Why do we use std::function in C++ rather than the original C function pointer?

Question

What is the advantage of std::function<T1(T2)> over the original T1 (*)(T2)?

It's more versatile, but also slower. You can't compare `::std::function` instances and it allocates from the heap. — user1095108, Jan 14 '14 at 17:56
@user1095108: It _may_ allocate from the heap. Just like the "small `string` optimization," an implementation may use a "small `function` optimization." — James McNellis, Jan 14 '14 at 18:02

score 37 · Accepted Answer · answered Jul 05 '12 at 21:42

37

std::function can hold more than function pointers, namely functors.

#include <functional>

void foo(double){}

struct foo_functor{
  void operator()(float) const{}
};

int main(){
  std::function<void(int)> f1(foo), f2((foo_functor()));
  f1(5);
  f2(6);
}

Live example on Ideone.

As the example shows, you also don't need the exact same signature, as long as they are compatible (i.e., the parameter type of std::function can be passed to the contained function / functor).

answered Jul 05 '12 at 21:42

Xeo

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Also, with `std::bind` it is possible to create delegates (i.e. encapsulate a member function pointer with an object) and store them in an std::function, without any difference in handling after creation. This is something that function pointers alone are not capable of. – Tamás Szelei Jul 05 '12 at 21:47
2

@fish: Well, what `std::bind` returns is a functor, so I wouldn't really special-case that. What can be seen as special is that you can just pass a member function pointer and the `std::function` will make it callable like `f(object, param);`. [Example on Ideone](http://ideone.com/haRtx) – Xeo Jul 05 '12 at 21:49
1

Another thing is (arguably) easier declarations. Function pointer declarations are a little hard follow due to their C heritage of *"declaration reflecting usage"*-rule. `std::function` objects have a declaration that looks more like a function declaration. (sorry I really should make an edit, but I see that the post is still being updated frequently at this moment. feel free to merge in these). – Tamás Szelei Jul 05 '12 at 21:50
yeah, well of course it does. I think it is worth mentioning because delegates are a common pattern in many languages and this is what makes it possible to fluidly replicate them in C++. – Tamás Szelei Jul 05 '12 at 21:52
As it is, the program compiles but doesn't do anything. Can I suggest to `cout` the argument in both the function and the functor? And I'd also add a call using a non-integer argument, like `f1(4.7);`. This would show the effect of storing a function accepting a `double` into an `std::function` accepting an `int` parameter. I would also include the output into the answer. I've already prepared the modified code [on Ideone](https://ideone.com/PD9rwW). I can edit your answer myself, if you agree, but I'll do it only with your permission. – Fabio says Reinstate Monica Mar 12 '19 at 12:59

score 17 · Answer 2 · answered Jul 05 '12 at 21:40

17

std::function can hold function objects (including lambdas), as well as function pointers with the correct signature. So it is more versatile.

answered Jul 05 '12 at 21:40

Benjamin Lindley

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Side question: I'm assuming the lambdas are called virtually, right? – user541686 Jul 05 '12 at 21:41
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@Mehrdad: What is your definition of *called virtually*? – David Rodríguez - dribeas Jul 05 '12 at 21:42
@DavidRodríguez-dribeas: An indirect call, i.e. like a `virtual` function. – user541686 Jul 05 '12 at 21:44
How is a `virtual` function indirect? – Michael Dorst Jul 05 '12 at 21:45
@Mehrdad: It can do that, it could also use a `void*` + a function pointer to a templated function. – Xeo Jul 05 '12 at 21:45
3

@Mehrdad: `std::function<>` needs to add an indirection to perform the type erasure, so yes, there is one extra level of indirection con calling a lambda through `std::function` (well, calling a lambda or any other thing through `std::function`) – David Rodríguez - dribeas Jul 05 '12 at 21:46
I thought that `virtual` just meant that it was inheritable? – Michael Dorst Jul 05 '12 at 21:48
And you can `std::bind` it like you're Lady Heather. – Captain Obvlious Jul 06 '12 at 01:33
2

@anthropomorphic: No, `virtual` means it is *overridable* in derived classes. – celtschk Aug 10 '14 at 07:42

score 11 · Answer 3 · edited Aug 10 '14 at 07:38

11

Apart from the cleaner look and a more descriptive syntax, std::function can store any callable object:

functions
lambda expressions
bind expressions
functors

Not to mention that storing, copying, and binding objects to member functions is much easier and more intuitive.

edited Aug 10 '14 at 07:38

Michael Dorst

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answered Jan 14 '14 at 18:04

Shoe

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Bind expressions simply return functors, so having both functors and bind expressions listed is redundant. – Michael Dorst Aug 10 '14 at 07:32

Why do we use std::function in C++ rather than the original C function pointer?

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