TITLE: copying a file's contents into a string

(Newsgroups: comp.lang.c++, 22 Mar 2000) 

LEWIS: Ryder Lewis <rlewis@liveoaktelecom.com>

> I'm trying to copy the contents of a file into a string, and have had
> very bad performance problems using the method in the code below. Is
> there a way to copy the contents of the file all at once, instead of
> appending the file one char at a time to the string?

KUEHL: Dietmar Kuehl <dietmar.kuehl@claas-solutions.de>

Just read the file when constructing the string:

  int main() {
    std::ifstream                  in(filename);
    std::istreambuf_iterator<char> beg(in), end;
    std::string                    str(beg, end);
  }

However, unless the library implementation uses some specializations
this is not likely to be as efficient as possible. If memory is not to
much of an issue and if it is acceptable if temporarily twice the size
of a string is occupied, you can do this which is probably the fastest
approach without hacking up some stream buffer:

  int main() {
    std::ifstream  in(filename);
    std::string    str;
    {
      std::ostringstream out;
      out << in.rdbuf();
      str = out.str();
    }
  }

The 'std::ostringstream' is used in a separate scope to have the
allocated memory released when it is no longer used: Otherwise the
'out' would hold a copy of the file whic is not really used but which
would occupy possible lots of memory. ... However, it would be really
nice if the first version would be really efficient without the
additional requirement of temporarily using lots of memory. Looks like
something I can hack at least for my implementation of the standard C++
library...



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