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()>([](){}) ) { /* ... */ }

+1: Good question, but what about this little fix? `void f(int x, int y){ f(x, y, [](){}); }` (where `void f(int, int, std::function)` is your original function without optional parameter) — Zeta, Jul 24 '13 at 19:25
You should not make your `noop` a `std::function`, make it `auto noop = []{};`. I suspect VS2012 is in the wrong here. And the canonical noop is `[]{}` -- save `()`! Or `<:]{%>` (the clown noop) for maximal amusement. — Yakk - Adam Nevraumont, Jul 24 '13 at 19:29
@Yakk: That's a [*bearded* lambda](http://stackoverflow.com/q/15736282/500104). Get your terminology right! :P — Xeo, Jul 24 '13 at 19:43
@Xeo know how hard it is to *search* for that on the internet? — Yakk - Adam Nevraumont, Jul 24 '13 at 19:49
@Zeta I agree that I could manually overload the function, but I think I'd prefer a named "noop" variable as a default argument (less typing, less error-prone) — nonagon, Jul 24 '13 at 23:03

score 3 · Accepted Answer · answered Jul 24 '13 at 19:28

3

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).

answered Jul 24 '13 at 19:28

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C++11 lambdas as default function arguments

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