Lambdas. They added it a while back.

  [things-to-capture](params) -> return-type { function-body; }

The return type is optional [0] since it can be detected automatically. The things-to-capture have a couple variants:

  [&foo]...

Captures variable foo by reference.

  [foo]...

Makes a copy of foo.

You can also capture "everything" using [&] or [=]. Handy if you don't want to make an explicit list of captured variables. You can also mix it:

  [&foo, bar]...

Captures foo by reference and a copy of bar.

https://en.cppreference.com/w/cpp/language/lambda

[0] Additionally, lambdas may not have a return value. Like this one:

  int n = 0;
  auto counter = [&n] () { n++; }
  counter();
  // now n == 1