dummy() function - What is that supposed to be?

Question

I read this question here on SO and stumbled over the best voted answer, which used code like this to call a lambda recursively:

std::function
           


        

Answer 1

Let's simplify the declaration a bit by using simpler types and expressions. We'll use int instead of std::function, 42 instead of the lambda, and f += 1 instead of f(3):

int f{42}, dummy((f += 1, 0));

To make it even more obvious, we can also use braces instead of parentheses for the second initialisation:

int f{42}, dummy{(f += 1, 0)};

This way, it should be clearer. It's a declaration which declares two variables: f and dummy. f is initialised with 42, and dummy is initialised with this expression: (f += 1, 0). That one's using the comma operator to first evaluate f += 1, discard the result, and then use the value 0 to initalise dummy.

Going back to the full (nonsimplified) declaration:

The type of both variables f and dummy is std::function. First f is initialised with a lambda. Then, dummy is initialised using a comma expression. The left-hand side of that expression, f(3), is evaluated and forgotten. The right-hand side, nullptr, is then used to initialise dummy. Initialising a std::function with nullptr results in creating an empty std::function object (the same as a default-constructed one).

The whole purpose of dummy is to introduce some extra context on the same line (= in the same declaration) in which f could be invoked.