C ++ std :: sort（）-无法对访问成员变量的类类型的向量进行排序

Question

Entry.h :

//returns the sum of all non mega entry percentages
float sumOfNonMegaEntryPct(vector<Number>& arg1_Numbers);

Entry.cpp :

//returns the sum of all mega entry percentages
float Entry::sumOfMegaEntryPct(vector<MegaNumber>& arg1_MegaNumbers)
{
    float sumPct = 0.00f;
     for (MegaNumber c : megaEntry)
     {
         sumPct = sumPct + arg1_MegaNumbers[c.getID()].getOccurencePct();
     }

     return sumPct;
}

Lotto.h :

public:
//compares two entries, used for sorting algorithm, sorts by nonmega number
bool compareEntry_sumPct_nonMega(Entry arg1, Entry arg2); 

    protected:
    vector<Numbers> numbers;
    vector<MegaNumbers> megaNumbers;

Lotto.cpp :

#include "lotto.h"

//sorts nonmega numbers by sum of their pct, used for sort algorithm
bool Lotto::compareEntry_sumPct_nonMega(Entry arg1, Entry arg2)
{
    bool b = arg1.sumOfNonMegaEntryPct(numbers) < arg2.sumOfNonMegaEntryPct(numbers);
     return b;
}

Source.cpp :

vector<Entry> copyGameEntry = game.getPlayEntry();
sort(copyGameEntry.begin(), copyGameEntry.end(),
     bind(&Lotto::compareEntry_sumPct_nonMega, game));

This is just a part of the code, but I think it's enough to make sense. When compiling, I get the error(s):

Severity Code Description Project File Line Error C2451 conditional expression of type 'std::_Unforced' is illegal Lottery Sort e:\\program files (x86)\\microsoft visual studio 14.0\\vc\\include\\algorithm 3133

Severity Code Description Project File Line Error C2675 unary '!': 'std::_Unforced' does not define this operator or a conversion to a type acceptable to the predefined operator Lottery Sort e:\\program files (x86)\\microsoft visual studio 14.0\\vc\\include\\algorithm 3118

Question:

What could be the problem ?

Answer 1

You are using std::bind incorrectly. You need to use placeholders for the unbound arguments:

using namespace std::placeholders;
sort(copyGameEntry.begin(), copyGameEntry.end(),
     bind(&Lotto::compareEntry_sumPct_nonMega, game, _1, _2));

NB this bind expression will copy the game object, so you should either use std::ref(game) or just &game instead, to avoid an unnecessary copy.

Or use a lambda function:

sort(copyGameEntry.begin(), copyGameEntry.end(),
     [&game](Entry& l, Entry& r) {
       return game.compareEntry_sumPct_nonMega(l, r);
     });

Answer 2

There are alternatives to invoke std::sort:

#include <algorithm>
#include <vector>

struct X
{
    int value;
    bool operator < (const X& other) const { return value < other.value; }
    static bool less(const X& a, const X& b) { return a.value < b.value; }
};

struct Holder
{
    bool less(const X& a, const X& b) const { return a.value < b.value; }
};

int main ()
{
    Holder holder;
    std::vector<X> values;

    // No stateful comparison
    std::sort(values.begin(), values.end());

    // No stateful comparison
    std::sort(values.begin(), values.end(), X::less);

    // Stateful comparison
    struct Less {
        const Holder& holder;
        Less(const Holder& holder) : holder(holder) {}
        bool operator ()(const X& a, const X& b) const { return holder.less(a, b); }
    };
    std::sort(values.begin(), values.end(), Less(holder));

    // Stateful comparison
    std::sort(values.begin(), values.end(), [&holder](const X& a, const X& b) {
        return holder.less(a, b);
    });

    // Stateful comparison
    using namespace std::placeholders;
    std::sort(values.begin(), values.end(), std::bind(&Holder::less, holder, _1, _2));
}

Likely, in your case you are missing std::placeholders

Answer 3

Your error is related to: bind(&Lotto::compareEntry_sumPct_nonMega, game) .

You need to specify std::placeholders when you call function std::bind , which will be replaced by the arguments of the called, returned function object, in the specified order.

You could verify using the following try - catch block:

try {
    std::sort(copyGameEntry.begin(), copyGameEntry.end(),
              bind(&Lotto::compareEntry_sumPct_nonMega, game)

} catch (std::bad_function_call& e) {
    std::cout << "ERROR: Bad function call\n";

}

In your case you need to add:

using namespace std::placeholders;
auto func_obj = bind(&Lotto::compareEntry_sumPct_nonMega, game, _1, _2);

then, the func_obj (_1, _2) will be called, internally by sort() , as:

func_obj(copyGameEntry[i], copyGameEntry[i+1]);

Alternatively , you can try using something like:

struct Holder{
    bool less(const Entry& a, const Entry& b) const { 
        return a.sumOfNonMegaEntryPct(numbers) < b.sumOfNonMegaEntryPct(numbers); 
    }
} holder;

struct Less {
    const Holder& holder;
    Less(const Holder& holder) : holder(holder) {}
    bool operator ()(const Entry& a, const Entry& b) const { return holder.less(a, b); }
};

std::sort(copyGameEntry.begin(), copyGameEntry.end(), Less(holder));