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operator overloading

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Revision as of 07:50, 10 June 2014 by 129.16.147.118 (Talk)

Customizes the C++ operators for operands of user-defined types.

Contents

Syntax

Overloaded operators are functions with special function names:

operator op (1)
operator type (2)
operator new (3)
operator delete (4)
operator "" suffix-identifier (5) (since C++11)
1) overloaded operator

Overloaded operators

When an operator appears in an expression, and at least one of its operands has a class type or an enumeration type, then overload resolution is used to determine the user-defined function to be called among all the functions whose signatures match the following:

Expression As member function As non-member function Example
@a (a).operator@ ( ) operator@ (a) !std::cin calls std::cin.operator!()
a@b (a).operator@ (b) operator@ (a, b) std::cout << 42 calls std::cout.operator<<(int)
a=b (a).operator= (b) cannot be non-member std::string s; s = "abc"; calls std::string.operator=(const char*)
a[b] (a).operator[](b) cannot be non-member std::map<int, int> m; m[1] = 2; calls m.operator[](int)
a-> (a).operator-> ( ) cannot be non-member std::unique_ptr<S> ptr(new S); ptr->bar() calls ptr.operator->()
a@ (a).operator@ (0) operator@ (a, 0) std::vector<int>::iterator i = v.begin(); i++ calls i.operator++(0)

Note: for overloading user-defined conversion functions, user-defined literals, allocation and deallocation see their respective articles.

Overloaded operators (but not the built-in operators) can be called using function notation:

std::string str = "Hello, ";
str.operator+=("world");                       // same as str += "world";
operator<<(operator<<(std::cout, str) , '\n'); // same as std::cout << str << '\n';

Restrictions

  • The operators :: (scope resolution), . (member access), .* (member access through pointer to member), and ?: (ternary conditional) cannot be overloaded
  • New operators such as **, <>, or &| cannot be created
  • The overloads of operators &&, ||, and , (comma) lose their special properties: short-circuit evaluation and sequencing.

Canonical implementations

The language puts no restrictions on what the overloaded operators do, or on the return type (it does not participate in overload resolution), but in general, overloaded operators are expected to behave as similar as possible to the built-in operators: operator+ is expected to add, rather than multiply its arguments, operator= is expected to assign, etc. The related operators are expected to behave similarly (operator+ and operator+= do the same addition-like operation). The return types are limited by the expressions in which the operator is expected to be used: for example, assignment operators return by reference to make it possible to write a=b=c=d, because the built-in operators allow that. Commonly overloaded operators have the following typical, canonical forms:[1]

Assignment operator

The assignment operator (operator=) has special properties: see copy assignment and move_assignment for details. To summarize, the canonical "universal assignment operator" implementation is

T& T::operator=(T arg) { // copy/move constructor is called to construct arg
    swap(arg);    // resources exchanged between *this and arg
    return *this;
}  // destructor is called to release the resources formerly held by *this

When there are resources that can be reused in assignment, for example, if the class owns a heap-allocated array, then copy-assignment between arrays of the same size can avoid allocation and deallocation:

T& operator=(const T& other) // copy assignment
{
    if (this != &other) { // self-assignment check expected
        if (/* storage cannot be reused (e.g. different sizes) */)
        { 
            delete[] mArray;            // destroy storage in this
            mArray = new int[/*size*/]; // create storage in this
        }
        // copy data from other's storage to this storage
    }
    return *this;
}
T& operator=(T&& other) // move assignment
{
    assert(this != &other); // self-assignment check not required
    delete[] mArray;        // delete this storage
    mArray = other.mArray;  // move
    other.mArray = nullptr; // leave moved-from in valid state
    return *this;
}

Stream extraction and insertion

The overloads of operator>> and operator<< that take a std::istream& or std::ostream& as the left hand argument are known as insertion and extraction operators. Since they take the user-defined type as the right argument (b in a@b), they must be implemented as non-members.

std::ostream& operator<<(std::ostream& os, const T& obj)
{
  // write obj to stream
  return os;
}
std::istream& operator>>(std::istream& is, T& obj)
{
  // read obj from stream
  if( /* T could not be constructed */ )
    is.setstate(std::ios::failbit);
  return is;
}

These operators are sometimes implemented as friend functions.

Function call operator

When a user-defined class overloads the function call operator, operator(), it becomes a FunctionObject type. Many standard algorithms, from std::sort to std::accumulate accept objects of such types to customize behavior. There are no particularly notable canonical forms of operator(), but to illustrate the usage

struct Sum {
    int sum;
    Sum() : sum(0) {}
    void operator()(int n) { sum += n; }
};
Sum s = std::for_each(v.begin(), v.end(), Sum());

Increment and decrement

When the postfix increment and decrement appear in an expression, the corresponding user-defined function (operator++ or operator--) is called with an integer argument 0. Typically, it is implemented as T operator++(int), where the argument is ignored. The postfix increment and decrement operator is usually implemented in terms of the prefix version:

struct X {
    X& operator++() {
        // actual increment takes place here
        return *this;
    }
    X operator++(int) {
        X tmp(*this); // copy
        operator++(); // pre-increment
        return tmp;   // return old value
    }
};

Although canonical form of preincrement/predecrement returns a reference, as with any operator overload, the return type is user-defined; for example the overloads of these operators for std::atomic return by value.

Binary arithmetic operators

Binary operators are typically implemented as non-members to maintain symmetry (for example, when adding a complex number and an integer, if operator+ is a member function of the complex type, then only complex+integer would compile, and not integer+complex). Since for every binary arithmetic operator there exists a corresponding compound assignment operator, canonical forms of binary operators are implemented in terms of their compound assignments:

class X {
  X& operator+=(const X& rhs) // compound assignment (does not need to be a member,
  {                           // but often is, to modify the private members)
    // addition of rhs to *this takes place here
    return *this; // return the result by reference
  }
};
inline X operator+(X lhs, const X& rhs) // first arg by value, second by const ref
{
  return lhs += rhs; // reuse compound assignment and return the result by value
}

Relational operators

Standard algorithms such as std::sort and containers such as std::set expect operator< to be defined, by default, for the user-provided types. Typically, operator< is provided and the other relational operators are implemented in terms of operator<.

inline bool operator< (const X& lhs, const X& rhs){ /* do actual comparison */ }
inline bool operator> (const X& lhs, const X& rhs){return rhs < lhs;}
inline bool operator<=(const X& lhs, const X& rhs){return !(lhs > rhs);}
inline bool operator>=(const X& lhs, const X& rhs){return !(lhs < rhs);}

Likewise, the inequality operator is typically implemented in terms of operator==:

inline bool operator==(const X& lhs, const X& rhs){ /* do actual comparison */ }
inline bool operator!=(const X& lhs, const X& rhs){return !(lhs == rhs);}

Array subscript operator

User-defined classes that provide array-like access typically define two overloads for operator[]: one which can be called in rvalue context such as x = a[i];, and one which can be used in lvalue context, such as a[i] = x;.

struct T {
    value_t& operator[](std::size_t idx) {
         /* actual access, e.g. return mVector[idx]; */
    };
    const value_t& operator[](std::size_t idx) const {
        // either actual access, or reuse non-const overload
        // for example, as follows:
        return const_cast<T&>(*this)[idx];
    };
};

To provide multidimensional array access semantics, e.g. to implement a 3D array access a[i][j][k] = x;, operator[] has to return a reference to a 2D plane, which has to have its own operator[] which returns a reference to a 1D row, which has to have operator[] which returns a reference to the element. To avoid this complexity, some libraries opt for overloading operator() instead, so that 3D access expressions have the Fortran-like syntax a(i,j,k) = x;

Example

#include <iostream>
 
class Fraction
{
    int gcd(int a, int b) {return b==0 ? a : gcd(b,a%b); }
    int n, d;
 public:
    Fraction(int n, int d = 1) : n(n/gcd(n,d)), d(d/gcd(n,d)) {}
    int num() const { return n; }
    int den() const { return d; }
    Fraction& operator*=(const Fraction& rhs) {
        int new_n = n*rhs.n / gcd(n*rhs.n, d*rhs.d);
        d = d*rhs.d / gcd(n*rhs.n, d*rhs.d);
        n = new_n;
        return *this;
    }
};
std::ostream& operator<<(std::ostream& out, const Fraction& f){
   return out << f.num() << '/' << f.den() ;
}
bool operator==(const Fraction& lhs, const Fraction& rhs) {
    return lhs.num() == rhs.num() && lhs.den() == rhs.den();
}
bool operator!=(const Fraction& lhs, const Fraction& rhs) {
    return !(lhs == rhs);
}
Fraction operator*(Fraction lhs, const Fraction& rhs)
{
    return lhs *= rhs;
}
 
int main()
{
   Fraction f1(3,8), f2(1,2), f3(10,2);
   std::cout << f1 << '*' << f2 << '=' << f1*f2 << '\n'
             << f2 << '*' << f3 << '=' << f2*f3 << '\n'
             << 2  << '*' << f1 << '=' << 2 *f1 << '\n';
}

Output:

3/8*1/2=3/16
1/2*5/1=5/2
2*3/8=3/4

See Also

Common operators
assignment increment
decrement
arithmetic logical comparison member
access
other

a = b
a += b
a -= b
a *= b
a /= b
a %= b
a &= b
a |= b
a ^= b
a <<= b
a >>= b

++a
--a
a++
a--

+a
-a
a + b
a - b
a * b
a / b
a % b
~a
a & b
a | b
a ^ b
a << b
a >> b

!a
a && b
a || b

a == b
a != b
a < b
a > b
a <= b
a >= b

a[b]
*a
&a
a->b
a.b
a->*b
a.*b

a(...)
a, b
(type) a
? :

Special operators

static_cast converts one type to another compatible type
dynamic_cast converts virtual base class to derived class
const_cast converts type to compatible type with different cv qualifiers
reinterpret_cast converts type to incompatible type
new allocates memory
delete deallocates memory
sizeof queries the size of a type
sizeof... queries the size of a parameter pack (since C++11)
typeid queries the type information of a type
noexcept checks if an expression can throw an exception (since C++11)
alignof queries alignment requirements of a type (since C++11)

References

  1. Operator Overloading on StackOverflow C++ FAQ