Why does ostringstream strip NULL?

Question

I have a string whose last part(suffix) needs to be changed several times and I need to generate new strings. I am trying to use ostringstream to do this as I think, using streams will be faster than string concatenations. But when the previous suffix is greater than the later one, it gets messed up. The stream strips off null characters too.

#include<iostream>
#include<sstream>

using namespace std;

int main()
{
  ostringstream os;
  streampos pos;
  os << "Hello ";
  pos = os.tellp();
  os << "Universe";
  os.seekp(pos);
  cout<<  os.str() << endl;
  os << "World\0";
  cout<<  os.str().c_str() << endl;
  return 0;
}

Output

Hello Universe
Hello Worldrse

But I want Hello World. How do I do this? Is there anyother way to do this in a faster manner?

Edit: Appending std::ends works. But wondering how it works internally. Also like to know if there are faster ways to do the same.

No so sure that stringstream will be the most efficient - you should consider one of the variants of `std::string::replace()` — Nim, Jul 14 '11 at 08:14

MSalters · Answer 1 · 2011-07-14T08:49:23.917

The string "World" is already null-terminated. That's how C strings work. "World\0" has two \0 characters. Therefore, operator<<(ostream&, const char*) will treat them the same, and copy all characters up to \0. You can see this even more clearly, if you try os << "World\0!". The ! will not be copied at all, since operator<< stopped at the first \0.

So, it's not ostringstream. It's how C strings aka const char* work.

score 3 · Accepted Answer · answered Jul 14 '11 at 08:00

It doesn't strip anything. All string literals in C++ are terminated by NUL, so by inserting one manually you just finish the string, as far as anyone processing it is concerned. Use ostream::write or ostream::put, if you need to do that — anything that expects char* (with no additional argument for size) will most likely treat it specially.

os.write("World\0", 6);

score 0 · Answer 3 · answered Jul 14 '11 at 09:44

Why do you think a stream operation is faster than a string? And why build the string before outputting to cout?

If you want a prefix to your output you could just do it like this

const std::string prefix = "Hello ";

std::cout << prefix << "Universe" << std::endl;
std::cout << prefix << "World" << std::endl;

Why does ostringstream strip NULL?

3 Answers3