private:

int compare_val;

public:

LessThanValue() = delete;

LessThanValue(int v) : compare_val(v) { }

bool operator() (int i) {

return i < this->compare_val;

}

std::vector<int> ns{1, 5, 6, 0, 7, 2, 3, 8, 3, 0, 2, 1};

std::cout << "ns = { ";

for (auto&& i : ns) {

std::cout << i << ' ';

}

std::cout << "}\n";

std::cout << "Greater than 4 (function pointer)\n";

for (auto&& i : iter::filter(greater_than_four, ns)) {

std::cout << i << '\n';

}

std::cout << "Less than 4 (lambda)\n";

for (auto&& i : iter::filter([] (const int i) { return i < 4; }, ns)) {

std::cout << i << '\n';

}

LessThanValue lv(4);

std::cout << "Less than 4 (callable object)\n";

for (auto&& i : iter::filter(lv, ns)) {

std::cout << i << '\n';

}

// filter(seq) with no predicate uses the truthiness of the values

std::cout << "Nonzero ints filter(ns)\n";

for (auto&& i : iter::filter(ns)) {

std::cout << i << '\n';

}

std::cout << "odd numbers\n";

for (auto&& i : iter::filter([] (const int i) {return i % 2;}, ns)) {

std::cout << i << '\n';

}