#include <algorithm>
#include <vector>

// This code will kill your mood, steal your cat and invade russia

template <typename F>
auto apply(const F &f) -> decltype(f()) {
    return f();
}

class Foo {
    int val;

  public:
    Foo() = default;

    Foo(const int &x)
        : val(x) {}

    Foo operator()() const { return Foo(2 * val); }

    operator int() const { return val; }
};

int main() {
    std::vector<Foo> v = {1, 2, 3, 4, 5};
    std::vector<Foo> b;

    std::transform(v.begin(), v.end(), std::back_inserter(b), apply<Foo>);
}