Using sort and function object , but the function object cannot modify member variables

Question

I wrote the code like the below, I don't know why the member variable flag did't equal 1 after sort process? Can someone give me suggestion.

Code:

class Func{
public:
    int flag;
    Func()
    {
        flag = 0;
    }
    bool operator()(int &l, int &r)
    {
        if(l==r)
        {
            flag = 1;   
        }
        return l > r;
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    vector<int> a;
    int b[] = {11,8,7,6,4,3,4,1};
    for(int i = 0; i <sizeof(b)/sizeof(b[0]); i++)
    {
        a.push_back(b[i]);
    }

    Func FuncObj = Func();
    std::sort(begin(a), end(a), FuncObj);
    return 0;
}

You say flag was not 1 after sorting, but your code never reads flag at all. Post your actual code that demonstrates the problem. — John Zwinck
– John Zwinck, Commented Aug 19, 2017 at 6:19
Flag equals to 1 only if r and l are equals, but this might not happen during sorting, even the fact that you have two 4 in the data set. — Kamen Stoykov
– Kamen Stoykov, Commented Aug 19, 2017 at 6:21
Off topic: Rather than ` std::vector<int> a; int b[] = {11,8,7,6,4,3,4,1};` and a for loop to copy, assign directly to the vector: std::vector<int> a = {11,8,7,6,4,3,4,1}; — user4581301
– user4581301, Commented Aug 19, 2017 at 6:24
On topic: What's the difference between passing by reference vs. passing by value? — user4581301
– user4581301, Commented Aug 19, 2017 at 6:25
Because FuncObj is passed by value. std::sort operates on a copy of FuncObj, flag is set to ` in the copy of FuncObj and FuncObj is unchanged. — user4581301
– user4581301, Commented Aug 19, 2017 at 6:35

user4581301 · Accepted Answer · 2017-08-19 07:20:32Z

1

According to reliable documentation, std::sort accepts the function object by value. This means the Func being used by std::sort is a copy of FuncObj. This means that when 4 is compared with 4, the copy's flag variable is set to 1, and FuncObj is unchanged. The copy is destroyed when std::sort returns, so this flag is lost.

The simplest solution define int flag as static int flag to that all instances of Func share the same Flag.

If this is not an option, different Funcs must have their own flag, my inclination was to go with a std::shared_ptr. Each default constructed Func would have had it's own flag and shared this flag with any copies made.

However Benjamin Lindley reminds of std::reference_wrapper and this provides a much more convenient solution for the problem. The wrapper is passed by value and copied rather then the Func it references, allowing the source Func to be modified inside std::sort.

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>

class Func{
public:
    int flag;
    Func(): flag(0)
    {
    }
    bool operator()(int &l, int &r)
    {
        if(l==r)
        {
            flag = 1;
        }
        return l > r;
    }
};

int main()
{
    std::vector<int> a = {11,8,7,6,4,3,4,1};

    Func FuncObj = Func();
    std::sort(begin(a), end(a), std::reference_wrapper<Func>(FuncObj));
    std::cout << FuncObj.flag;
    return 0;
}

edited Aug 19, 2017 at 7:20

answered Aug 19, 2017 at 6:56

user4581301

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2 Comments

Benjamin Lindley Over a year ago

The most straightforward, and idiomatic way to solve this is with a std::reference_wrapper. This is precisely the purpose for which it exists.

user4581301 Over a year ago

Can argue with that, but I'd be stupid to. You have raisins, @BenjaminLindley . One more edit coming up.

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