operator overloading

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

Syntax

Overloaded operators are functions with special function names:

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:

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.
• The overload of operator -> must either return a raw pointer or return an object (by reference or by value), for which operator -> is in turn overloaded.
• It is not possible to change the precedence, grouping, or number of operands of operators.

Canonical implementations

Other than the restrictions above, the language puts no other constraints 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
            /* reset size to zero and mArray to null, in case allocation throws */
            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 pre-increment/pre-decrement 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 {
 public:
  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
  }
 
  // friends defined inside class body are inline and are hidden from non-ADL lookup
  friend X operator+(X lhs,       // passing lhs by value helps optimize chained a+b+c
                    const X& rhs) // otherwise, both parameters may be const references.
  {
     lhs += rhs; // reuse compound assignment
     return lhs; // return the result by value (uses move constructor)
  }
};

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, and expect it to implement strict weak ordering (thus satisfying the Compare concept). An idiomatic way to implement strict weak ordering for a structure is to use lexicographical comparison provided by std::tie:

struct Record {
    std::string name;
    unsigned int floor;
    double weight;
    friend bool operator<(const Record& l, const Record& r) {
        return std::tie(l.name, l.floor, l.weight)
             < std::tie(r.name, r.floor, r.weight); // keep the same order
    }
};

Typically, once operator< is provided, 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 that allows both reading and writing typically define two overloads for operator[]: const and non-const variants:

struct T {
          value_t& operator[](std::size_t idx)       { return mVector[idx]; };
    const value_t& operator[](std::size_t idx) const { return mVector[idx]; };
};

If the value type is known to be a built-in type, the const variant should return by value.

Where direct access to the elements of the container is not wanted or not possible or distinguishing between l-value c[i] = v; and r-value v = c[i]; usage, operator[] may return a proxy. see for example std::bitset::operator[].

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';
}

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

See Also

• Operator precedence
• Alternative operator syntax

References

1. Operator Overloading on StackOverflow C++ FAQ