C++ std::partition compile error

Question

I'm trying to partition an array in order to do a Quick Sort. I have not coded it fully as I'm getting a compile error in the partition part. I'm not sure what to make of the error listing.

Here is the header for a weighted point struct

#ifndef WEIGHTED_POINT_H
#define WEIGHTED_POINT_H

struct WeightedPoint {

    _int64 pointWeight;
    int xCoordinate;
    int ycoordinate;

};

bool compareWeightedPoints(WeightedPoint p1, WeightedPoint p2);

#endif

Here is the CPP for it

#include "WeightedPoint.hpp"

bool compareWeightedPoints(WeightedPoint p1, WeightedPoint p2) {
    return p1.pointWeight < p2.pointWeight;
}

Here is the main method

#include <algorithm>
#include <vector>

#include "WeightedPoint.hpp"

const int VECTOR_SIZE = 100;

WeightedPoint* partitionWeightedPoints(WeightedPoint* first, WeightedPoint* last) {

    WeightedPoint* key = first;
    WeightedPoint* middle = std::partition(first + 1, last, [=](const WeightedPoint *data)->bool {return data->pointWeight < key->pointWeight; }) - 1;

}

void fillWeightedPointsVector(std::vector<WeightedPoint> inputData) {

    int counter = 0;
    while (counter++ < VECTOR_SIZE) {
        inputData.push_back({ std::rand(), std::rand(), std::rand() });
    }

}

int main() {

    std::vector<WeightedPoint> inputData;
    fillWeightedPointsVector(inputData);
    WeightedPoint* arrayToSort = inputData.data();
    int numberOfElements = inputData.size();
    partitionWeightedPoints(&arrayToSort[0], &arrayToSort[numberOfElements - 1]);
}

Here is the compilation error

1>------ Build started: Project: TestPartition, Configuration: Debug Win32 ------
1>  WeightedPoint.cpp
1>  MainSource.cpp
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\algorithm(2141): error C2664: 'bool partitionWeightedPoints::<lambda_a5c326daa6382dcb3f8cf628cbfa8999>::operator ()(const WeightedPoint *) const' : cannot convert argument 1 from 'WeightedPoint' to 'const WeightedPoint *'
1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\algorithm(2164) : see reference to function template instantiation '_BidIt std::_Partition<_Iter,_Pr>(_BidIt,_BidIt,_Pr,std::bidirectional_iterator_tag)' being compiled
1>          with
1>          [
1>              _BidIt=WeightedPoint *
1>  ,            _Iter=WeightedPoint *
1>  ,            _Pr=partitionWeightedPoints::<lambda_a5c326daa6382dcb3f8cf628cbfa8999>
1>          ]
1>          c:\users\gopalmenon\documents\visual studio 2013\projects\testpartition\testpartition\mainsource.cpp(11) : see reference to function template instantiation '_FwdIt std::partition<WeightedPoint*,partitionWeightedPoints::<lambda_a5c326daa6382dcb3f8cf628cbfa8999>>(_FwdIt,_FwdIt,_Pr)' being compiled
1>          with
1>          [
1>              _FwdIt=WeightedPoint *
1>  ,            _Pr=partitionWeightedPoints::<lambda_a5c326daa6382dcb3f8cf628cbfa8999>
1>          ]
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\algorithm(2146): error C2664: 'bool partitionWeightedPoints::<lambda_a5c326daa6382dcb3f8cf628cbfa8999>::operator ()(const WeightedPoint *) const' : cannot convert argument 1 from 'WeightedPoint' to 'const WeightedPoint *'
1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1>  Generating Code...
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

I'm not sure what to make of it. I'm not too good at C++. Any help would be much appreciated.

Answer 1

std::partition expects the predicate to take a const reference (ie const WeightedPoint& in this case), but your predicate signature takes a const pointer. This is where your current compile error comes from. To remove that, for example change

[=](const WeightedPoint *data)->bool {return data->pointWeight < key->pointWeight; }

to

[=](const WeightedPoint& data)->bool {return data.pointWeight < key->pointWeight; }