C++ weird expressions are compiled just fine

Question

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

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

Can someone explain them to me?

Answer 1

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

[](){}(); 

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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 

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

Answer 2

• 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.