基于范围的for循环

语法：

 for ( for-range-declaration : expression )
statement 

注意一般用auto表达类型。不需要修改时常用引用类型

例子：

 // range-based-for.cpp
 // compile by using: cl /EHsc /nologo /W4
 #include <iostream>
 #include <vector>
 using namespace std;

 int main()
 {
     // Basic 10-element integer array.
     int x[] = { , , , , , , , , ,  };

     // 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;

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

     for( auto y : x ) { // Copy of 'x', almost always undesirable
         cout << y << " ";
     }
     cout << endl;

     for( auto &y : x ) { // Type inference by reference.
         // Observes and/or modifies in-place. Preferred when modify is needed.
         cout << y << " ";
     }
     cout << endl;

     for( const auto &y : x ) { // Type inference by reference.
         // Observes in-place. Preferred when no modify is needed.
         cout << y << " ";
     }
     cout << endl;
     cout << "end of integer array test" << endl;
     cout << endl;

     // Create a vector object that contains 10 elements.
     vector<double> v;
     for (int i = ; i < ; ++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;
     cout << "end of vector test" << endl;
 }