为什么连续的 istream_iter 对象会增加迭代流? [英] Why do successive istream_iter objects increment the iterated stream?

问题描述

为什么下面的代码输出c?

Why does the following code output c?

// main.cpp
#include <iostream>
#include <sstream>
#include <iterator>

int main( int argc, char* argv[] )
{
  std::string p( "abcdefghijklmnopqrstuvwxyz" );
  std::stringstream ss(p);
  std::istream_iterator<char> i( ss );
  std::istream_iterator<char> j( ss );
  std::istream_iterator<char> k( ss );
  std::cout << *k << std::endl;

  return 0;
}

.

$ g++ --version
g++ (GCC) 9.2.1 20190827 (Red Hat 9.2.1-1)
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ -g ./main.cpp && ./a.out
c

这有点像每个连续的 istream_iterator 实例都隐式地迭代 stringstream 的内部内容".为什么每个 istream_iterator 实例不从其 istream_type 的开头开始?

It's sort of like each successive istream_iterator instance is implicitly iterating "something internal" to the stringstream. Why doesn't each istream_iterator instance start at the start of its istream_type?

推荐答案

是的，istream_iterator 的构造函数执行读取，并且 i、j、k 使用相同的流所以它们是互动的.

Yes, the constructor of istream_iterator performs reading, and i, j, k use the same stream so they're interactive.

(强调我的)

3) 初始化迭代器，将流的地址存储在一个数据中成员，并且 从输入流执行第一次读取到初始化缓存值数据成员.

3) Initializes the iterator, stores the address of stream in a data member, and performs the first read from the input stream to initialize the cached value data member.

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