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c++istringstream

std::istringstream from std::string without copying

I've been using this:

ifstream in("file.txt")
string line;    
getline(in,line);
istringstream iss(line);
...

for some simple parsing. I would like to avoid unnecessary copying in order to improve performance so I tried:

ifstream in("huge_line.txt");
string line;
getline(in,line);
istringstream ss;
ss.rdbuf()->pubsetbuf(const_cast<char*>(line.c_str()), line.size());
...

and it seems to do the job (significantly improve performance, that is). My question is, is this safe given the const_cast? I mean, as long as I'm working with an istrinstream, the internal buffer should never get written to by the istringstream class, so the ss variable should remain in a valid state as long as the line variable is valid and unchanged, right?

Solution

  • The const_cast is safe, because the underlying buffer of std::string is not const. And yes, as long as line does not expire while ss is being read from, your program should be fine.