How to copy a prefix of an input stream to a different stream in C++?

Question

There's a neat trick that can be used to copy file contents in C++. If we have an std::ifstream input; for one file and an std::ofstream output; for a second file, the contents of input can be copied to output like this:

output << input.rdbuf();

This solution copies the entirety of the first file (or at least the entirety of the input stream that hasn't been consumed yet). My question is, how can I copy only a prefix (n first bytes) of an input stream to an output stream?

After looking through a bit of documentation, my idea was to somehow shorten the stream of the input and then copy it to the output stream:

input.rdbuf()->pubsetbuf(input.rdbuf()->eback(), length_to_output);
output << input.rdbuf();

The problem is, that this won't compile as eback is not public. This is only supposed to illustrate my idea. I know that I could just copy the entire input into a string and then copy a substring of it into the output, but I am worried that it will be less time- and memory-efficient. As such, I thought about using streams instead, like above.

Answer 1

I tried different solutions, including the one presented by @Some programmer dude, and ultimately decided to go with a manual read and write loop. Below is the code that I used (based on this, with small modifications) and at the bottom are the benchmark results:

bool stream_copy_n(std::istream& in, std::ostream& out, std::size_t count) noexcept
{
    const std::size_t buffer_size = 256 * 1024; // a bit larger buffer
    std::unique_ptr<char[]> buffer = std::make_unique<char[]>(buffer_size); // allocated on heap to avoid stack overflow
    while(count > buffer_size)
    {
        in.read(buffer.get(), buffer_size);
        out.write(buffer.get(), buffer_size);
        count -= buffer_size;
    }

    in.read(buffer.get(), count);
    out.write(buffer.get(), count);

    return in.good() && out.good(); // returns if copy was successful 
}

The benchmark results (when copying an entire file 1GB file) acquired using the built in Unix time command, real time:

Method	Time
Linux C function sendfile	0.59
std::filesystem::copy_file	0.60
Unix command cp	0.69
Manual read and write loop presented above	0.78
output << input.rdbuf()	0.96
std::copy_n(std::istreambuf_iterator<char>(input), std::filesystem::file_size(inputFilePath), std::ostreambuf_iterator<char>(output));	3.28
std::copy_n(std::istream_iterator<char>(input), std::filesystem::file_size(inputFilePath), std::ostream_iterator<char>(output));	27.37

Despite the fact that it is not the fastest, I chose the read-write loop as it uses stream objects and isn't exclusive to only copying files.