6.12a For-each loops

原完整教程链接：6.12a For-each loops

1.
/* The for-each statement has a syntax that looks like this:

   for (element_declaration : array)
      statement;
*/

// Let’s take a look at a simple example that uses a for-each loop to 
// print all of the elements in an array named fibonacci:

int main()
{
    int fibonacci[] = { 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 };
    for (int number : fibonacci) // iterate over array fibonacci
       std::cout << number << ' '; // we access the array element for this iteration through variable number
 
    return 0;
}
// This prints:
// 0 1 1 2 3 5 8 13 21 34 55 89

2.
/* 
   Because element_declaration should have the same type as the 
   array elements, this is an ideal case in which to use the auto 
   keyword, and let C++ deduce the type of the array elements for us.
*/
int main()
{
    int fibonacci[] = { 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 };
    for (auto number : fibonacci) // type is auto, so number has its type deduced from the fibonacci array
       std::cout << number << ' ';
 
    return 0;
}

3.
/*
   Each array element iterated over will be copied into variable 
   element. (此处可以按照函数利用括号传递参数的形式来理解，即把
   for (auto element: array) 想象成 function(auto element) {})
   Copying array elements can be expensive, and most of 
   the time we really just want to refer to the original element. 
   Fortunately, we can use references for this:
*/
   int array[5] = { 9, 7, 5, 3, 1 };
   for (auto &element: array) // The ampersand makes element a reference to
                              // the actual array element, preventing a copy 
                              // from being made
        std::cout << element << ' ';

4.
// And, of course, it’s a good idea to make your element const if 
// you’re intending to use it in a read-only fashion:
   int array[5] = { 9, 7, 5, 3, 1 };
   for (const auto &element: array) // element is a const reference to 
                                    // the currently iterated array element
        std::cout << element << ' ';

5.
**For-each doesn’t work with pointers to an array!**
/*
   In order to iterate through the array, for-each needs to know how 
   big the array is, which means knowing the array size. Because 
   arrays that have decayed into a pointer do not know their size, 
   for-each loops will not work with them!
*/
#include <iostream>
 
int sumArray(int array[]) // array is a pointer
{
    int sum = 0;
    for (const auto &number : array) // compile error, the size of array 
                                     // isn't known
        sum += number;
 
    return sum;   
}
 
int main()
{
     int array[5] = { 9, 7, 5, 3, 1 };
     std::cout << sumArray(array); // array decays into a pointer here
     return 0;
}
/*
   总结：在通过参数列表传递数组名的时候，数组名其实被“退化”成
   了一个指针。平时我们总认为数组名就等价于数组中第一个元素的
   指针，其实是不够准确的。数组名除了包含第一个元素的指针，还
   包含了数组的大小。因此在一些需要知道数组大小的上下文环境
   中，如for-each loop，我们通过参数列表传递的数组名被隐式地“退
   化”成了一个指针，丢掉了数组大小这个信息，便会出现编译错误。
*/