8.4. Operator overloads

There are many reasons to want to overload the builtin operator functions for user defined types.

Case 1

Operator overloads are required for some containers in the STL. Notably set, which requires any type used in a set overloads operator<, or the operator defined for the set if a custom comparison function is defined. Similarly, map requires operator< and unordered_map requires both that and operator==.

Classes that do not overload these comparison operators can’t be used in these containers.

Overloading these operators is useful in any conditional expression, but only makes sense when creating a class with data members that are comparable.

Case 2

C++ provides many builtin types and operators to manipulate them. We want new user defined types to be as simple and intuitive to use as the builtin types.

For example, finding the roots of a quadratic equation:

double root1 = (-b + sqrt(b*b - (4 * a * c))) / (2 * a);

When all the variables are builtin types, this looks familiar. But without operator overloads, a rational number class would be a pain to read and use.

Complex  root = Complex(0,0);
Rational a    = Rational(2,3);
Rational b    = Rational(6,1);
Rational c    = Rational(5,3);
Complex tmp   = sqrt(b.multiply(b)
                      .minus(c.multiply(a.multiply(4))));
root.setComplex(b.invSign().plus(tmp.divide(a.multiply(2))));

When everything has to be a named function, and the standard operators can only be used on builtin types, the result is not as clean as we would like.

What we want here is the ability to use familiar semantics on user defined types.

Rational a {2,3}, b{6,1}, c{5,3};
Complex root1 = (-b + sqrt(b*b - (4 * a * c))) / (2 * a);

Overloading the standard operator functions make this possible.

In C++, operators are overloaded in the form of functions with special names. For example, a+b and operator+(a,b) both call the same function.

As with other functions, overloaded operators can generally be implemented either as a member function of their left operand’s type or as non-member functions.

Most of the work in overloading operators is boiler-plate code. Not surprising, since most operators defer their actual work to plain functions. The programming community has already thought long and hard on techniques to make overloads efficient and easy to maintain.

8.4.1. Basic overloading guidelines

Most C++ operators can be overloaded. You cannot change the meaning of operators for built-in types in C++. Operators can only be overloaded when at least 1 operand is a user-defined type. Other rules of overloads still apply: overloads for a specific function signature can only be used once.

Not all operators can be overloaded in C++. Those that cannot be overloaded are: operator ., operator ::, operator sizeof, operator typeid, operator .*, and operator ? :.

When it comes to operator overloading in C++, there are basic guidelines you should follow. As with all such guidelines, there are exceptions. The goal of operator overloading is to make classes easier to use and to make them behave more like builtin types. Keep that in mind when overloading.

  1. Whenever the meaning of an operator is not obviously clear and undisputed, it should not be overloaded.

    Instead, provide a function with a well-chosen name. It is hard to understand the semantics behind the operator unless the use of the operator in the application domain is well known and undisputed. Contrary to popular belief, this is hardly ever the case.

  2. Always stick to the well-known semantics for the operator.

    C++ poses no limitations on the semantics of overloaded operators. Your compiler will happily accept code that implements the binary + operator to subtract from its right operand. However, the users of such an operator would never suspect the expression a + b to subtract a from b. Of course, this supposes that the semantics of the operator in the application domain is undisputed.

  3. Always provide all out of a set of related operations.

    Operators are related to each other and to other operations.

    • If your type supports a + b, then users will expect to be able to call a += b, too.

    • If it supports prefix increment ++a, then a++ is likely expected also.

    • If they can check whether a < b, then most users expect to also to be able to check a > b.

    • If copy-construction is allowed, then assignment is expected.

The remaining sections describe the specific techniques and function signatures for some of the most common operator overloads.

8.4.2. Assignment operator

The copy assignment operator is called when an object appears on the left side of an assignment expression.

The canonical copy-assignment operator is expected to perform no action on self-assignment, and to return the lhs by reference:

T& T::operator=(const T& other)
{
  // copy data from other to
  // current instance
  return *this;
}

In those situations where copy assignment cannot benefit from resource reuse (it does not manage a heap-allocated array and does not have a member that does, such as a member std::vector or std::string), there is a popular convenient shorthand: the copy-and-swap assignment operator, which takes its parameter by value swaps with the parameter, and lets the destructor clean it up.

// copy/move constructor is called to construct other
T& T::operator=(T other) noexcept
{
    using std::swap;
    swap(*this, other); // exchange resources between *this and other
    return *this;
}

When the function ends, the destructor of other is called to release the resources formerly held by *this. A custom swap function for the user defined type T is required for this technique to work:

friend void swap(T& first, T& second)
{
  using std::swap;

  // by swapping the members of two objects,
  // the two objects are effectively swapped
  swap(first.size_, second.size_);
  swap(first.value_, second.value_);
  // repeat for each member
}

8.4.3. Insertion and extraction operators

The bitshift operators << and >>, although still used in hardware interfacing for the bit-manipulation functions they inherit from C, have become more prevalent as formatted stream operators in C++.

The overloads of operator >> and operator << that take a std::istream reference or std::ostream reference as the left hand argument are known as insertion and extraction operators.

Since these operators change their left argument (they alter the stream), they should, according to the rules of thumb, be implemented as members of their left operand’s type. However, their left operands are streams from the standard library, and while most of the stream output and input operators defined by the standard library are indeed defined as members of the stream classes, when you implement output and input operations for your own types, you cannot change the standard library’s stream types. So clearly, these overloads cannot be stream member functions.

It is common, however, to see C++ examples posted on the internet that define these overloads as friends. The advantage of making these functions friends is they have access to the private data of the class, if needed. The disadvantage of making these functions friends is that you may decide to stream data out of a class that is otherwise not accessible.

Why is that bad?

Josh Bloch in Effective Java, dedicates an entire section to this topic. Effective Java discusses the toString() overload, but it serves a similar purpose to operator << in the Java language.

When you create an output function that streams data out of your class that is not available through any other function, then some users will use your stream function just to get their hands on your private data.

  • If your data needs to be part of the output stream, then make a function to access it.

  • If you have functions that provide access to every private member that is part of the stream, then you don’t need it to be a member function or a friend.

That’s why you need to implement these operators for your own types as non-member non-friend functions. The canonical forms are:

std::ostream& operator<<(std::ostream& os, const T& rhs)
{
  // write rhs to stream

  return os;
}

std::istream& operator>>(std::istream& is, T& rhs)
{
  // read rhs from stream

  if( /* could not construct T from stream */ ) {
    is.setstate(std::ios::failbit);
  }

  return is;
}

8.4.4. Function call operator

When a user-defined class overloads the function call operator, operator(), it becomes a FunctionObject type. A function object is a class that can be called as if it was a function. Many standard algorithms, from sort` to accumulate accept objects of such types to customize behavior. Prior to the C++11 additions of function and lambda expressions, function objects were an important way to pass functions to algorithms.

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());

Note

Unlike lambda expressions and function pointers, when passing a function object to an algorithm, the function call operator must be included.

A function call overload can be overloaded to take any number of additional arguments, including zero. A single class can contain more than 1 function call operator overload, subject to the other rules of function overloading.

8.4.5. Relational operators

Standard algorithms such as std::sort and containers such as set expect operator < to be defined, by default, for the user-provided types, and expect it to implement strict std::weak_ordering. Strict weak ordering defines members of a set as comparable to each other. The general signature for these non-member functions is:

inline bool operator<(const T& lhs, const T& rhs)
{
   // compare the data in left-hand side and right-hand side objects
   // for less than
}

inline bool operator==(const T& lhs, const T& rhs)
{
   // compare the data in left-hand side and right-hand side objects
   // for equality
}

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;
};

inline bool operator<(const Record& lhs, const Record& rhs)
{
   // parameters passed to each tie must be in the same order
   // or this will always return false
   return std::tie(lhs.name, lhs.floor, lhs.weight)
        < std::tie(rhs.name, rhs.floor, rhs.weight);
}

If some of the data required for the comparison is private and has no function to access the data members, then you may need to make your relational operators friends.

If you do need to define operator< as a member function, then the left-hand side of the operator will be the this pointer. The signature of the operator overload changes:

bool operator<(const T& rhs) const
{
   /* do actual comparison with *this */
}

Note that this form of the overload must be const in order to compile as a member function.

Once you have defined operator< and operator==, there is no need to rewrite the comparison logic again. It is much better to implement the remaining comparison functions in terms of < and ==.

// note the operands swapped inside the function body
inline bool operator> (const T& lhs, const T& rhs){ return   rhs < lhs; }

inline bool operator<=(const T& lhs, const T& rhs){ return !(lhs > rhs); }
inline bool operator>=(const T& lhs, const T& rhs){ return !(lhs < rhs); }

inline bool operator!=(const T& lhs, const T& rhs){ return !(lhs == rhs); }

Note

Since C++20, all 6 comparison operators are defined if the three-way comparison operator operator<=> is defined, and that operator, in turn, is generated by the compiler if it is defined as defaulted:

struct Record
{
    std::string name;
    unsigned int floor;
    double weight;
    auto operator<=>(const Record&) = default;
};
// records can now be compared with ==, !=, <, <=, >, and >=

8.4.6. 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 addition doesn’t behave in a way most people expect:

complex a = {1,1};
int b = 3;
complex c = a+b;  // compiles
complex d = b+a;  // error

As a member function, only complex+integer would compile, 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 T
{
 public:
  T& operator+=(const T& rhs) // compound assignment (does not need to be a member,
  {                           // but often is, to modify the private members)
    // add rhs to *this
    return *this;             // return the result by reference
  }
};

The normal addition is often implemented as a non-friend non-member:

T operator+(      T lhs,    // passing lhs by value helps optimize chained a+b+c
            const T& rhs)   // otherwise, both parameters may be const references
{
  lhs += rhs; // reuse compound assignment
  return lhs; // return the result by value (uses move constructor)
}

The remaining binary arithmetic operators are implemented using the same pattern.

8.4.7. Increment and decrement operators

Unlike many of the operator overloads in this section, the increment and decrement operators are unary operators – only one operand, the current object, is involved.

The postfix increment and decrement operator is usually implemented in terms of the prefix version:

struct T
{
    // prefix increment
    T& operator++()
    {
        // do the actual increment here
        return *this;
    }

    // postfix increment
    const T operator++(int)
    {
        T tmp(*this); // copy
        operator++(); // pre-increment
        return tmp;   // return old value
    }
};

There are a couple of things to notice about postfix increment:

  • It returns a constant copy of the previous object value.

    Any modifications made to the returned object after calling operator++(int) would be modifying a temporary object. This is always bad. Returning a const object prevents accidental changes to a temporary object. This overload should never return a reference.

  • It takes a ‘dummy parameter’ of type int.

    The ‘dummy’ parameter simply allows two functions with the same name – operator++ to have different overloads and therefore different behaviors. 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.

8.4.8. Conversion operators

C++ allows you to create operators to convert between your type and other ADT’s. A conversion function is declared like a non-static member function or member function template with

  • no parameters,

  • no explicit return type, and

  • with the name of the form:

    // implcit conversion
operator int() const { /* return int version of type */ }

// explicit conversion
explicit operator int() const { /* return int version of type */ }

Suppose we want to concatenate a Rational to a string?

Rational a {2,3};
std::string s = {"A = "};
s += a;                   // will not compile

Your compiler may present something like:

error: no viable overloaded '+='
candidate function not viable:
no known conversion from 'Rational' to
'const std::__1::basic_string<char>' for 1st argument
_LIBCPP_INLINE_VISIBILITY basic_string& operator+=(const basic_string...

Defining a conversion operator for std::string allows this conversion to happen:

Rational::operator std::string() const {
   std::stringstream ss;
   ss << numerator << '/' << denominator;
   return ss.str();
}

A class constructor taking a single argument can also be seen as converting its argument into the type:

Rational (int x) {
   numerator = x;
   denominator = 1;
}

And an expression like this works:

Rational r = 3;

The conversion both of these previous cases, for the string and the int happened implicitly. Often, we don’t want types to always implicitly convert to a user-defined type:

void func (Rational r);

int main () {
   func(3);   // this works
}

The call to func() works because Rational has a conversion constructor that converts int values to Rational ones. C++ provides a keyword explicit that requires a cast - it inhibits the implicit conversion of a user defined type.

explicit Rational (int x) { . . .

explicit operator std::string() const { . . .

And the previous expressions need casts or an explicit constructor call:

Rational r = Rational(3);       // constructor
func(Rational(3));              // constructor
func(static_cast<Rational>(3)); // cast
func((Rational)3);              // c-style cast

// explicitly convert a Rational to string
// using functional conversion syntax
std::string s = string(Rational(3));

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