Seed random in 64-Bit

问题:

With the iPhone 5S update I want my app to be able to support the new 64-Bit processor.

However, using 64-Bit may cause truncation if a larger data type is casted into a smaller one, as in the case of casting a long into an int. Most of the time this can be easily fixed by just using the bigger data type, but in the case of random number generators which are sometimes seeded by using the "time(NULL)" function I cannot do that.

The current code is simple:

srandom(time(NULL));

But in XCode 5 with 64-Bit it is causing the following error: Implicit conversion loses integer precision: 'time_t' (aka 'long') to 'unsigned int'. This is because "time(NULL)" returns a long integer and "srandom" requires an unsigned int. Therefore there are two options:

1. Convert the long integer to an unsigned int
2. Replace "time(NULL)" with another function which does the same job but returns an unsigned int.

Which one would you recommend and what function should I use to do it?

NOTE: I use random() instead of arc4random() because I also need to be able to seed the random number generator in order to get a repeatable outcome.

回答1:

time() typically returns the number of seconds since the epoch (not counting leap seconds), which means if you use it more than once in a second (or two people run the program at the same time) then it will return the same value, resulting in a repeated sequence even when you don't want it. I recommend against using time(NULL) as a seed, even in the absence of a warning (or error with -Werror) caused by the truncation.

You could use arc4random() to get a random seed instead of a seed based on time. It also happens to return an unsigned 32-bit value which will fix the error you're seeing.

srandom(arc4random());

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You might consider moving to Objective-C++ so that you can use the standard C++ <random> library, which is much more powerful and flexible, and which also enables simpler and more direct expression of many ideas, than these other libraries

C++ <random> documentation

回答2:

On iOS, just use arc4random(3) and don't worry about seeding.