What's this C++ syntax that puts a brace-surrounded block where an expression is expected?

Question

I came across this weird C++ program.

#include 
using namespace std;
int main()
{
  int a = ({int x; cin >> x; x;});
  cout << a;         


        

Answer 1

It's a GCC extension. Compile your code with the -pedantic flag if you want to get rid of stuff like this (and you really do want to).