How is “int main(){(([](){})());}” valid C++?

Question

I recently came across the following esoteric piece of code.

int main(){(([](){})());}

Reformat it as follows to make it more readable:

Answer 1

The code essentially calls an empty lambda.

Let's start from the beginning: [](){} is an empty lambda expression.

Then, in C and C++, you can wrap expressions in parens and they behave exactly the same^† as if written without them, so that's what the first pair of parens around the lambda does. We're now at ([](){}).

Then, () after the first wrapping parens calls the (empty) lambda. We're now at ([](){})()

The whole expression is wrapped in parens again and we get (([](){})()).

At last, ; ends the statement. We arrive at (([](){})());.

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† There are some corner cases at least in C++, like with T a_var; there's a difference between decltype(a_var) and decltype((a_var)).