CODEWITHSUNDEEP
c++c++11
Alessandro Jacopson
Alessandro Jacopson

Reputation: 18665

Idiomatic C++ for remove_if

I have this class

class Point2D
{
public:
 bool isValid();
 // ...
private:
 double x_, y_;
};

I have a std::vector< Point2D > and I would like to remove the invalid points, now I do like this:

bool invalid ( const Point2D& p )
{
 return !p.isValid();
}

void f()
{
 std::vector< Point2D > points;
 // fill points
 points.erase( std::remove_if( points.begin(), points.end(), invalid ), points.end() );
 // use valid points
}

Is there a standard way of doing this (beautifully), for example without the need of "duplicating" the functionality of the class method Point2D::isValid?

Maybe using C++11 lambda (I am not very familiar with lambda)?

Upvotes: 10

Views: 4828

Answers (6)

Yakov Galka
Yakov Galka

Reputation: 72489

If Boost is OK for you, use what @Randall Flagg suggested together with boost::remove_erase_if:

boost::remove_erase_if(points, !boost::bind(&Point2D::isValid, _1));

Upvotes: 5

Xeo
Xeo

Reputation: 131799

The lambda version won't be any cleaner either, but it has another important advantage: locality. You see the code where you use it:

points.erase( std::remove_if( points.begin(), points.end(),
              [](const Point2D& p){
                return !p.isValid();
              }), points.end() );

Note, that you need to change isValid to make it a const function, otherwise you can't call it on a reference-to-const (const Point2D&).
Another option would be to implement operator! for your class:

class Point2D{
  // ... as before
public:
  bool isValid() const;

  bool operator!() const{
    return !isValid();
  }
};

Note, both functions are const. Now you could implement a generic negating functor:

struct negate{
  template<class T>
  bool operator()(T const& t){
    return !t;
  }
};

And use that:

points.erase( std::remove_if( points.begin(), points.end(), negate()), points.end() );

Upvotes: 10

Frerich Raabe
Frerich Raabe

Reputation: 94339

You can do what you want using a combination of std::mem_fun_ref and std::not1:

points.erase( std::remove_if( points.begin(), points.end(),
                              std::not1( std::mem_fun_ref( &Point2D::isValid ) ) ),
              points.end() );

For what it's worth, the only "idiomatic" part about this is the erase-remove idiom.

Upvotes: 6

n. m. could be an AI
n. m. could be an AI

Reputation: 119877

Try this:

points.erase(std::remove_if(points.begin(), 
                            points.end(),
                            std::not1(std::mem_fun_ref(&Point2D::isValid))), 
             points.end());

Upvotes: 16

Randall Flagg
Randall Flagg

Reputation: 320

Not totally standard but nearly : you can use boost::bind and do the following

points.erase( std::remove_if( points.begin(), points.end(),
  !boost::bind(&Point2D::isValid, _1 )), points.end() );

By the way you should declare the isValid method const.

Upvotes: 11

RedX
RedX

Reputation: 15175

I think you are looking for not1

Edit: Looking at your example closer i don't think you can do it any other way, since isValid() is a member function.

Upvotes: 2

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