defaults(const base_of_five_defaults&) = default;
    base_of_five_defaults(base_of_five_defaults&&) = default;
    base_of_five_defaults& operator=(const base_of_five_defaults&) = default;
    base_of_five_defaults& operator=(base_of_five_defaults&&) = default;
    virtual ~base_of_five_defaults() = default;
};

however, this can be avoided if the objects of the derived class are not dynamically allocated, or are dynamically allocated only to be stored in a std::shared_ptr (such as by std::make_shared): shared pointers invoke the derived class destructor even after casting to std::shared_ptr<Base>.