现代C++ 基于范围的for和for_each语句

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.