C++ In a Nutshell

Name

for_each function template — Calls a function for each item in a range

Synopsis

template<typename InIter, typename Func>
  Function for_each(InIter first, InIter last, Func f);

The for_each function template calls f for each item in the range [first, last), passing the item as the sole argument to f. It returns f.

Example

Example 13-1 shows how the use for_each to test whether a sequence is sorted. The is_sorted object remembers the previous item in the sequence, which it compares with the current item. The overloaded bool operator returns true if the sequence is sorted so far or false if the sequence is out of order. The example takes advantage of the fact that for_each returns the f parameter as its result.

Example 13-1. Using for_each to test whether a list is sorted

#include <iostream>
#include <algorithm>
#include <list>
  
template<typename T>
class is_sorted
{
public:
  is_sorted(  ) : first_time(true), sorted(true) {}
  void operator(  )(const T& item) {
    // for_each calls operator(  ) for each item.
    if (first_time)
      first_time = false;
    else if (item < prev_item)
      sorted = false;
    prev_item = item;
  }
  operator bool(  ) { return sorted; }
private:
  bool first_time;
  bool sorted;
  T prev_item;
};
  
int main(  )
{
  std::list<int> l;
  l.push_back(10);
  l.push_back(20);
  ...  if (std::for_each(l.begin(  ), l.end(  ), is_sorted<int>(  )))
    std::cout << "list is sorted" << '\n';
}

Technical Notes

Complexity is linear: f is called exactly last - first times.

See Also

copy function template, accumulate in <numeric>