c++ 标准模板库 STL 算法之 for_each 函数的使用用法详解

std::for_each

template <class InputIterator, class Function>   Function for_each (InputIterator first, InputIterator last, Function fn);

Apply function to range

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

The behavior of this template function is equivalent to:

123456789

template<class InputIterator, class Function>  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 andlast, including the element pointed by first but not the element pointed bylast.

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.

Applies a specified function object to each element in a forward order within a range and returns the function object.

template<class InputIterator, class Function>    Function for_each(       InputIterator _First,        InputIterator _Last,        Function _Func    );

Parameters

---

_First

An input iterator addressing the position of the first element in the range to be operated on.

_Last

An input iterator addressing the position one past the final element in the range operated on.

_Func

User-defined function object that is applied to each element in the range.

Return Value

---

A copy of the function object after it has been applied to all of the elements in the range.

Remarks

---

The algorithm for_each is very flexible, allowing the modification of each element within a range in different, user-specified ways. Templatized functions may be reused in a modified form by passing different parameters. User-defined functions may accumulate information within an internal state that the algorithm may return after processing all of the elements in the range.

The range referenced must be valid; all pointers must be dereferenceable and, within the sequence, the last position must be reachable from the first by incrementation.

The complexity is linear with at most (_Last – _First) comparisons.

头文件：

<algorithm>

说明：

for_each算法范围 [_First, _ Last) 中的每个元素调用函数_Fn1，并返回输入的参数 _Func。此函数不会修改序列中的任何元素。

// for_each example#include <iostream>     // std::cout#include <algorithm>    // std::for_each#include <vector>       // std::vector#include <list>void myfunction (int i){  // function:  std::cout << ' ' << i;}struct myclass{           // function object type:  void operator() (int i) {std::cout << ' ' << i;}} myobject;void test_my(){    std::vector<int> 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';    std::cout << "myvector contains:";    for_each (myvector.begin(), myvector.end(), myclass());    std::cout << '\n';}/******运行结果：myvector contains: 10 20 30myvector contains: 10 20 30myvector contains: 10 20 30******///print为仿函数struct print{    int count;    print(){count = 0;}    void operator()(int x)    {        std::cout << x << std::endl;        ++count;    }};void test_print(){    using namespace std;    list<int> testlist;    //初始化    for ( size_t i = 1991; i <2013 ; ++i)    {        testlist.push_back(i);    }    //遍历testlist元素并打印    print p = for_each(testlist.begin(), testlist.end(), print());    //打印testlist元素个数    cout << "元素个数是："<<p.count << endl;}/******运行结果：1991199219931994199519961997199819992000200120022003200420052006200720082009201020112012元素个数是：22******/class Multi_By_x{    private:        int multiplier;    public:        Multi_By_x(const int& k) : multiplier(k) { }        void operator()(const int& x) { std::cout << x * multiplier << " " << std::endl; }};void test_Multi_By_x(){  int sequence[6] = {2007,2010,2011,2012,2013,2014};  using std::vector;  vector<int>  v(sequence, sequence+6);  Multi_By_x Multi2(2);  for_each(v.begin(),v.end(),Multi2);   // Apply function}/*************运行结果：401440204022402440264028*************/int main (){    test_my();    std::cout<<"\n***************************************************\n";    test_print();    std::cout<<"\n***************************************************\n";    test_Multi_By_x();    std::cout<<"\n***************************************************\n";    return 0;}/***************myvector contains: 10 20 30myvector contains: 10 20 30myvector contains: 10 20 30***************************************************1991199219931994199519961997199819992000200120022003200420052006200720082009201020112012元素个数是：22***************************************************401440204022402440264028***************************************************Process returned 0 (0x0)   execution time : 0.039 sPress any key to continue.****************/

再看一下MSDN上边给出的例子程序：

#include <vector>#include <algorithm>#include <iostream>// The function object multiplies an element by a Factortemplate <class Type>class MultValue{private:   Type Factor;   // The value to multiply bypublic:   // Constructor initializes the value to multiply by   MultValue ( const Type& _Val ) : Factor ( _Val )   {   }   // The function call for the element to be multiplied   void operator ( ) ( Type& elem ) const   {      elem *= Factor;   }};// The function object to determine the averageclass Average{private:   long num;      // The number of elements   long sum;      // The sum of the elementspublic:   // Constructor initializes the value to multiply by   Average ( ) : num ( 0 ) , sum ( 0 )   {   }   // The function call to process the next elment   void operator ( ) ( int elem )   {      num++;      // Increment the element count      sum += elem;   // Add the value to the partial sum   }   // return Average   operator double ( )   {      return  static_cast <double> (sum)/static_cast <double> (num);   }};int main( ){   using namespace std;   vector <int> v1;   vector <int>::iterator Iter1;   // Constructing vector v1   int i;   for ( i = -4 ; i <= 2 ; i++ )   {      v1.push_back(  i );   }   cout << "Original vector  v1 = ( " ;   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )      cout << *Iter1 << " ";   cout << ")." << endl;   // Using for_each to multiply each element by a Factor   for_each ( v1.begin ( ) , v1.end ( ) , MultValue<int> ( -2 ) );   cout << "Multiplying the elements of the vector v1\n "        <<  "by the factor -2 gives:\n v1mod1 = ( " ;   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )      cout << *Iter1 << " ";   cout << ")." << endl;   // The function object is templatized and so can be   // used again on the elements with a different Factor   for_each (v1.begin ( ) , v1.end ( ) , MultValue<int> (5 ) );   cout << "Multiplying the elements of the vector v1mod\n "        <<  "by the factor 5 gives:\n v1mod2 = ( " ;   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )      cout << *Iter1 << " ";   cout << ")." << endl;   // The local state of a function object can accumulate   // information about a sequence of actions that the   // return value can make available, here the Average   double avemod2 = for_each ( v1.begin ( ) , v1.end ( ) ,Average ( ) );   cout << "The average of the elements of v1 is:\n Average ( v1mod2 ) = "        << avemod2 << "." << endl;}/*****************运行结果如下：Original vector  v1 = ( -4 -3 -2 -1 0 1 2 ).Multiplying the elements of the vector v1 by the factor -2 gives: v1mod1 = ( 8 6 4 2 0 -2 -4 ).Multiplying the elements of the vector v1mod by the factor 5 gives: v1mod2 = ( 40 30 20 10 0 -10 -20 ).The average of the elements of v1 is: Average ( v1mod2 ) = 10.Process returned 0 (0x0)   execution time : 0.122 sPress any key to continue.******************/