CS253 Lambda Functions

Lambda

This is the Greek letter lambda.

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

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.

Here’s a boring ordinary function:

double half(double d) { return d/2.0; }

int main() {
    cout << half(4.2) << '\n';
}

2.1

double half(double d) { return d/2.0; }

int main() {
    double (*p)(double) = half;
    cout << (*p)(10.2) << ' ' << p(8.6) << '\n';
}

5.1 4.3

p is a variable of type “pointer to function taking a double and returning a double”.
Or, p is a pointer to function taking a double and returning a double.
So, *p is a function taking a double and returning a double.
For convenience, you can use a pointer to a function to call the function.
Some people find such declarations difficult to deal with.
- Not me, though!

double half(double d) { return d/2.0; }

int main() {
    auto p = half;
    cout << (*p)(10.2) << ' ' << p(8.6) << '\n';
}

5.1 4.3

auto sure made that declaration easier!

It seems a shame to have to name the function half.
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 …

auto p = [](double d) -> double { return d/2.0; };
cout << p(8.6) << '\n';

4.3

Let’s omit that return-type declaration:

auto p = [](double d) { return d/2.0; };
cout << p(8.6) << '\n';

4.3