What do <: and :> mean when declaring a lambda? [duplicate]

Question

I have stumbled across following lambda syntax, which I do not understand:

#include <iostream>

template<typename Callback>
void do_it(Callback callback) {
        callback();
}

template<typename T>
void p() {
        std::cout << __PRETTY_FUNCTION__ << std::endl;
}

int main() {
        auto a = <:&:> { };
        p<decltype(a)>();
        do_it(<:&:> { std::cout << "Hello" << std::endl; }); //this
}

Program above produces an output:

void p() [with T = main()::__lambda0]
Hello

Could you explain, what does <:&:> {/* ... */} mean? Is it possible to declare this way a lambda which takes an argument?

Answer 1

<: and :> are digraphs. They get translated to [ and ] respectively. As such, your code is equivalent to:

auto a = [&] { };
do_it([&] { std::cout << "Hello" << std::endl; }); 

So it's just a lambda which captures everything by reference.

To declare a lambda which takes an argument like this, just add a parameter list after the capture group:

auto a = <:&:> (AType) { };

---

If you want whoever reads your code to hate you, you can use a mix of digraphs and trigraphs wherever possible:

auto a = <:&??)??<%>;
//       ^ same as [&]{}