Initializing static default_random_engine

Question

I have a simple class which will be useful for me with generating random numbers using default_random_engine.

Random.h:

#include <random>

using namespace std;

class Random
{
    public:
        Random();
        ~Random() {}
    private:
        static default_random_engine _engine;
};

Random.cpp:

#include "Random.h"

Random::Random()
{
    _engine = default_random_engine{}; //Will this be initialized every time I create an object of type Random?
}

Will _engine be initialized every time I create an object of type Random? I just want it to stay like it is after the first initialization because I was told I can use the same engine multiple times.

If that `using namespace std;` is there for reasons other than making the code in this very question shorter: Please don't. Putting it in a `.cpp` is already bad enough, but in headers, it's rather unacceptable. — Baum mit Augen, May 05 '16 at 15:04

OMGtechy · Answer 1 · 2016-05-05T15:04:49.060

3

Will _engine be initialized every time I create an object of type Random?

Yes.

I just want it to stay like it is after the first initialization

In which case, you should define it like so:

#include "Random.h"

std::default_random_engine Random::_engine;

Without this, even your original version won't compile (as you've not defined the static member).

edited May 05 '16 at 15:04

answered May 05 '16 at 14:56

OMGtechy

7,935
8
48
83

He needs to define it like that regardless. In order to get his desired result, he simply needs to not reassign it in his constructor (or anywhere else). – Benjamin Lindley May 05 '16 at 15:02
@BenjaminLindley true, I'll add this to my answer, thanks! :) – OMGtechy May 05 '16 at 15:04

score 0 · Accepted Answer · answered May 05 '16 at 15:02

0

The link @NathanOliver provided in my question helped me understand, which is to make a global variable of that type in the source file:

#include "random.h"

default_random_engine Question::_engine = default_random_engine{};

Random::Random() {  }

answered May 05 '16 at 15:02

Eyal

1,649
3
25
49

Initializing static default_random_engine

2 Answers2