stringstream 和 str 不同步 [英] stringstream and str not synchronized

问题描述

我正在用 C++ 编写一个简单的解析器.我想用 std::ws 删除前导空格.

I'm writing a simple parser in c++. I would like to remove leading whitespaces with std::ws.

bool            Parser::readWhiteSpace()
{
  std::cout << "Before : str=[" << this->_ss.str() << "], peek=[" << (char)this->_ss.peek() << ']'<< std::endl;
  this->_ss >> std::ws;
  std::cout << "After : str=[" << this->_ss.str() << "], peek=[" << (char)this->_ss.peek() << ']'<< std::endl;                                                                                                                                          

  return (true);
}

输出是:

Before : str=[   something], peek=[ ]
After :  str=[   something], peek=[s]

我不明白为什么流和流中的 str 不同步.它不应该影响 str 吗?

I don't understand why the stream and the str from the stream are not synchronized. Is it not supposed to affect the str ?

推荐答案

字符串流有一个指针，即输出位置指示符，它指向下一个"字符.通过修剪前导空白，后备缓冲区本身不会被修改，但是这个位置指示器会增加.std::ws 读取一个字符直到它变成一个空格，因此你最后一次查看会发现这个指示符指向 s.

The string stream has a pointer, the output position indicator, which points at the "next" character. By trimming leading whitespace, the backing buffer itself is not modified, but this position indicator is incremented. std::ws reads a character until it's a whitespace, thus your last peek would find this indicator pointing to s.

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