Examples

Example: FlagNth. Here is the classic example from [Sutter02], which is intended to remove the third element from a container v:

class FlagNth {


public:


  FlagNth( size_t n ) : current_(0), n_(n) {}





  // evaluate to true if and only if this is the n-th invocation


  template<typename T>


  bool operator()( const T& ) {return ++current_ == n_; }                  // bad: non-const





private:


  size_t current_, n_;


};





// … later …





v.erase( remove_if( v.begin(), v.end(), FlagNth(3) ) );

This is not guaranteed to remove the third element, even though that was intended. In most real-world STL implementations, it erases both the third and the sixth elements. Why? Because remove_if is commonly implemented in terms of find_if and remove_copy_if, and it passes along a copy of the predicate to each of those functions, possibly after it has already itself done some work that affects the predicate's state.

Conceptually, this example is perverse because remove_if only guarantees that it will remove all elements satisfying some criterion. It does not document the order in which elements in the range will be visited or removed, so the code is relying on an assumed, but undocumented and unsatisfied, assumption.

The correct way to remove a particular element is to iterate to it and then call erase.