Why not just use random_device?

Question

I am a bit confused about the c++11 random library.

What I understand: we need two separate concepts:

• random engine (which can be pseudo (need seed) or

Answer 1

This article is a good point to start.

I'm going to synthesize just few points:

• It Has Unknown Cost.

  How costly it is to read a number from this “device”? That is unspecified. It could, for example, be reading from /dev/random on a Linux system, which can block for an extended period waiting for entropy (which is itself problematic for a variety of reasons).

For my personal experience I've notified that std::random_device is usually slower than a simple Pseudo-randomic algorithm. That could be no true in general, but usually it does. That because it may involve physical devices, or other hardware than the simple CPU.

• It Might Actually Be Deterministic .

  C++11's std::random_device is not required to be nondeterministic! Implementations can and do implement it as a simple RNG with a fixed seed, so it produces the same output for every run of the program.