how can I read exactly 128 bytes from an fstream into a string object?

Question

How do I read exactly 128 bytes from an fstream into a string object?

I wrote some code to read the first 128 bytes of a file and print it and then the last 128 bytes of the file and print that. The last part works, since you can easily iterate to EOF, but how do I get exactly 128 bytes from the front? The code below doesn't work since you can't add 128 to an ifstream iterator, it's not indexable, only incrementable (it seems).

Sure I could make an iterator and *++ it 128 times, but there must be a single line straightforward way to do it, right?

#include <iostream>
#include <fstream>
#include <string>

int main(int argc, char **argv)
{
    std::ifstream ifs ("input.txt",std::ifstream::in | std::ifstream::binary);

    if (ifs.good())
    {
    // read first 128 bytes into a string
        ifs.seekg(0,std::ifstream::beg);
        std::string first128((std::istreambuf_iterator<char>(ifs)),
                             (std::istreambuf_iterator<char>(ifs))+128);

        std::cout << first128 << std::endl;

    // read last 128 bytes into a string
        ifs.seekg(-128,std::ifstream::end);
        std::string last128((std::istreambuf_iterator<char>(ifs)),
                            std::istreambuf_iterator<char>());

        std::cout << last128 << std::endl;

        return 0;
    }

    return 1;
}

Answer 1

char buffer[129];
ifs.read (buffer,128);
buffer[128] = '\0';
first128 = buffer;

How about this then:

template <typename Itr, typename Out>
void copy_n(Itr it, size_t count, Out out)
{
    for(size_t i=0;i<count;++i)
      out = *it++;
} 

...

std::string first128; 
std::istreambuf_iterator<char> it(ifs);
copy_n( it, 128,
  std::back_inserter<std::string>(first128) );

Answer 2

My answer uses an intermediate buffer, but perhaps you will be happy that it uses iterators to initialize the string from the buffer.

std::vector<char> buffer(128); // create a buffer
ifs.read( &buffer[0], buffer.size() ); // read to buffer
std::string first128( buffer.begin(), buffer.end() ); // copy from vector

To me it seems like they got a little too cute with the implementation of iostreams. Trying to use iterators for stream I/O is overly complicated.

By the way, I suspect the implementation that you were attempting will, under the covers, do a variety of intermediate buffering (perhaps some in the kernel, some in the library) as well as re-allocating and copying the string several times as it grows.

One other idea: Do you really need the result in a standard string? You might just work from the vector -- avoiding the final step of copying to a string. Or, if you are feeling adventurous, you could create your own string class that does allow you to expose the internal buffer in the same way that vector does.

Answer 3

I think that the following related question has the answer you're seeking.