11. Operators

Lec 11: Operators

When you are using overloaded operators, you are actually calling member functions:

##include <iostream>
##include <vector>
##include <string>
using namespace std;

void human_readable_form() {
    vector<string> v{"Hello", "World"};
    cout << v[0];
    v[1] += "!";
}

void member_function_form() {
    vector<string> v{"Hello", "World"};
    cout.operator<<(v.operator[](0));
    v.operator[](1).operator+=("!");
}

void non_member_function_form() {
    // Note: there are some operators that can't be written in non-member-function form.
    vector<string> v{"Hello", "World"};
    operator<<(cout, operator[](v, 0));
    operator+=(operator[](v, 1), "!");
}

A Simple Implementation

// ...

class StringVector {
private:
    vector<string> data;
public:
    StringVector () = default;

    StringVector& operator+= (const string& str);
    StringVector& operator+= (const StringVector& vec);

    // Note: don't use `string& operator[] const (const int& index);`
    // Since `const` modifier doesn't allow you to return a
    // non-const reference to a member variable
    string& operator[] (const int& index);

    int size();

};

int StringVector::size()
{
    return data.size();
}

// Member function style
StringVector& StringVector::operator+= (const string& str)
{
    data.push_back(str);
    return *this;
}

StringVector& StringVector::operator+= (const StringVector& vec)
{
    data.insert(data.end(), vec.data.cbegin(), vec.data.cend());
    return *this;
}

// Non-member function style


string& StringVector::operator[] (const int& index)
{
    return data[index];
}

Note: We use const <type>& here, since:

1. const value can't be implicitly converted to non-const one, so we'd better use const
2. e.g. strvec += "Hello", "Hello" here is const (and also constexpr)
3. Use reference to avoid unnecessary copy.

General Rule of Thumb

1. Some operators must be implemented as members, e.g., []() -> =, due to C++ semantics.
2. Some operators must be implemented as non-members, e.g., <, if you are writing a class for the right-hand side (rhs), not the left-hand side (lhs).
3. If it's a unary operator, e.g., ++, implement it as a member.
4. If binary operator treats both operands equally (e.g., both unchanged), implement as non-member (maybe friend). Examples: +, <
5. If binary operator does not treat both operands equally (changes lhs), implement as a member (allows easy access to lhs private members). Examples: +=

friend modifier

Imagine you have a Fraction class:

class Fraction {
private:
    int denom;
    int nom;
};

And you want to use cout to output it. By "General Rule of Thumb", you are writing a class for the rhs (i.e. Fraction instead of ostream), you have to use non-member.

ostream& operator<< (ostream& os, const Fraction& f)
{
    // Error! `nom` and `denom` are private!
    os << f.nom << "/" << f.denom;
    return os;
}

However, nom and denom are private members in Fraction, so it seems that you can't use them in non-members without extra modifier(s).

That's why we have to introduce friend modifier for adding non-member functions to the friends list of the target class.

class Fraction {
private:
    int denom;
    int nom;
    friend ostream& operator<< (ostream& os, const Fraction& f);
};

Const-ness and others

Take operator<< as example:

1. ostream& return type, because

2. we have to chain <<'s, so ostream and & (instead of void)

3. it will change, so non-const

4. we aren't creating new objects, so &

5. ostream& os, because

6. it will change, so non-const

7. const Fraction& f, because

8. it will merely be copied and not change, so const

Principle of Least Astonishment (POLA)

"If a necessary feature has a high astonishment factor, it may be necessary to redesign the feature".

1. Design operators primarily to mimic conventional usage.
2. example 1: += returns a reference to the lhs object
3. counterexample 1: overloading ,
4. counterexample 2: overloading + for Time with sec, min, hour, ..., since we don't know whether +1 is adding a sec or a day, etc
5. Use nonmember functions for symmetric operators.
6. to avoid non-symmetric cases like
   • "char" + str: invalid
   • str + "char": valid
7. Always provide all out of a set of related operators.
8. 
9. e.g. Implement all ==, !=, <, >, <=, >=, <=> (if the class supports) and not just some of them