c++ - 基于范围的 for 循环等价物

Question

所以根据n2243基于范围的 for 循环等同于:

{
     auto && __range = ( expression );

     for ( auto __begin = std::Range<_RangeT>::begin(__range),
                  __end = std::Range<_RangeT>::end(__range);
          __begin != __end;
          ++__begin )
     {
         for-range-declaration = *__begin;
         statement
     }    
}

然后它说 2 If the header <iterator_concept> is not included prior to a use of the range-based for statement, the program is ill-formed.所以我质疑这是最新的。我也很好奇什么std::Range是或者如果它纯粹是一个实现细节。我能找到的最接近的是 n3350 .

这answer依赖此信息并说:

Range for is as fast as possible since it caches the end iterator[citation], uses pre-increment and only dereferences the iterator once.

so if you tend to write:

for(iterator i = cont.begin(); i != cont.end(); i++) { /**/ }

Then, yes, range-for may be slightly faster, since it's also easier to write there's no reason not to use it (when appropriate).

P.S. I said it's as fast as possible, it isn't however faster than possible. You can achieve the exact same performance if you write your manual loops carefully.

我很好奇它现在是否真的有所作为。据我所知，它只是语法糖。例如，在您可以执行 auto it = s.rbegin(); it != s.rend(); ++it 的循环中，它需要返回反向迭代器的样板代码，其中基于范围的 for 循环需要 begin。和 end .如果它所节省的只是打字，那么它还提供了哪些其他优势，因为它仅期望begin和 end ？我很好奇我上面引用的答案是否仍然有效，因为这篇论文是 2007 年的。

Answer 1

正如@DyP 所说，这部分在最终标准中发生了一些变化。 C++11 6.5.4 基于范围的for语句[stmt.ranged]:

1 For a range-based for statement of the form

  for ( for-range-declaration : expression ) statement

let range-init be equivalent to the expression surrounded by parentheses

  ( expression )

and for a range-based for statement of the form

  for ( for-range-declaration : braced-init-list ) statement

let range-init be equivalent to the braced-init-list. In each case, a range-based for statement is equivalent to

  {
    auto && __range = range-init;
    for ( auto __begin = begin-expr,
               __end = end-expr;
          __begin != __end;
          ++__begin ) {
      for-range-declaration = *__begin;
      statement
    }
  }

where __range, __begin, and __end are variables defined for exposition only, and _RangeT is the type of the expression, and begin-expr and end-expr are determined as follows:

• if _RangeT is an array type, begin-expr and end-expr are __range and __range + __bound, respectively, where __bound is the array bound. If _RangeT is an array of unknown size or an array of incomplete type, the program is ill-formed;

• if _RangeT is a class type, the unqualified-ids begin and end are looked up in the scope of class _RangeT as if by class member access lookup (3.4.5), and if either (or both) finds at least one declaration, begin-expr and end-expr are __range.begin() and __range.end(), respectively;

• otherwise, begin-expr and end-expr are begin(__range) and end(__range), respectively, where begin and end are looked up with argument-dependent lookup (3.4.2). For the purposes of this name lookup, namespace std is an associated namespace.

[ Example:

  int array[5] = { 1, 2, 3, 4, 5 };
  for (int& x : array)
    x *= 2;

—end example ]

2 In the decl-specifier-seq of a for-range-declaration, each decl-specifier shall be either a type-specifier or constexpr.