c++ - 为什么连续的 istream_iter 对象会递增迭代流？

Question

为什么下面的代码会输出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 实例都在隐式迭代 stringstream 的“内部内容”。为什么每个 istream_iterator 实例不是从其 istream_type 的开头开始？

Answer 1

是的，constructor of istream_iterator执行读取，i、j、k 使用相同的流，因此它们是交互的。

(强调我的)

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.