c++ - 如何从字符串流中的同一位置读取两次？

Question

我有一个正在读取的 stringstream 实例。在从流中获取数据的某个时刻，我需要读取一个可能存在也可能不存在的标识符。逻辑是这样的:

std::string identifier;
sstr >> identifier;
if( identifier == "SomeKeyword" )
    //process the the rest of the string stream using method 1
else
   // back up to before we tried to read "identifier" and process the stream using method 2

如何实现上述逻辑？

Answer 1

使用流的tellg()和 seekg()方法，例如:

std::string identifier;
std::stringstream::pos_type pos = sstr.tellg();
sstr >> identifier;
if (identifier == "SomeKeyword")
{
    //process the rest of the string stream using method 1
}
else
{
    // back up to before we tried to read "identifier
    sstr.seekg(pos);
    // process the stream using method 2
}