现代C++中强调，使用基于范围的 for 循环（Visual studio 2012之后的），相比于旧版的 for 循环更整洁和易于使用，并且不容易发生意外错误。让我们一睹为快。

当然，使用前需要包含头文件：

#include <algorithm>

1 基于范围的for语句

基于范围的for语句（Range-based for Statement），其语句形式为：

for ( for-range-declaration : expression )

    statement

看一段示例代码：

#include <iostream>

#include <vector>

using namespace std;

int main()

{

    cout << "Test 1 : ";

    // Basic 10-element integer array.

    int x[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

    // Range-based for loop to iterate through the array.

    for( int y : x ) { // Access by value using a copy declared as a specific type.

                       // Not preferred.

        cout << y << " ";

    }

    cout << endl;

    cout << "Test 2 : ";

    // The auto keyword causes type inference to be used. Preferred.

    for( auto y : x ) { // Copy of 'x', almost always undesirable

        cout << y << " ";

    }

    cout << endl;

    cout << "Test 3 : ";

    for( auto &y : x ) { // Type inference by reference.

        // Observes and/or modifies in-place. Preferred when modify is needed.

        cout << y << " ";

    }

    cout << endl;

    cout << "Test 4 : ";

    for( const auto &y : x ) { // Type inference by reference.

        // Observes in-place. Preferred when no modify is needed.

        cout << y << " ";

    }

    cout << endl;

    cout << "Test 5 : ";

    // Create a vector object that contains 10 elements.

    vector<double> v;

    for (int i = 0; i < 10; ++i) {

        v.push_back(i + 0.14159);

    }

    // Range-based for loop to iterate through the vector, observing in-place.

    for( const auto &j : v ) {

        cout << j << " ";

    }

    cout << endl;

}

输出结果为：

Test 1 : 1 2 3 4 5 6 7 8 9 10

Test 2 : 1 2 3 4 5 6 7 8 9 10

Test 3 : 1 2 3 4 5 6 7 8 9 10

Test 4 : 1 2 3 4 5 6 7 8 9 10

Test 5 : 0.14159 1.14159 2.14159 3.14159 4.14159 5.14159 6.14159 7.14159 8.14159 9.14159

2 for_each语句

for_each语句，其语句形式为:

template<class InputIterator, class Function>

   Function for_each(

      InputIterator _First,

      InputIterator _Last,

      Function _Func

   );

其中，_First是迭代器对应起始元素位置，_Last是迭代器对应的结束位置，必须是序列中可以访问的位置，_Func是用户定义的函数对象，将在迭代器范围内的所有元素，均应用到该函数中。

看一段代码示例：

// alg_for_each.h

#include <vector>

#include <algorithm>

#include <iostream>

// The function object multiplies an element by a Factor

template <class Type>

class MultValue

{

private:

   Type Factor;   // The value to multiply by

public:

   // 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 average

class Average

{

private:

   long num;      // The number of elements

   long sum;      // The sum of the elements

public:

   // 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);

   }

};

// main.cpp

#include "alg_for_each.h"

void main( )

{

   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;

}

运行结果为：

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.