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.

Doesn't compile for me, even when I add sensible definitions for key and container. Could you supply an example without any missing parts? — yesterday
It is a compiler extension gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html — yesterday
default is a keyword. Even if the statement expression is removed, how can it compile it != container.end() ? it->second : default;? — yesterday
Better question: How is this not standard yet? It's been available in Rust for ages now, albeit with less silly syntax. — 1 hour ago

score 13 · Accepted Answer · 2023-08-13 21:08:56Z

13

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;
}();

edited 1 hour ago

answered yesterday

Jarod42

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1

2

It is still not the same as lambdas since a return in statement expressions returns from the enclosing function and not the statement expression.

– Daniel

1 hour ago

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supercatsupercat 77.4k9 gold badges166 silver badges210 bronze badges · Accepted Answer · 2023-08-12 19:50:24Z

Even before C89 was published, the authors of gcc invented an extension called a "statement expression" which would have been a useful part of the Standard language. It takes a compound statement whose last statement is an expression, and executes everything therein, and then treats the value of the last expression as the value of the statement expression as a whole.

While some other compilers have options to support such gcc extensions, the Standard’s refusal to recognize features that aren’t widely use, combined with programmers’ reluctance to use features that aren’t recognized by the Standard, have created a decades-long "chicken and egg" problem.

You could've mentioned a full capture lambda as did Jarod.

– Red.Wave

13 hours ago — 13 hours ago

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