What is the meaning is this code block [=,&i]()mutable {}()?

Question

I was taking a c++ test and there was a strange code block below which I didn't understand. Here, i is an int and code is a char:

[=,&i]()mutable
{
  i++;
  code = 'b';
  std::cout << "i:" <<i<<"""code:"<<code <<cout::endl;
}();

I don't know how to interpret this; it doesn't look like a typical C++ code block. I've searched online for information about this but can't find anything else on this sort of code style.

What does this code mean?

Answer 1

This is a lambda function, a feature added to the language in 2011.

• The = means that copies of variables from the outside are available inside.
• The &i means that, despite the above, i in particular is actually available by reference.
• The mutable keyword allows the code copy to be modified inside the function body.
• The trailing () "runs" the function immediately after it's declared.

As written, I can't see any particular reason to use a lambda for this. It looks like someone has succeeded at their goal: to confuse you.