Why can a struct defined inside a function not be used as functor to std::for_each?

Posted by Oswald on Stack Overflow See other posts from Stack Overflow or by Oswald
Published on 2010-12-29T01:47:37Z Indexed on 2010/12/29 1:54 UTC
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The following code won't compile. The compiler complains about *no matching function for call to for_each*. Why is this so?

#include <map>
#include <algorithm>

struct Element
{
    void flip() {}
};

void flip_all(std::map<Element*, Element*> input)
{
    struct FlipFunctor
    {
        void operator() (std::pair<Element* const, Element*>& item)
        {
            item.second->flip();
        }
    };

    std::for_each(input.begin(), input.end(), FlipFunctor());
}

When I move struct FlipFunctor before function flip_all, the code compiles.

Full error message:

no matching function for call to ‘for_each(std::_Rb_tree_iterator<std::pair<Element* const, Element*> >, std::_Rb_tree_iterator<std::pair<Element* const, Element*> >, flip_all(std::map<Element*, Element*, std::less<Element*>, std::allocator<std::pair<Element* const, Element*> > >)::FlipFunctor)’

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