C++ weird expressions are compiled just fine

Why are the following valid C++ expressions? These aren't lambdas

[]{}();
{}[]{};

Can someone explain them to me?

The first is a lambda with no parameter list and a subsequent call. []{} is equivalent to [](){} so the whole line is equivalent to

[](){}();

The second is a pair of braces, which introduce and then immediately close a scope, followed by an unused lambda definition with no parameter list:

{
  // empty scope
}
[]{}; // lambda

You can refer to http://en.cppreference.com/w/cpp/language/lambda for the variations on lambda definition syntax.

• This one is a lambda call

  []{}();
  

  it is equivalent to

  [](){}();
  

• The second is an empty scope, followed by a (unused) lambda.

Parens are optional for lambda without parameter.