How to read a fixed number of bytes from a c++ std::istream

Question

How to read a fixed number of bytes from a std::istream without doing any extraction? For example, I have a variable sz of type size_t and I would like to read sizeof(size_t) bytes from the istream.

void foo(std::istream& is) {
  if(is.rdbuf()->in_avail() < sizeof(size_t)) return;
  // how to read to sz from istream is without extraction (advancing pointers)
  size_t sz;
}

Answer 1

You can only peek the next character without extracting.

As such, you should change your strategy: instead of trying to avoid extraction, extract the characters that you need, and then restore the state of the stream. That is possible if the stream supports seeking:

• use tellg to get the current position
• extract the bytes
• use seekg to jump to the earlier position

Otherwise, you may need to implement a buffer of your own to do whatever you're trying to achieve by "reading without extracting".