How to call stl::nth_element with a member function inside this class?

Question

I want to use the function nth_element with my own sorting function (which should have access to the data of the object) within a class. Currently, I am doing the following:

class Foo
{
 public:
   glm::vec3 *points;
   int nmbPoints;

   bool idxPointCompareX(int a, int b);
   void bar();
}

bool Foo::idxPointCompareX(int a, int b)
{return points[a].x < points[b].x;)

void Foo::bar()
{
   stl::vector<int> idxPointList;
   for(int i = 0; i < nmbPoints; i++) idxPointList.push_back(i);  

   stl::nth_element(idxPointList.first(),idxPointList.first()+nmbPoints/2,idxPointList.end(), idxPointCompareX);
}

Of course, this did not work and I got the error: "reference to non-static member function must be called". After that, I had a look at Reference to non-static member function must be called, How to initialize std::function with a member-function? and some other questions on here. I understand why this did not work, but I'm unsure how to solve this.

Can someone help me and tell me how to solve this problem?

Jonathan Wakely · Accepted Answer · 2020-07-31T15:24:47.030

6

To take the address of a member function you need to use the correct syntax, i.e. &Foo::idxPointCompareX not just idxPointCompareX

But you also need a Foo object to call the function on, so you would need to bind one to it. Presumably you mean to call it on this so you could use std::bind:

using namespace std::placeholders;
stl::nth_element(begin, begin+n, end,
                 std::bind(&Foo::idxPointCompareX, this, _1, _2));

Or simpler, use a lambda function:

stl::nth_element(begin, begin+n, end, 
                 [this](int a, int b) { return idxPointCompareX(a, b);}
);

This creates a lambda function that captures this and passes its arguments on to the idxPointCompareX function on the captured this pointer.

edited Jul 31 '20 at 15:24

answered Nov 16 '15 at 09:39

Jonathan Wakely

153,269
21
303
482

As I mentioned already under his answer, I made a mistake while writing the question. idxPointCompareX uses an index to get access to the point data of 'points'. – DanceIgel Nov 16 '15 at 09:42
you're missing n'th element here. `stl::nth_element(begin, begin + smth, end, [this](int a, int b) { return idxPointCompareX(a, b);} );` – Алексей Неудачин Jul 30 '20 at 18:36

score 2 · Answer 2 · edited May 31 '16 at 15:18

2

idxPointCompareX is a member function, i.e. it cannot be called without some reference to a Foo object. Looking at its definition though, it seems that it doesn't need to be a member since it's purely defined in terms of its arguments.

You could make it a static function (i.e. a "class function") or a free function and then pass that to std::nth_element.

edited May 31 '16 at 15:18

Toby Speight

23,550
47
57
84

answered Nov 16 '15 at 09:35

Frerich Raabe

81,733
18
105
196

I am sorry I made a mistake in the question. I will edit it directly after posting this. – DanceIgel Nov 16 '15 at 09:40

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

You can't call an object method before it is created, so you have some options here:

Make the method static
Leave the constructor empty and move all into an init method where you call the compare part
Use a lambda

Examples:

Static method:

static bool idxPointCompareX(glm::vec3 a, glm::vec3 b)
{return a.x < b.x;)

Init method:

Foo::bar()
{
   stl::vector<int> idxPointList;
   for (int i = 0; i < nmbPoints; i++)
       idxPointList.push_back(i);  
}

Foo::init()
{
    stl::nth_element(idxPointList.first(),
                     idxPointList.first()+nmbPoints/2,idxPointList.end(),
                     idxPointCompareX);
}

Lambda:

Foo::bar()
{
   stl::vector<int> idxPointList;
   for (int i = 0; i < nmbPoints; i++)
       idxPointList.push_back(i);  

   stl::nth_element(idxPointList.first(),
                    idxPointList.first()+nmbPoints/2,idxPointList.end(),
                    [](int a, int b){return points[a].x < points[b].x;));
}

I would go for the lambda version myself.

How to call stl::nth_element with a member function inside this class?

3 Answers3

Examples: