What is ({}) called in C++?

Question

I just read code like this:

auto value = ({
  auto it = container.find(key);
  it != container.end() ? it->second : default_value;
});

What is this ({}) called? I don't think I've ever seen this before.

Answer 1

It is a gcc extension, so not standard C++,

it is statement expression.

Since C++11, you might use Immediately Invoked Function Expressions (IIFE) with lambda instead in most cases:

auto value = [&](){
  auto it = container.find(key);
  return it != container.end() ? it->second : default_value;
}();