STL for_each

内容

记录for_each的使用方法。

参考资料

• http://www.cplusplus.com/reference/algorithm/for_each/
• Standard for C++ 11

C++标准中的内容

摘自《25.2.4 For each [alg.foreach]》。

template
Function for_each(InputIterator first, InputIterator last, Function f);

1. Requires: Function shall meet the requirements of MoveConstructible (Table 20). [ Note: Function
   need not meet the requirements of CopyConstructible (Table 21). —end note ]
2. Effects: Applies f to the result of dereferencing every iterator in the range [first,last), starting
   from first and proceeding to last - 1. [ Note: If the type of first satisfies the requirements of a
   mutable iterator, f may apply nonconstant functions through the dereferenced iterator.—end note ]
3. Returns: std::move(f).
4. Complexity: Applies f exactly last - first times.
5. Remarks: If f returns a result, the result is ignored.

for_each的算法

本章是第一个参考页面的内容。

函数原型

template 
   Function for_each (InputIterator first, InputIterator last, Function fn);

功能

对迭代器[first, last)区间的每个元素，都调用函数fn.

Applies function fn to each of the elements in the range [first,last).

The behavior of this template function is equivalent to:

template
  Function for_each(InputIterator first, InputIterator last, Function fn)
{
  while (first!=last) {
    fn (*first);
    ++first;
  }
  return fn;      // or, since C++11: return move(fn);
}

Parameters

• first, last: Input iterators to the initial and final positions in a sequence. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
• fn: Unary function that accepts an element in the range as argument. This can either be a function pointer or a move constructible function object. Its return value, if any, is ignored.

说明：这里提到sequence的迭代器，但并非说只能是sequence container的迭代器。如前面摘录的c++ standard，并没有要求这一点。在下面的示例中，我们在http://www.cplusplus.com/reference/algorithm/for_each/](http://www.cplusplus.com/reference/algorithm/for_each/)的基础上，增加了set的例子，也说明了这一点。

Example

代码

#include      // std::cout
#include     // std::for_each
#include        // std::vector
#include           // std::set

void myfunction (int i) {  // function:
  std::cout << ' ' << i;
}

struct myclass {           // function object type:
  void operator() (int i) {std::cout << ' ' << i;}
} myobject;

int main () {
  std::vector myvector;
  myvector.push_back(10);
  myvector.push_back(20);
  myvector.push_back(30);

  std::cout << "myvector contains:";
  for_each (myvector.begin(), myvector.end(), myfunction);
  std::cout << '\n';

  // or:
  std::cout << "myvector contains:";
  for_each (myvector.begin(), myvector.end(), myobject);
  std::cout << '\n';

  //set
  std::set values(myvector.begin(), myvector.end());
  std::cout << "set contains:";
  for_each(values.begin(), values.end(), myfunction);
  std::cout << '\n';

  return 0;
}

Output

myvector contains: 10 20 30
myvector contains: 10 20 30
set contains: 10 20 30