Don't copy when you can swap

Recently I worked on a cpp code beatifier, it reads in a cpp file, analysis the content and beautify the format, then write the file back. In my code there's a function like this:

void beautify_a_file(const string& cpp_file)
{
   string content;
   // load the content
   {
      ifstream ifs(cpp_file.c_str());
      string tmp((std::istreambuf_iterator<char>(ifs)), std::istreambuf_iterator<char>());
      content=tmp; // this is bad, two large strings are constructed and then tmp is destroied
   }

   process(content);

   // write it back
   {
      ofstream ofs(cpp_file.c_str());
      ofs<<content;
   }
}

Let's look at the line content=tmp; this is a very time consuming operation, for the string here could be really big. A better way is to use string::swap, the method interchange of the two string's buffer ownership and no time-onsuming buffer copy is involved.

The fix: Change:

content=tmp;

to:

content.swap(tmp);

in <string> the std::swap is overloaded to be using string::swap, so you can also use:

std::swap(content, tmp);

Nearly all std containers have their own swap implementations, use them, instead of copy consructor!

转载于:https://www.cnblogs.com/tactoth/archive/2009/09/12/1565530.html