How to encapsulate custom iterator in function using boost-range - c++

Question

Lately I was using boost-range to create ranges over elements satisfying certain criteria. In all cases I'm using the same kind of filtered range all the time, so that I tried to encapsulate this behaviour in an external function.
This was the point where my problems started. Consider the following example.
#include <boost/range/adaptor/filtered.hpp>
#include <iostream>
#include <vector>
auto myFilter = [](const std::vector<int>& v, int r) {
return v | boost::adaptors::filtered([&r](auto v) { return v%r == 0; });
};
int main(int argc, const char* argv[])
{
using namespace boost::adaptors;
std::vector<int> input{ 1, 2, 3, 4, 5, 6, 7, 8, 9 };
for (auto& element : input | filtered([](auto v) {return v % 2 == 0; } ))
{
std::cout << "Element = " << element << std::endl;
}
std::cout << std::endl;
for (auto& element : myFilter(input,4))
{
std::cout << "Element = " << element << std::endl;
}
return 0;
}
The first for-loop behaves as expected printing 4 and 8. The second for-loop however prints just 4. Why is that?
My second idea was to implement a class having a begin() and end() function. This should be a thin wrapper around a range object.
This was the solution, after fiddling out the type of the range iterator.
struct MyFilter {
MyFilter(const std::vector<int>& c, int r) : c(c), r(r), f([&r](auto v) { return v%r == 0; }) {
}
boost::range_detail::filtered_range<std::function<bool(int)>, std::vector<int>>::iterator begin() {
return rng.begin();
}
boost::range_detail::filtered_range<std::function<bool(int)>, std::vector<int>>::iterator end() {
return rng.end();
}
std::vector<int> c;
int r;
std::function<bool(int)> f;
boost::range_detail::filtered_range < std::function<bool(int)>, std::vector<int>> rng=c | boost::adaptors::filtered(f);
};
Usage should be something like:
for (auto& element : MyFilter(input, 4)) {
std::cout << "Element = " << element << std::endl;
}
Unfortunately, it prints again just the 4. Whichs is quite strange to me??
Now, I got the solution by myself. I have to remove the "&" in my lambda function to make it work!

Answer 1

In:
auto myFilter = [](const std::vector<int>& v, int r) {
return v | boost::adaptors::filtered([&r](auto v) { return v%r == 0; });
};
It returns another range adaptor while r captured by reference becomes a dangling reference. To fix it capture r by value:
auto myFilter = [](const std::vector<int>& v, int r) {
return v | boost::adaptors::filtered([r](auto v) { return v%r == 0; });
}; ^
+--- capture by value