Hurb Sutter: For me, easily the biggest news of the meeting was that we voted lambda functions and closures into C++0x. I think this will make STL algorithms an order of magnitude more usable, and it will be a great boon to concurrent code where it's important to be able to conveniently pass around a piece of code like an object, to be invoked wherever the program sees fit (e.g., on a worker thread).

Here is the report.

Here is an example:

double min_salary = ....
....
double u_limit = 1.1 ∗ min_salary;
std::find_if(employees.begin(), employees.end(),
    [&](const employee& e) {
       return e.salary() >= min_salary && e.salary() < u_limit;
    });

Here are some of the finer points of the syntax and semantics:

1. An lambda expression has the general form [...](...)...{...}
2. The [...] portion serves as a lambda introducer, it also indicates which local variables, or this, are captured from the environment
3. The optional (...)... portion declares the parameters, thrown exceptions and return types of the lambda expression
4. The {...} portion contains the code of the lambda expression
5. Lambda expressions evaluates to closure objects, which are function objects whose classes are implicitly defined and named. There's no duck typing here. The duck typing happens in the template system

The simplest lambda expression in C++ is thus

[]{ }

The most useful lambda expression will be introduced by [&], which captures every free variable in the lambda expression by reference. The [=] introducer introduces a lambda expression that captures everything by value. The [] captures nothing.

The lambda introducer may also contain an explicit list of variable names or this, in which case they are captured by the lambda expression whether explicitly referenced by the lambda expression body or not. I haven't figured out how this could be used except that it may make local variables live longer.

In C++, the closure feature is more of a syntactic sugar for function objects.

One irony here is that while in the past, we have strived to turn for loops into for_each statements, closures will make those for_each statements look (mostly) just like the for loops.

Responses[4]

Posted by Weiqi Gao on March 30, 2008 8:30:06 AM CDT #