How can I find out how many bytes are available from a std::istream?

Question

If I wanted to read() the content of a std::istream in to a buffer, I would have to find out how much data was available first to know how big to make the buffer. And to get the number of available bytes from an istream, I am currently doing something like this:

std::streamsize available( std::istream &is )
{
    std::streampos pos = is.tellg();
    is.seekg( 0, std::ios::end );
    std::streamsize len = is.tellg() - pos;
    is.seekg( pos );
    return len;
}

And similarly, since std::istream::eof() isn't a very useful fundtion AFAICT, to find out if the istream's get pointer is at the end of the stream, I'm doing this:

bool at_eof( std::istream &is )
{
    return available( is ) == 0;
}

My question:

Is there a better way of getting the number of available bytes from an istream? If not in the standard library, in boost, perhaps?

Answer 1

For std::cin you don't need to worry about buffering because it is buffered already --- and you can't predict how many keys the user strokes.

For opened binary std::ifstream, which are also buffered, you can call the seekg(0, std::ios:end) and tellg() methods to determine, how many bytes are there.

You can also call the gcount() method after reading:

char buffer[SIZE];

while (in.read(buffer,SIZE))
{
  std::streamsize num = in.gcount();
  // call your API with num bytes in buffer 
}

For text input reading via std::getline(inputstream, a_string) and analyzing that string afterwards can be useful.

Answer 2

Posting this as an answer, as it seems to be what the OP wants.

I would have to find out how much data was available first to know how big to make the buffer - not true. See this answer of mine (second part).