Sorting a vector of objects in C++

Question

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&)]

Why isn't this correct, my compiler gives me that error.
And i want my function to be global.
Thank you.

score 6 · Accepted Answer · answered Mar 10 '11 at 20:12

6

Is sortingVector a non-static member of some class? The error message suggests that it is, in which case you will need to wrap it (using boost::bind, for example) into a binary operation that doesn't take a this parameter. You might want to make sortingVector a static member or a free function instead.

answered Mar 10 '11 at 20:12

Jeremiah Willcock

30,161
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yes, that was the problem, i didint know that. I'll keep that in mind next time, thank you – Adrian Mar 10 '11 at 20:15

score 4 · Answer 2 · answered Mar 10 '11 at 20:14

4

This is what a compilable example should look like:

Because you did not provide the exact code that was generating the errors people have given a couple of different types of answer. As a result it is generally a good idea to generate a compilable example the displays the problem.

#include <algorithm>
#include <vector>
#include <string>

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

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

int main()
{
    std::vector<Keyword>    topKeywords;

    std::sort(topKeywords.begin(), topKeywords.end(), sortingVector);
}

Generally the compiler can do a better job of optimising (I am told) if you use a functor rather than a function pointer.

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

answered Mar 10 '11 at 20:14

Martin York

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2

... because it will instantiate `std::sort` with a template argument specific to your functor type, whose `operator()` therefore can easily be inlined into that instantiation. With a function pointer, the instantiation of `std::sort` is only for the function pointer type, not for the exact function, so inlining a call to the particular function actually pointed to by the pointer is a harder job. It can't be inlined into the instantiation, only into an inlined replacement of the call to the instantiation. At least, that's what I'm told ;-) – Steve Jessop Mar 10 '11 at 20:39
But if the function is fixed at compile time then I see no reason that the function can not be inlined. The trouble for a compiler is proving that a function pointer is not modified at runtime, so even though we know it is not changed can the compiler prove it. – Martin York Mar 10 '11 at 23:28
You see no reason why the comparator cannot be inlined into (say) `std::sort`, or you see no reason why `std::sort` can't be inlined into `main`? In any case, what's relevant isn't whether it *can* be inlined (of course it can, provided that `std::sort` is inlined), it's whether it actually will be inlined, and there are more potential obstacles to inlining in the case of the function pointer. That's all, I don't think anyone is claiming that the function pointer case is *guaranteed* to be slower than the functor. – Steve Jessop Mar 10 '11 at 23:50
As for "can it prove it", that particular part is basically the same problem the compiler faces when it inlines a function and you've passed it (say) an integer literal as a parameter. Decent optimizers do propagate the value through the inlined code, and can for example do dead code elimination for conditionals involving the previously-variable-parameter-now-constant-value. So it's certainly not beyond the wit of a normal compiler to determine what function is actually called, provided `std::sort` is inlined. – Steve Jessop Mar 10 '11 at 23:58

score 1 · Answer 3 · edited May 23 '17 at 12:26

1

I believe right example here -> Sorting a vector of custom objects

edited May 23 '17 at 12:26

Community

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answered Mar 10 '11 at 20:12

Darius Kucinskas

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Yes, but using a function for comparison is fine too. – tauran Mar 10 '11 at 20:17

score 1 · Answer 4 · answered Mar 10 '11 at 20:14

1

Put std:: in front of your sort call. And #include <algorithm> at the top of the source file.

answered Mar 10 '11 at 20:14

detunized

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score 0 · Answer 5 · answered Mar 10 '11 at 20:10

0

You probably want to call std::sort instead of plain sort and might have to include the appropriate header file (algorithmunless I'm mistaken).

answered Mar 10 '11 at 20:10

Timo Geusch

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yes, Ive included, looks like i had to make that function static – Adrian Mar 10 '11 at 20:16

score 0 · Answer 6 · answered Mar 10 '11 at 20:13

0

are you using namespace std; ? if not you want std::sort()

answered Mar 10 '11 at 20:13

Brian Roach

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score 0 · Answer 7 · answered Mar 10 '11 at 20:14

#include <vector>
#include <algorithm>
#include <string>

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

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

int main()
{
    std::vector<Keyword> topKeywords(100);

    // imagine topKeywords initialization here

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

    return 0;
}

Compiles fine on my machine (gcc version 4.4.3).

score 0 · Answer 8 · answered Mar 10 '11 at 20:14

0

You have multiple sortingVector functions - thus the <unresolved overloaded function type> part of your error.

answered Mar 10 '11 at 20:14

Erik

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Sorting a vector of objects in C++

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