输入流上基于范围的循环

本文介绍了输入流上基于范围的循环的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

要遍历输入流，通常会像这样使用std::istream_iterator:

To iterate over an input stream, we would usually use a std::istream_iterator like so:

typedef std::istream_iterator<std::string> input_iterator; std::ifstream file("myfile"); for (input_iterator i(file); i != input_iterator(); i++) { // Here, *i denotes each element extracted from the file }

如果我们可以使用基于范围的for语句来迭代输入流，那就太好了.但是，对于类类型的对象，基于范围的for要求对象具有begin()和end()成员函数(第6.5.4节，添加了粗体):

It'd be nice if we could use the range-based for statement to iterate over input streams. However, for objects of class type, range-based for requires the object to have begin() and end() member functions (§6.5.4, bold emphasis added):

• 如果_RangeT是数组类型，则 begin-expr 和 end-expr 分别是__range和__range + __bound，其中是数组绑定.如果_RangeT是未知大小的数组或类型不完整的数组，则程序格式错误；

  • 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;

    如果_RangeT是类类型，则在好像按类成员访问查找(3.4.5)，并且如果一个(或两个)都找到至少一个声明，则 begin-expr 和 end-expr 为__range.begin()和__range.end()分别；

    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;

    否则， begin-expr 和 end-expr 分别为begin(__range)和end(__range)，其中begin和end与参数相关的查找(3.4.2).出于此名称查找的目的，名称空间std是关联的名称空间.

    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.

    输入流不具有这些成员函数(它们不是Container)，因此基于范围的for不适用于它们.无论如何，这是有道理的，因为您将需要某种方式来指定要提取的类型(在上述情况下为std::string).

    The input streams don't have these member functions (they are not Containers) and so range-based for won't work on them. This makes sense anyway because you would need some way to specify the type to extract (std::string in the case above).

    但是，如果我们知道要提取的内容，是否可以为输入流定义我们自己的begin()和end()函数(也许是std::begin()和std::end()的特殊化或重载)，以便如上所述由类成员访问查找找到?

    But if we know what we want to extract, is it possible to define our own begin() and end() functions (perhaps specializations or overloads of std::begin() and std::end()) for input streams such that they would be found by class member access lookup as described above?

    从6.5.4节开始(至少对我而言)尚不清楚，如果先前的查找失败，那么是否将使用依赖于参数的查找来查找函数.要考虑的另一件事是，std::ios_base及其派生词已经有一个名为end的成员，该成员是进行搜索的标志.

    It's unclear (at least to me) from §6.5.4 whether the functions will then be looked up with argument-dependent lookup if the previous lookup fails. Another thing to consider is that std::ios_base and its derivatives already have a member called end which is a flag for seeking.

    这是预期的结果:

    std::ifstream file("myfile"); for (const std::string& str : file) { // Here, str denotes each element extracted from the file }

    或者:

    std::ifstream file("myfile"); for (auto i = begin(file); i != end(file); i++) { // Here, *i denotes each element extracted from the file }
    推荐答案

    一种显而易见的方法是为流使用简单的装饰器，以提供类型和必要的接口.这是这样的样子:

    An obvious approach is to use a simple decorator for your stream providing the type and the necessary interface. Here is how this could look like:

    template <typename T> struct irange { irange(std::istream& in): d_in(in) {} std::istream& d_in; }; template <typename T> std::istream_iterator<T> begin(irange<T> r) { return std::istream_iterator<T>(r.d_in); } template <typename T> std::istream_iterator<T> end(irange<T>) { return std::istream_iterator<T>(); } for (auto const& x: irange<std::string>(std::ifstream("file") >> std::skipws)) { ... }