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larspensjo

Purpose of std::function

6 posts in this topic

What is the purpose of this class template?

I suppose it simplifies the syntax for function pointers. But does it give you anything else but syntactic sugaring?
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I think it's worth noticing [font=courier new,courier,monospace]std::function[/font] is correctly mapped even when capture lists are used with lambdas.
Function pointers cannot as far as I recall , and sure [i]interfaces [/i]cannot.
So they're really the only "complete" way to deal with lambdas, a thing which I consider to be a bit ugly, but that's it.
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Thanks for the answers. There is a case I am specifically interested in, but I couldn't find out if it is now possible. I want to use closures with callbacks. Take this example, with a tree walker and a callback for every node in the tree:
[CODE]class Tree {
Walk(void (*callback)(Node*));
};[/CODE]
It is common to also have an extra argument of the type "void *" that will be forwarded to the callback function. I think this is a typical case that a closure would be used, is it now possible?

Something like:[CODE]
void Callback1(Node*, int);
void Callback2(Node*, int, float);

Tree *t;
auto cb1 = std::bind(Callback1, _1, 77);
t->Walk(cb1);
auto cb2 = std::bind(Callback1, _1, 77, 1.0f);
t->Walk(cb2);
[/CODE]If possible, what is the recommended way to implement this?
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[quote name='Brother Bob' timestamp='1352987704' post='5001222']
If you make [i]Tree::Walk[/i] take an [i]std::function<void(Node *)>[/i], you can then do [i]t->Walk([](Node *n){Callback1(n, 77);})[/i]. The benefit, as stated above, is that you can pass anything that is callable with a [i]Node[/i]-pointer and have no return value: a function pointer, a closure and an old-style function object, and your code doesn't have to know or take special action for the different types of callable objects.
[/quote]

I see, it is that simple! This is something I should be able to use to great advantage.
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