std::istringstream from std::string without copying

Question

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?

`ss.rdbuf()->pubsetbuf(&line.front(), line.size());` for short version, also avoids ugly cast — Xeverous, Jan 06 '18 at 17:21
I wonder why `istringstream` doesn't have a constructor to move from a string — M.M, Mar 30 '18 at 02:54

score 4 · Accepted Answer · answered Jul 03 '13 at 15:55

4

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.

answered Jul 03 '13 at 15:55

David G

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score -1 · Answer 2 · answered Mar 30 '18 at 02:18

-1

The effect of ss.rdbuf()->pubsetbuf is implementation-defined and hence doesn't necessarily do what you expect.

So, the effect of your altered code doesn't need to be equivalent to the initial one.

answered Mar 30 '18 at 02:18

0xbadf00d

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std::istringstream from std::string without copying

2 Answers2

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