generating random enums

Question

How do I randomly select a value for an enum type in C++? I would like to do something like this.

enum my_type(A,B,C,D,E,F,G,h,J,V);
my_type test(rand() % 10);

But this is illegal... there is not an implicit conversion from int to an enum type.

static_cast is a good friend in such situations. – Edward Strange Jun 08 '10 at 16:10 — Edward Strange, Jun 08 '10 at 16:10

zildjohn01 · Accepted Answer · 2010-06-08T18:55:15.147

30

How about:

enum my_type {
    a, b, c, d,
    last
};

void f() {
    my_type test = static_cast<my_type>(rand() % last);
}

edited Jun 08 '10 at 18:55

answered Jun 08 '10 at 15:58

zildjohn01

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1

+1 for less hard-coding, but I recommend the C++ style static_cast. – Bill Jun 08 '10 at 16:52
4

This won't work if the enum values aren't contiguous. – einpoklum Aug 15 '16 at 13:44
@einpoklum Can you give an example? – Sven van den Boogaart Oct 17 '17 at 14:08
@SvenvandenBoogaart: `a = 1, b = 2, c = 4, d = 16`. The `rand()` will give you invalid values. – einpoklum Oct 17 '17 at 18:48
... also, never use `rand()`, it is [considered harmful](https://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful). – einpoklum Oct 17 '17 at 18:49
Use of C++ random number facilities now preferred – Bart Louwers Jun 24 '19 at 10:35

score 9 · Answer 2 · answered Jun 08 '10 at 16:03

9

There is no implicit conversion, but an explicit one will work:

my_type test = my_type(rand() % 10);

answered Jun 08 '10 at 16:03

Mike Seymour

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Some individual mentioned in the accepted answer that implicit conversion one won't work if the enum values aren't contiguous. Does it also apply to this? – knoxgon Sep 27 '17 at 06:25
@VolkanGüven Yes. This is the same as the accepted answer. – lmat - Reinstate Monica Dec 14 '21 at 03:37

score 8 · Answer 3 · answered Jun 24 '19 at 10:48

Here is how I solved a similar problem recently. I put this in an appropiate .cc file:

static std::random_device rd;
static std::mt19937 gen(rd());

Inside the header that defines the enum:

enum Direction
{
    N,
    E,
    S,
    W
};
static std::vector<Direction> ALL_DIRECTIONS({Direction::N, Direction::E, Direction::S, Direction::W});

And to generate a random direction:

Direction randDir() {
    std::uniform_int_distribution<size_t> dis(0, ALL_DIRECTIONS.size() - 1);
    Direction randomDirection = ALL_DIRECTIONS[dis(gen)];
    return randomDirection;
}

Don't forget to

#include <random>

generating random enums

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