Overloading Operators

This lesson is a quick introductory tour of operator overloading within C++. It is not intended for those who are entirely new to programming. Rather, the people who may find it useful include:

• those who have completed our introductory course, but want a quick review
• those who are already familiar with programming in another language, but new to C++
• those who have used C++ in the past, but would benefit from a refresher

It summarises several lessons from our introductory course. Anyone looking for more thorough explanations or additional context should consider Chapter 8 of that course.

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We can overload operators like + and *= for our custom types. Operators are implemented in the same way as functions, with a specific naming convention.

For example, if we had a custom type called Vector, and we wanted to give Vectors the ability to be added to other Vectors using the + operator, we would define a function called operator+ as follows:

1Vector operator+(const Vector& a,
2                 const Vector& b){
3  return Vector{
4    a.x + b.x, a.y + b.y, a.z + b.z};
5}
6

1Vector a { 1.f, 2.f, 3.f };
2Vector b { 1.f, 1.f, 1.f };
3
4// { 2.f, 3.f, 4.f }
5Vector Sum { a + b };
6

Operators can also be implemented as part of the struct or class that defines our type:

1struct Vector {
2  float x;
3  float y;
4  float z;
5
6  Vector operator*(float Multiplier){
7    return Vector{
8      x * Multiplier,
9      y * Multiplier,
10      z * Multiplier
11    };
12  }
13};
14

1Vector a { 1.f, 2.f, 3.f };
2
3// { 2.f, 4.f, 6.f }
4Vector Result { a * 2.f };
5

Unary and Binary Operators

Multiple operators can have the same function name. For example, operator- can be a binary operation, ie, it operates on two objects. This would normally be used to represent arithmetic subtraction (eg VectorA - VectorB)

But, - can also be a unary operator, acting upon only one object. This would normally be used for negation (eg, -VectorA)

The compiler can infer whether we are overloading a unary or binary operator from the number of parameters.

• When defined outside the class, a binary operator will have two parameters. The left operand will be the first parameter; the right operand will be the second parameter.
• When defined within the class, a binary operator will have one parameter. The left operand will be the current object; the right operand will be the parameter.
• When defined outside the class, a unary operator will have one parameter. That parameter will be the operand.
• When defined within the class, a unary operator will have no parameters. The current object will be the operand.

1#include <iostream>
2
3struct Vector {
4  float x;
5  float y;
6  float z;
7
8  Vector operator-(){
9    std::cout << "Unary - \n";
10    return Vector{-x, -y, -z};
11  }
12
13  Vector operator-(const Vector& Other){
14    std::cout << "Binary - \n";
15    return Vector{
16      x - Other.x,
17      y - Other.y,
18      z - Other.z
19    };
20  }
21};
22
23Vector operator+(const Vector& a){
24  std::cout << "Unary + \n";
25  return a;
26}
27
28Vector operator+(const Vector& a,
29                 const Vector& b){
30  std::cout << "Binary + \n";
31  return Vector{
32    a.x + b.x, a.y + b.y, a.z + b.z};
33}
34
35int main(){
36  Vector A;
37  -A;
38  A - A;
39  +A;
40  A + A;
41}
42

1Unary - 
2Binary - 
3Unary + 
4Binary +
5

The this Pointer

For many operators, such as ++ and *=, it is expected that the operator will return a reference to the object it operated on. This is important for allowing operators to be chained on our object, in expressions such as (MyObject *= 2) *= 3. With statements like this, we want subsequent operators to act upon the original object, not a copy of it.

Our previous examples didn’t allow this. However, within a class or struct function, the this keyword returns a pointer to the current object - that is, the object that the function or operator was called upon. Therefore, we can use this to ensure our operators return the exact same object they were used on:

1#include <iostream>
2
3struct Vector {
4  float x;
5  float y;
6  float z;
7
8  Vector& operator*=(float Multiplier){
9    x *= Multiplier;
10    y *= Multiplier;
11    z *= Multiplier;
12    return *this;
13  }
14};
15
16int main(){
17  Vector A{1.f, 1.f, 1.f};
18  (A *= 2) *= 3;
19  std::cout << A.x;
20}
21

1x component: 6
2

Postfix and Prefix Operators

Operators like -- and ++ can be used either before or after their operand, and each usage has different behavior.

For example, ++MyInteger would increment the integer and return it. MyInteger++ would increment the integer but return a different integer, which has the value of our original integer before it was incremented.

For our custom types, overloading the prefix ++ works as expected:

1struct Vector {
2  float x;
3  float y;
4  float z;
5
6  // ++MyVector (Prefix Operator)
7  Vector& operator++(){
8    ++x;
9    ++y;
10    ++z;
11    return *this;
12  }
13};
14

Overloading the postfix operator has a slightly inelegant syntax. To let the compiler distinguish between which function handles the prefix operator and which handles the postfix, we add an extra unused int parameter to the postfix handler:

1#include <iostream>
2
3struct Vector {
4  float x;
5  float y;
6  float z;
7
8  // ++MyVector (Prefix Operator)
9  Vector& operator++(){
10    std::cout << "Prefix ++ \n";
11    ++x;
12    ++y;
13    ++z;
14    return *this;
15  }
16
17  // MyVector++ (Postfix Operator)
18  Vector operator++(int){
19    std::cout << "Postfix ++ \n";
20    Vector Original{x, y, z};
21    ++x;
22    ++y;
23    ++z;
24    return Original;
25  }
26};
27
28int main(){
29  Vector A;
30  ++A;
31  A++;
32}
33

1Prefix ++ 
2Postfix ++
3

More Object Capabilities

Later in this course, we cover a range of further abilities we can add to our custom types. This includes how we can:

• Overload the function call operator, () to let our custom types act like functions (functors).
• Implement iterators to let our custom objects work like arrays and other collections, including compatibility with standardized algorithms and native syntax such as range-based for loops.
• Set up user-defined conversions, to let our objects be implicitly or explicitly converted to other types, including built-in C++ types
• Implement three-way comparisons and default comparisons using a single function, thereby allowing our objects to be sorted and compared using boolean operators, such as <= and !=
• Customize copy and move semantics, for control over how our objects react to low-level operations like being passed to a function, or copied to a new variable