Thread: problem with boost random func (again)

Two things about the error reported:

main.obj : error LNK2005: "class boost::random::mersenne_twister<unsigned long,32,624,397,31,2567483615,11,7,2636928640,15,4 022730752,18,3346425566> rng" (?rng@@3V?$mersenne_twister@K$0CA@$0CHA@$0BIN@$0BP @$0JJAILANP@$0L@$06$0JNCMFGIA@$0P@$0OPMGAAAA@$0BC@ $0MHHGGGNO@@random@boost@@A) already defined in http.obj

The first tells you that rng is the problem. mt19937 is in fact a typedef for that long an awkward template class you see there. By this time the linker already has no knowledge of typedefs name substitution. That is simply an aid for us humans.

The second tells you that there is a redefinition of rng. rng is being defined twice. And this is the actual error. At this point you should know the difference between declaring and defining a name.

int val;

Defines an integer named val. In other words, it creates the variable and allocates storage for it.

extern int val;

Declares an integer named val. The definition will happen somewhere else. In other words it simply introduces this name into the scope delegation its definition to some cpp file that includes the header where this extern is.

You should never define variables in header files for the reason you just found out. Changes are you will come across multiple definitions of a name. And the rules are multiples declarations are ok, as long as there is just one definition

You provide little information as to why you want to move that definition to a header file. But no matter what, the solution is to prepend extern

On the particular case of a mersenne twister implementation, I don't see a problem with creating as many instances as needed when you want to deal with random numbers. Two different functions using it should each declare their own MT generator at function scope.

rnd.cpp:

Code:

namespace rnd {
namespace {
mt19937 rng;
}

void my_srand() {
	detail::rng.seed( time_seed() );
}

int my_rand(int min, int max) {
	uniform_int<> dist(min, max);
	variate_generator<mt19937&, uniform_int<> > die(detail::rng, dist);

	return die();
}
}

rnd.hpp:

Code:

namespace rnd {
  void my_srand();
  int my_rand(int min, int max);
}