Difference between revisions of "cpp/language/rule of three"

Revision as of 13:05, 7 August 2023

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 manages 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).

#include <cstddef>
#include <cstring>
#include <iostream>
 
class rule_of_three
{
    char* cstring; // raw pointer used as a handle to a
                   // dynamically-allocated memory block
 
    rule_of_three(const char* s, std::size_t n) // to avoid counting twice
        : cstring(new char[n]) // allocate
    {
        std::memcpy(cstring, s, n); // populate
    }
 
public:
    explicit rule_of_three(const char* s = "")
        : rule_of_three(s, std::strlen(s) + 1) {}
 
    ~rule_of_three() // I. destructor
    {
        delete[] cstring; // deallocate
    }
 
    rule_of_three(const rule_of_three& other) // II. copy constructor
        : rule_of_three(other.cstring) {}
 
    rule_of_three& operator=(const rule_of_three& other) // III. copy assignment
    {
        if (this == &other)
            return *this;
 
        std::size_t n{std::strlen(other.cstring) + 1};
        char* new_cstring = new char[n];            // allocate
        std::memcpy(new_cstring, other.cstring, n); // populate
        delete[] cstring;                           // deallocate
        cstring = new_cstring;
 
        return *this;
    }
 
    operator const char* () const // accessor
    {
        return cstring;
    }
};
 
int main()
{
    rule_of_three o1{"abc"};
    std::cout << o1 << ' ';
    auto o2{o1}; // II. uses copy constructor
    std::cout << o2 << ' ';
    rule_of_three o3("def");
    std::cout << o3 << ' ';
    o3 = o2; // III. uses copy assignment
    std::cout << o3 << '\n';
}   // <- I. all destructors are called 'here'

abc abc def abc

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 (or = default or = delete declared) 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:
    explicit rule_of_five(const char* s = "") : cstring(nullptr)
    { 
        if (s)
        {
            std::size_t n = std::strlen(s) + 1;
            cstring = new char[n];      // allocate
            std::memcpy(cstring, s, n); // populate 
        } 
    }
 
    ~rule_of_five()
    {
        delete[] cstring; // deallocate
    }
 
    rule_of_five(const rule_of_five& other) // copy constructor
        : rule_of_five(other.cstring) {}
 
    rule_of_five(rule_of_five&& other) noexcept // move constructor
        : cstring(std::exchange(other.cstring, nullptr)) {}
 
    rule_of_five& operator=(const rule_of_five& other) // copy assignment
    {
        return *this = rule_of_five(other);
    }
 
    rule_of_five& operator=(rule_of_five&& other) noexcept // move assignment
    {
        std::swap(cstring, other.cstring);
        return *this;
    }
 
// alternatively, replace both assignment operators with 
//  rule_of_five& operator=(rule_of_five other) noexcept
//  {
//      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^[1].

This rule also appears in the C++ Core Guidelines as C.20: If you can avoid defining default operations, do.

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^[2].

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 makes the class prone to slicing, which is why polymorphic classes often define copy as deleted (see C.67: A polymorphic class should suppress public copy/move in C++ Core Guidelines), which leads to the following generic wording for the Rule of Five:

C.21: If you define or =delete any default operation, define or =delete them all.

External links