Immutable sub vector of immutable vector - c++

Question

Let's say I have one const vector<string> with some elements in it and I'd like to create a const subset of that vector, how can I do that?
Ideally it would be cool if C++ supported something like the code below, unfortunately it doesn't. Anybody know of a work around?
#include <fstream>
#include <vector>
#include <string>
#include <iostream>
#include <algorithm>
using namespace std;
int main() {
ifstream infile("input.txt"); // contains apple, orange, banana in separate lines
istream_iterator<string> eos;
istream_iterator<string> input(infile);
const vector<string> stuff(input, eos);
const vector<string> a_stuff(stuff.copy_if([](const string& s) { return s[0] == 'a'; }));
return 0;
}

Answer 1

Assuming you want to make a copy that contains the subset, you can use a function and assign the result to a const vector<T>
#include <vector>
#include <algorithm>
#include <iterator>
template <typename T, typename Pred>
std::vector<T> vector_copy_if(std::vector<T> const & vec, Pred && pred)
{
std::vector<T> result;
std::copy_if(vec.begin(), vec.end(), std::back_inserter(result), pred);
return result;
}
int main()
{
std::vector<int> const some_numbers { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
std::vector<int> const even_numbers = vector_copy_if(some_numbers, [](int x) { return x % 2 == 0; });
}

Answer 2

It is also possible to use a boost::filter_iterator to achieve the same result:
#include <vector>
#include <string>
#include <boost/iterator/filter_iterator.hpp>
using namespace std;
int main()
{
const vector<string> v {"apple", "banana", "orange"};
auto filter= [] (string s){return s.length() > 0 && s[0] == 'a';};
auto start = boost::make_filter_iterator(filter, v.begin(), v.end());
auto end = boost::make_filter_iterator(filter, v.end(), v.end());
const vector<string> cv (start, end);
}