Algorithm for generating a unique ID in C++?
What can be the best algorithm to generate a unique id in C++? The length ID should be a 32 bit unsigned integer.
Answer
Getting a unique 32-bit ID is intuitively simple: the next one. Works 4 billion times. Unique for 136 years if you need one a second. The devil is in the detail: what was the previous one? You need a reliable way to persist the last used value and an atomic way to update it.
How hard that will be depends on the scope of the ID. If it is one thread in one process then you only need a file. If it is multiple threads in one process then you need a file and a mutex. If is multiple processes on one machine then you need a file and a named mutex. If it is multiple processes on multiple machines then you need to assign a authoritative ID provider, a single server that all machines talk to. A database engine is a common provider like that, they have this built-in as a feature, an auto-increment column.
The expense of getting the ID goes progressively up as the scope widens. When it becomes impractical, scope is Internet or provider too slow or unavailable then you need to give up on a 32-bit value. Switch to a random value. One that's random enough to make the likelihood that the machine is struck by a meteor is at least a million times more likely than repeating the same ID. A goo-ID. It is only 4 times as large.