struct Keyword
{
    std::string keyword;
    int numUses;
};  

bool sortingVector(const Keyword& key1, const Keyword& key2)
{
    return key1.numUses < key2.numUses;
}

sort(topKeywords.begin(), topKeywords.end(), sortingVector);



: no matching function for call to 'sort(std::vector<Keyword>::iterator, std::vector<Keyword>::iterator, <unresolved overloaded function type>)'
        c:\mingw\bin\../lib/gcc/mingw32/4.5.0/include/c++/bits/stl_algo.h:5236:18: note: candidate is: void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<Keyword*, std::vector<Keyword> >, _Compare = bool (NewsAggregatorImpl::*)(const Keyword&, const Keyword&)]

Por que isso não está correto, meu compilador me dá esse err
E eu quero que minha função seja globa
Obrigado