Question

I am reading about boost::function and I am a bit confused about its use and its relation to other C++ constructs or terms I have found in the documentation, e.g. here.

In the context of C++ (C++11), what is the difference between an instance of boost::function, a function object, a functor, and a lambda expression? When should one use which construct? For example, when should I wrap a function object in a boost::function instead of using the object directly?

Are all the above C++ constructs different ways to implement what in functional languages is called a closure (a function, possibly containing captured variables, that can be passed around as a value and invoked by other functions)?

Answer 1

A function object and a functor are the same thing; an object that implements the function call operator operator(). A lambda expression produces a function object. Objects with the type of some specialization of boost::function/std::function are also function objects.

Lambda are special in that lambda expressions have an anonymous and unique type, and are a convenient way to create a functor inline.

boost::function/std::function is special in that it turns any callable entity into a functor with a type that depends only on the signature of the callable entity. For example, lambda expressions each have a unique type, so it’s difficult to pass them around non-generic code. If you create an std::function from a lambda then you can easily pass around the wrapped lambda.