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How to pass a comparator as a parameter for a function?

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0

I have a function in a sorting header for mergeSort

This is my code:

template

void mergeSort(vector<Comparable> & a, vector<Comparable> & tmpArray, int left int right, Comparator cmp)

 if (cmp(left,right))
 
 int center = (left + right) / 2;
 mergeSort(a, tmpArray, left, center);
 mergeSort(a, tmpArray, center + 1, right);
 merge(a, tmpArray, left, center + 1, right);
 

I want to use this comparator and pass in the parameters in my mainDriver.cpp

class CompareXCoordinate 
public:
 bool operator()(const Point & p1, const Point & p2) const
 
 return (p1.getX() < p2.getX());
 
;

Currently I am passing it like this:

Points is vector of Point objects
tempArr is an empty vector

mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate::operator())

I get an error of C3867: non-standard syntax, use & to create pointer to member

Is that the proper way to pass a comparator or is it a different syntax?

c++ mergesort cartesian-coordinates

share|improve this question

edited Mar 9 at 1:21

πάντα ῥεῖ

73.8k1077144

asked Mar 9 at 1:15

Amor DiazAmor Diaz

164

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  See the implemenation of merge sort here. 1) Better to use iterators, not full blown container types such as vector -- this makes it much more flexible. 2) Make the comparison a template argument and pass an instance of the comparison object (if necessary).
  
  – PaulMcKenzie
  Mar 9 at 1:21
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Pass an instance of the CompareXCoordinate class.
  
  – πάντα ῥεῖ
  Mar 9 at 1:22
  

  
  
  
  

add a comment | 

0

I have a function in a sorting header for mergeSort

This is my code:

template

void mergeSort(vector<Comparable> & a, vector<Comparable> & tmpArray, int left int right, Comparator cmp)

 if (cmp(left,right))
 
 int center = (left + right) / 2;
 mergeSort(a, tmpArray, left, center);
 mergeSort(a, tmpArray, center + 1, right);
 merge(a, tmpArray, left, center + 1, right);
 

I want to use this comparator and pass in the parameters in my mainDriver.cpp

class CompareXCoordinate 
public:
 bool operator()(const Point & p1, const Point & p2) const
 
 return (p1.getX() < p2.getX());
 
;

Currently I am passing it like this:

Points is vector of Point objects
tempArr is an empty vector

mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate::operator())

I get an error of C3867: non-standard syntax, use & to create pointer to member

Is that the proper way to pass a comparator or is it a different syntax?

c++ mergesort cartesian-coordinates

share|improve this question

edited Mar 9 at 1:21

πάντα ῥεῖ

73.8k1077144

asked Mar 9 at 1:15

Amor DiazAmor Diaz

164

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  See the implemenation of merge sort here. 1) Better to use iterators, not full blown container types such as vector -- this makes it much more flexible. 2) Make the comparison a template argument and pass an instance of the comparison object (if necessary).
  
  – PaulMcKenzie
  Mar 9 at 1:21
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Pass an instance of the CompareXCoordinate class.
  
  – πάντα ῥεῖ
  Mar 9 at 1:22
  

  
  
  
  

add a comment | 

I have a function in a sorting header for mergeSort

This is my code:

template

void mergeSort(vector<Comparable> & a, vector<Comparable> & tmpArray, int left int right, Comparator cmp)

 if (cmp(left,right))
 
 int center = (left + right) / 2;
 mergeSort(a, tmpArray, left, center);
 mergeSort(a, tmpArray, center + 1, right);
 merge(a, tmpArray, left, center + 1, right);
 

I want to use this comparator and pass in the parameters in my mainDriver.cpp

class CompareXCoordinate 
public:
 bool operator()(const Point & p1, const Point & p2) const
 
 return (p1.getX() < p2.getX());
 
;

Currently I am passing it like this:

Points is vector of Point objects
tempArr is an empty vector

mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate::operator())

I get an error of C3867: non-standard syntax, use & to create pointer to member

Is that the proper way to pass a comparator or is it a different syntax?

c++ mergesort cartesian-coordinates

share|improve this question

edited Mar 9 at 1:21

πάντα ῥεῖ

73.8k1077144

asked Mar 9 at 1:15

Amor DiazAmor Diaz

164

void mergeSort(vector<Comparable> & a, vector<Comparable> & tmpArray, int left int right, Comparator cmp)

 if (cmp(left,right))
 
 int center = (left + right) / 2;
 mergeSort(a, tmpArray, left, center);
 mergeSort(a, tmpArray, center + 1, right);
 merge(a, tmpArray, left, center + 1, right);
 

class CompareXCoordinate 
public:
 bool operator()(const Point & p1, const Point & p2) const
 
 return (p1.getX() < p2.getX());
 
;

mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate::operator())

share|improve this question

edited Mar 9 at 1:21

πάντα ῥεῖ

73.8k1077144

asked Mar 9 at 1:15

Amor DiazAmor Diaz

164

share|improve this question

edited Mar 9 at 1:21

πάντα ῥεῖ

73.8k1077144

asked Mar 9 at 1:15

Amor DiazAmor Diaz

164

edited Mar 9 at 1:21

πάντα ῥεῖ

73.8k1077144

edited Mar 9 at 1:21

πάντα ῥεῖ

73.8k1077144

asked Mar 9 at 1:15

Amor DiazAmor Diaz

164

asked Mar 9 at 1:15

Amor DiazAmor Diaz

164

1 Answer
1

active

oldest

votes

1

operator() is an instance method, so you need to pass an instance of the CompareXCoordinate class as the comparator, not the operator() itself:

mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate());

Though, your class does not act on any non-static data members (that would make sense if you wanted the use of < or > to be configurable), so you could just use a standalone function instead of a class:

bool CompareXCoordinate(const Point & p1, const Point & p2)

 return (p1.getX() < p2.getX());


mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate);

Or, if you are using C++11 or later, you can use a lambda instead:

mergeSort(points, tempArr, points.begin(), points.end(),
 [](const Point & p1, const Point & p2)
 return (p1.getX() < p2.getX());
 
);

share|improve this answer

edited Mar 9 at 1:36

answered Mar 9 at 1:33

Remy LebeauRemy Lebeau

342k19268461

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  No need for (…) around the return expr.
  
  – Marcelo Cantos
  Mar 9 at 1:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MarceloCantos technically, yes, but that is a matter of personal style.
  
  – Remy Lebeau
  Mar 9 at 1:36
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Sure, but why introduce text that takes more effort and adds nothing to readability?
  
  – Marcelo Cantos
  Mar 9 at 2:22
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MarceloCantos it may not add readability for you, but it can for other people. If you don't want to use it, that is your choice. Other people can choose differently.
  
  – Remy Lebeau
  Mar 9 at 5:29
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It doesn't add readability for anyone. Return is a statement. Its expression will never appear in a context requiring precedence disambiguation.
  
  – Marcelo Cantos
  Mar 9 at 23:20
  

  
  
  
  

 | 
show 2 more comments

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1

operator() is an instance method, so you need to pass an instance of the CompareXCoordinate class as the comparator, not the operator() itself:

mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate());

Though, your class does not act on any non-static data members (that would make sense if you wanted the use of < or > to be configurable), so you could just use a standalone function instead of a class:

bool CompareXCoordinate(const Point & p1, const Point & p2)

 return (p1.getX() < p2.getX());


mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate);

Or, if you are using C++11 or later, you can use a lambda instead:

mergeSort(points, tempArr, points.begin(), points.end(),
 [](const Point & p1, const Point & p2)
 return (p1.getX() < p2.getX());
 
);

share|improve this answer

edited Mar 9 at 1:36

answered Mar 9 at 1:33

Remy LebeauRemy Lebeau

342k19268461

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  No need for (…) around the return expr.
  
  – Marcelo Cantos
  Mar 9 at 1:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MarceloCantos technically, yes, but that is a matter of personal style.
  
  – Remy Lebeau
  Mar 9 at 1:36
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Sure, but why introduce text that takes more effort and adds nothing to readability?
  
  – Marcelo Cantos
  Mar 9 at 2:22
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MarceloCantos it may not add readability for you, but it can for other people. If you don't want to use it, that is your choice. Other people can choose differently.
  
  – Remy Lebeau
  Mar 9 at 5:29
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It doesn't add readability for anyone. Return is a statement. Its expression will never appear in a context requiring precedence disambiguation.
  
  – Marcelo Cantos
  Mar 9 at 23:20
  

  
  
  
  

 | 
show 2 more comments

1

operator() is an instance method, so you need to pass an instance of the CompareXCoordinate class as the comparator, not the operator() itself:

mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate());

Though, your class does not act on any non-static data members (that would make sense if you wanted the use of < or > to be configurable), so you could just use a standalone function instead of a class:

bool CompareXCoordinate(const Point & p1, const Point & p2)

 return (p1.getX() < p2.getX());


mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate);

Or, if you are using C++11 or later, you can use a lambda instead:

mergeSort(points, tempArr, points.begin(), points.end(),
 [](const Point & p1, const Point & p2)
 return (p1.getX() < p2.getX());
 
);

share|improve this answer

edited Mar 9 at 1:36

answered Mar 9 at 1:33

Remy LebeauRemy Lebeau

342k19268461

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  No need for (…) around the return expr.
  
  – Marcelo Cantos
  Mar 9 at 1:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MarceloCantos technically, yes, but that is a matter of personal style.
  
  – Remy Lebeau
  Mar 9 at 1:36
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Sure, but why introduce text that takes more effort and adds nothing to readability?
  
  – Marcelo Cantos
  Mar 9 at 2:22
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @MarceloCantos it may not add readability for you, but it can for other people. If you don't want to use it, that is your choice. Other people can choose differently.
  
  – Remy Lebeau
  Mar 9 at 5:29
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It doesn't add readability for anyone. Return is a statement. Its expression will never appear in a context requiring precedence disambiguation.
  
  – Marcelo Cantos
  Mar 9 at 23:20
  

  
  
  
  

 | 
show 2 more comments

operator() is an instance method, so you need to pass an instance of the CompareXCoordinate class as the comparator, not the operator() itself:

mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate());

Though, your class does not act on any non-static data members (that would make sense if you wanted the use of < or > to be configurable), so you could just use a standalone function instead of a class:

bool CompareXCoordinate(const Point & p1, const Point & p2)

 return (p1.getX() < p2.getX());


mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate);

Or, if you are using C++11 or later, you can use a lambda instead:

mergeSort(points, tempArr, points.begin(), points.end(),
 [](const Point & p1, const Point & p2)
 return (p1.getX() < p2.getX());
 
);

share|improve this answer

edited Mar 9 at 1:36

answered Mar 9 at 1:33

Remy LebeauRemy Lebeau

342k19268461

mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate());

bool CompareXCoordinate(const Point & p1, const Point & p2)

 return (p1.getX() < p2.getX());


mergeSort(points, tempArr, points.begin(), points.end(), Point::CompareXCoordinate);

mergeSort(points, tempArr, points.begin(), points.end(),
 [](const Point & p1, const Point & p2)
 return (p1.getX() < p2.getX());
 
);

share|improve this answer

edited Mar 9 at 1:36

answered Mar 9 at 1:33

Remy LebeauRemy Lebeau

342k19268461

answered Mar 9 at 1:33

Remy LebeauRemy Lebeau

342k19268461

answered Mar 9 at 1:33

Remy LebeauRemy Lebeau

342k19268461