CS253 Functors

Please pronounce carefully.

A Method

Consider this class:

class Foo {
  public:
    int thrice(int n) { return n*3; }
};

Foo f;
cout << f.thrice(42) << '\n';

int thrice(int n)
- return type: int
- name of the method: thrice
- argument list: (int n)

An operator function

Let’s use the unary ~ operator. It’s defined as bit-wise complement for integers.

class Foo {
  public:
    const char *operator~() { return "hello"; }
};

Foo f;
cout << ~f << '\n';

hello

const char *operator~()
- return type: const char *
- name of the method: operator~
- argument list: ()
Not to be confused with the destructor, which is named ~Foo.

Another operator function

Let’s use the unary postfix () operator, alias the “function call” operator:

class Foo {
  public:
    double operator()(int i, float f) { return i+f; }
};

Foo f;
cout << f(3, 1.2) << '\n';

4.2

double operator()(int i, float f)
- return type: double
- name of the method: operator()
- argument list: (int i, float f)
f is not a function! f is a object of type Foo! Sure acts like a function, though.

Sounds useless

😒

Hooray!
We’ve created an alternate way to write a function.
It has the advantage of being more confusing.
However, it can also have state.

Another operator function

Let’s write a functor that returns its argument+1:

class BiggerBy1 {
  public:
    int operator()(int i) { return i+1; }
};

BiggerBy1 b;
cout << b(17) << '\n';

int operator()(int i)
- return type: int
- name of the method: operator()
- argument list: (int i)

Another operator function

It’s easy to change that to increment by two, instead:

class BiggerBy2 {
  public:
    int operator()(int i) { return i+2; }
};

BiggerBy2 b;
cout << b(17) << '\n';

However, it would be tedious to have to write many such functors for all the different increment values we might want to use.

A stateful functor

Let’s give the functor a ctor argument:

class Bigger {
    const int increment;
  public:
    Bigger(int inc) : increment(inc) { }
    int operator()(int i) { return i+increment; }
};

Bigger b(4), c(12);
cout << b(17) << ' ' << b(100) << '\n'
     << b(1000) << ' ' << c(2000) << '\n';

21 104
1004 2012

The parentheses in b(4) are very different than the parentheses in b(17).
- b(4) constructs an object of type Bigger, named b.
- b(17) is b.operator()(17).

Another stateful functor

Another use of memory:

class Queue {
    int previous=0;
  public:
    int operator()(int n) {
        const auto save=previous;
        previous=n;
        return save;
    }
};

Queue q;
cout << q(12)  << '\n';
cout << q(100) << '\n';
cout << q(42)  << '\n';

0
12
100

Another stateful functor

Why does this fail?

class Queue {
    int previous=0;
  public:
    int operator()(int n) {
        const auto save=previous;
        previous=n;
        return save;
    }
};

Queue q;
cout << q(12)  << '\n'
     << q(100) << '\n'
     << q(42)  << '\n';

0
12
100

The order of the three operator() calls is unspecified.

Even Queueier!

Multi-element queue:

class Queue {
    deque<int> store;
  public:
    Queue(int init_size) : store(init_size) { }
    int operator()(int n) {
        store.push_back(n);
        const auto f = store.front();
        store.pop_front();
        return f;
    }
};

Queue q(3);
for (int i=10; i<20; i++)
    cout << q(i) << '\n';

Or …

class Total {
    int sum=0;
  public:
    int operator()(int n) {
        return sum += n;
    }
};

Total t;
for (int i=0; i<=10; i++)
    cout << "Add " << i << " yielding " << t(i) << '\n';

Add 0 yielding 0
Add 1 yielding 1
Add 2 yielding 3
Add 3 yielding 6
Add 4 yielding 10
Add 5 yielding 15
Add 6 yielding 21
Add 7 yielding 28
Add 8 yielding 36
Add 9 yielding 45
Add 10 yielding 55

Sorting with Functors

How does a container like set know the sorting order?
Sure, it’s easy to answer for int and string, but what about class Student?
- Sort by major?
- Sort by eId?
- Sort by last name, then first name, then CSUID?
Besides, even for int, you might want to sort in a different order.
You might even want multi-key sorting, like a phone book:
- primary key: last name
- secondary key: first name
- tertiary key: middle name
- quaternary key: street name
- quinary key: house number

The Idiot Savant

Imagine that you had a semi-brilliant assistant who was very good at sorting.
However, your assistant doesn’t know alphabetical order.
He can do everything that’s needed for sorting except actually determine if, say, “Jay” comes before “David”.
Therefore, he frequently comes to you, asking if this word comes before this other word.
set is like that. It knows the mechanics of sorting, but it doesn’t know what order to use. You need to help it.

Comparison functor

You provide set with a comparison functor.
The set code makes comparisons using the functor.
The functor only compares two items at a time.
- I say again, two items at a time. Only two. The functor compares—it doesn’t sort.
The functor returns true iff the first item should come before the second item.
Why can’t you use a comparison function, as opposed to a comparison functor?
Because the set template takes types, not objects.

Implicit/explicit

Implicit sorting criterion:

set<int> s = {31, 49, 41, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

23 26 31 41 49 53 58 84 93 97

Explicit sorting criterion:

set<int, less<int> > s = {31, 49, 41, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

23 26 31 41 49 53 58 84 93 97

What’s this > > folderol? An old problem with the >> token.

Reversed

Use greater()<int> to sort from biggest to smallest:

set<int, greater<int>> s = {31, 49, 41, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

97 93 84 58 53 49 41 31 26 23

Explicitly explicit

Use a comparison functor that behaves the same as less()<int>:

struct compare {                            // 😱 😱 struct‽ 😱 😱
    bool operator()(int a, int b) const {
        return a < b;
    }
};

set<int, compare> s = {31, 49, 41, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

23 26 31 41 49 53 58 84 93 97

Two-level

Use a comparison functor that does a two-level comparison, like last name/first name.

struct compare {
    bool operator()(int a, int b) const {
        int da = abs(a-45), db = abs(b-45);
        if (da != db)
            return da < db; // Primary sort: distance from 45
        return a < b;       // Secondary sort: value of number
    }
};

set<int, compare> s = {31, 49, 41, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

41 49 53 58 31 26 23 84 93 97

Ternary operator

Another way to write a two-level comparison functor:

struct compare {
    bool operator()(int a, int b) const {
        int da = abs(a-45), db = abs(b-45);
        return (da != db) ? da < db : a < b;
    }
};

set<int, compare> s = {31, 49, 41, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

41 49 53 58 31 26 23 84 93 97

Curious criteria

This sorts by different criteria:

struct compare {
    bool operator()(int a, int b) const {
        int last_a = a%10, last_b = b%10;
        if (last_a != last_b)
            return last_a < last_b; // Primary sort: by last digit
        return a < b;               // Secondary sort: value of number
    }
};

set<int, compare> s = {31, 49, 41, 26, 53, 58, 97, 93, 23, 84};
for (auto v : s)
    cout << v << ' ';

31 41 23 53 93 84 26 97 58 49

Multi-level

struct student {
    int id; string name; float gpa;
};
struct compare {
    bool operator()(const student &a, const student &b) const {
        return (a.gpa != b.gpa) ? a.gpa > b.gpa : a.id < b.id;
    }
};
set<student,compare> s = {
    {81234567, "Jack Applin",      3.9},
    {82828282, "Craig Partridge",  0.5},
    {84444444, "Homecoming Queen", 2.2},
    {83333333, "Joe Kolledge",     2.2},
};
for (const auto &v : s)
    cout << v.id << ' ' << v.gpa << ' ' << v.name << '\n';

81234567 3.9 Jack Applin
83333333 2.2 Joe Kolledge
84444444 2.2 Homecoming Queen
82828282 0.5 Craig Partridge

Not all

Of course, not all collections require a comparison functor.
vector, array, and list just go in the order you specify, and so have no comparison functor.
unordered_set needs only an equality comparison (and a hash functor), so its comparison functor defaults to equal_to(), not less().

Example #1

We’re going to build up to using functors and λ-expressions.

char embiggen(char c) {
    if ('a' <= c && c <= 'z')
        return c - 'a' + 'A';
    else
        return c;
}

int main() {
    string name = "Beverly Hills Chihuahua";
    for (char &c : name)    // & for reference
        c = embiggen(c);
    cout << name << '\n';
}

BEVERLY HILLS CHIHUAHUA

Example #2

Use a ternary expression, instead:

char embiggen(char c) {
    return ('a'<=c && c<='z') ?  c-'a'+'A' : c;
}

int main() {
    string name = "Beverly Hills Chihuahua";
    for (char &c : name)
        c = embiggen(c);
    cout << name << '\n';
}

BEVERLY HILLS CHIHUAHUA

Example #3

Use the transform() algorithm, rather than an explicit loop:

char embiggen(char c) {
    return ('a'<=c && c<='z') ?  c-'a'+'A' : c;
}

int main() {
    string name   = "Beverly Hills Chihuahua";
    string result = name;       // Why?
    transform(name.begin(), name.end(),
              result.begin(), embiggen);
    cout << result << '\n';
}

BEVERLY HILLS CHIHUAHUA

Example #4

This example uses the same buffer for input & output of transform().

char embiggen(char c) {
    return ('a'<=c && c<='z') ?  c-'a'+'A' : c;
}

int main() {
    string name = "Beverly Hills Chihuahua";
    transform(name.begin(), name.end(),
              name.begin(), embiggen);
    cout << name << '\n';
}

BEVERLY HILLS CHIHUAHUA

Example #5

This code uses an actual functor, as opposed to a function.

class embiggen {
  public:
    char operator()(char c) {
        return ('a'<=c && c<='z') ?  c-'a'+'A' : c;
    }
};

string name = "Beverly Hills Chihuahua";
embiggen biggifier;
transform(name.begin(), name.end(),
          name.begin(), biggifier);
cout << name << '\n';

BEVERLY HILLS CHIHUAHUA

Example #6

Create a temporary functor object:

class embiggen {
  public:
    char operator()(char c) {
        return ('a'<=c && c<='z') ?  c-'a'+'A' : c;
    }
};

string name = "Beverly Hills Chihuahua";

transform(name.begin(), name.end(),
          name.begin(), embiggen());

cout << name << '\n';

BEVERLY HILLS CHIHUAHUA

Example #7

Instead of a functor, a lambda-expression can also be used.

string name = "Beverly Hills Chihuahua";

transform(name.begin(), name.end(),
          name.begin(),
          [](char c){ return 'a'<=c && c<='z' ? c-'a'+'A' : c; }
          );

cout << name << '\n';

BEVERLY HILLS CHIHUAHUA

Example #8

Don’t re-invent the wheel:

string name = "Beverly Hills Chihuahua";

for (char &c : name)
    c = toupper(c);

cout << name << '\n';

BEVERLY HILLS CHIHUAHUA

CS253: Software Development with C++

Spring 2020

Functors

CS253 Functors

A Method

An operator function

Another operator function

Sounds useless

Another operator function

Another operator function

A stateful functor

Another stateful functor

Another stateful functor

Even Queueier!

Or …

Sorting with Functors

The Idiot Savant

Comparison functor

Implicit/explicit

Reversed

Explicitly explicit

Two-level

Ternary operator

Curious criteria

Multi-level

Not all

Example #1

Example #2

Example #3

Example #4

Example #5

Example #6

Example #7

Example #8