C++11 lambdas as default function arguments

Question

I'm writing a C+11 function that takes a callable as an argument, and I'd like to have that argument default to a no-op function. This is my best attempt so far:

const std::function<void()> noop= [](){};
void f( int x, int y, std::function<void()> fn= noop ) { /* ... */ }

I'm wondering whether the standard libraries provide a "noop" std function for me, or do I need to write my own as I have above? I'm also wondering if there's a way to avoid explicitly naming the "noop" function. For example:

void f( int x, int y, std::function<void()> fn= [](){} ) { /* ... */ }

won't compile (in Visual Studio 2012 Update 3), nor will:

void f( int x, int y, std::function<void()> fn= std::function<void()>([](){}) ) { /* ... */ }

Answer 1

I'm wondering whether the standard libraries provide a "noop" std function for me, or do I need to write my own as I have above?

No, there are no default noop functions. In this case, you have to create your own (like you did with noop functor object).