From:

"marco.guazzone@gmail.com" <marco.guazzone@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 24 Dec 2009 16:38:42 CST

Message-ID:

<0c8451a6-655c-4702-ba1b-4560eca16956@27g2000yqr.googlegroups.com>

I have problems in mixing static and dynamic polymorphism.

I need dynamic polymorphism in order to create "heterogeneous"

containers (e.g., std::vector<base*>).

Specifically, I have some "untouchable classes" (i.e., coming from

third party libraries) which represent

* random number generators

--- [code] ---

template <typename IntT, IntT a, IntT c, IntT m>

class linear_congruential

{

IntT operator()() {....} // Generate a random number

};

template <typename UIntT, size_t w, size_t n, size_t m, size_t r,

UIntT a, size_t u, size_t s, UIntT b, size_t t, UIntT c, size_t l>

class mersenne_twister

{

IntT operator()() {....} // Generate a random number

};

--- [/code] ---

* probability distributions

--- [code] ---

template <typename RealT>

class exponential {...};

template <typename RealT, typename GeneratorT>

RealT rand(exponential<RealT> const& d, GeneratorT& g) {...}

template <typename RealT>

class normal {...};

template <typename RealT, typename GeneratorT>

RealT rand(normal<RealT> const& d, GeneratorT& g) {...}

--- [/code] ---

Now since I need both to store different probability distributions

inside a vector and to decide what distribution to use at run-time,

I'm trying to create a set of adaptor and base classes in order to get

dynamic polymorphism.

Something like this:

--- [code] ---

namespace my {

template <typename RealT> class base_distribution {...};

template <typename DistributionT> class distribution_adaptor: public

base_distribution<typename DistributionT::real_type> {...};

} // Namespace my

int main() {

//...

std::vector<my::base_distribution<double>*> distrs;

distrs.push_back( new distribution_adaptor<normal>(normal(0,1)) );

distrs.push_back( new distribution_adaptor<exponential>(exponential

(1)) );

}

--- [/code] ---

The problem is how to glue with the "rand" free function during the

iteration of distrs vector?

Since "virtual template methods" are not allowed, a possible solution

might be to bind the random generator to the definition of the

base_distribution class (and of the distribution_adaptor as well):

--- [code] ---

namespace my {

//...

template <typename RealT, typename GeneratorT>

class base_distribution

{

virtual RealT rand(GeneratorT& rng) = 0;

};

template <typename DistributionT, typename GeneratorT>

class distribution_adaptor: public base_distribution<typename

DistributionT::real_type, GeneratorT>

{

typedef DistributionT distr_type;

typedef typename distr_type::real_type real_type;

real_type rand(GeneratorT& rng) { return ::my::rand(rng); }

};

} // Namespace my

int main()

{

//...

typedef linear_congruential<...> generator_type:

std::vector<my::base_distribution<double,generator_type>*> distrs;

distrs.push_back( new distribution_adaptor<normal,generator_type>

(normal(0,1)) );

distrs.push_back( new distribution_adaptor<exponential,generator_type>

(exponential(1)) );

generator_type rng(123456);

for (

std::vector<my::base_distribution<double,generator_type>*>::iterator

it = distrs.begin();

it != distrs.end();

++it

) {

(*it)->rand(rng);

}

--- [/code] ---

However this solution is very bad to me since it bind the random

generator to the design of the probability distribution; furthermore,

it does not allow to choose the random generator at run-time (actually

not needed, but maybe in the future)

So, do you have a more effective way to solve this problem?

Thank you very much in advance!!

NOTE: if you want I post the entire source code. I didn't do it since

this post is already very long.

Best Regards,

-- Marco

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