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Show Lecture.LambdaFunctions as a slide show.

CS253 Lambda Functions

Lambda

λ

This is the Greek letter lambda.

In C++, it refers to an anonymous function, an unnamed function, a function literal.

Functions

• Functions are not first-class objects in C++.
• You can’t have a variable of type “function”, or even of type “function taking no arguments and returning int”.
• You can’t copy a function.
• However, you can have a pointer to a function.

A function

Here’s a boring ordinary function:

bool odd(int n) { return n & 01; }

int main() {
    cout << odd(42) << ' ' << odd(43) << '\n';
}

0 1

Boolean values are displayed as 0 and 1 by default, unless you use cout << boolalpha.

Pointer to a function

bool odd(int n) { return n & 01; }

int main() {
    bool (*p)(int) = odd;
    cout << (*p)(42) << ' ' << p(43) << '\n';
}

0 1

• p is a variable of type “pointer to function taking a int and returning a bool”.
• Or, p is a pointer to function taking a int and returning a bool.
• So, *p is a function that takes a int and returns a bool.
• Some people (not me!) find such declarations hard to understand.
• For convenience, you can use a pointer to a function to call it.

auto is your friend

bool odd(int n) { return n & 01; }

int main() {
    auto p = odd;
    cout << (*p)(42) << ' ' << p(43) << '\n';
}

0 1

auto sure made that declaration easier!

There must be a better way.

• It seems a shame to have to name the function odd.
• Imagine if integers worked that way:
  • You couldn’t have an integer constant.
  • All you could have would be named integers: int foo=7;
• It’s nice to have integer constants, integer literals, unnamed integers.
• If only we could have unnamed functions …

λ-expressions

auto p = [](int n) -> bool { return n & 01; };
cout << p(42) << ' ' << p(43) << '\n';

0 1

• [](int n) -> int { return n & 01; } is a lambda-expression
  • [] is the capture-specification
    • Stuff can go inside [], but we’ll ignore that.
    • Think of it as “here comes a λ-function”.
  • (int n) is the argument list
  • -> bool specifies the return type
    • optional, if the compiler can figure it out
  • { return n & 01; } is the body
• I often forget the ; at the end of the first line.

λ-expressions

The return-type may be deduced using auto:

auto p = [](int n) { return n & 01; };
cout << p(42) << ' ' << p(43) << '\n';

0 1

What is the return-type of the lambda-expression pointed to by p?

Generic λ-expressions

Even the arguments can be auto:

auto twice = [](auto v) { return v + v; };

cout << twice(3) << '\n'
     << twice(2.34) << '\n'
     << twice("Jack"s) << '\n'
     << twice('!') << '\n';

6
4.68
JackJack
66

This is more like a templated function, in that it defines a family of functions.

Use

• Well, this is all very impressive, but what good is it?
• Surely, it’s easier just to use regular functions.
• Functors and λ-expressions are useful with algorithms.

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