C++ Cookbook

10.7. Copying a File

Problem

You need to copy one file to another in a portable manner, i.e., without using OS-specific APIs.

Solution

Use C++ file streams in <fstream> to copy data from one stream to another. Example 10-9 gives an example of a buffered stream copy.

Example 10-9. Copying a file

#include <iostream>
#include <fstream>

const static int BUF_SIZE = 4096;

using std::ios_base;

int main(int argc, char** argv) {

   std::ifstream in(argv[1],
      ios_base::in | ios_base::binary);  // Use binary mode so we can
   std::ofstream out(argv[2],            // handle all kinds of file
      ios_base::out | ios_base::binary); // content.
   
   // Make sure the streams opened okay...

   char buf[BUF_SIZE];

   do {
      in.read(&buf[0], BUF_SIZE);      // Read at most n bytes into
      out.write(&buf[0], in.gcount()); // buf, then write the buf to
   } while (in.gcount() > 0);          // the output.

   // Check streams for problems...

   in.close();
   out.close();
}

Discussion

Copying a file may appear to be a simple matter of reading from one stream and writing to another. But the C++ streams library is large, and there are a number of different ways to do the reading and the writing, so you should know a little about the library to avoid costly performance mistakes.

Example 10-9 runs fast because it buffers input and output. The read and write functions operate on entire buffers at a time—instead of a character-at-a-time copy loop—by reading from the input stream to the buffer and writing from the buffer to the output stream in chunks. They also do not do any kind ...