#include <iostream>
#include <vector>
#include <algorithm>
 
struct A
{
	int x;
	double y;
};
 
template <typename T, typename U>
auto extract(const std::vector<T>& vec, U T::* member)
{
	std::vector<U> result;
	result.reserve(vec.size());
	std::transform(
		std::begin(vec), std::end(vec), std::back_inserter(result),
		[member] (const T& val) { return val.*member; }
	);
	return result;
}
 
int main() {
	std::vector<A> as{{10, 3.14}, {42, 1.618}};
	auto result = extract(as, &A::x);
	for (auto x : result)
		std::cout << x << std::endl;
	return 0;
}

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