Untitled

Choose m elements randomly from a vector containing n elements
template<class fwditer>
bidiiter random_unique(fwditer begin, fwditer end, size_t num_random) {
    size_t left = std::distance(begin, end);
    while (num_random--) {
        fwditer r = begin;
        std::advance(r, rand()%left);
        std::swap(*begin, *r);
        ++begin;
        --left;
    }
    return begin;
}

struct rangegenerator {
    rangegenerator(int init) : start(init) { }

    int operator()() {
        return start++;
    }

    int start;
};

vector<T> numbers; // this is filled somewhere else

vector<int> indices(numbers.size());

generate(begin(indices), end(indices), rangegenerator(0));

random_shuffle(begin(indices), end(indices));

// then take the first n elements of indices and use them as indices into numbers