gistfile1.cpp

#include <vector>
#include <memory>
#include <cassert>

template<typename T>
struct idx_vector {
    typedef int index;
    
    std::vector<std::unique_ptr<T>> pointers_;
    std::vector<index> free_indices_;
    
    index add(T x) {
        if (free_indices_.empty()) {
            index i = free_indices_.back();
            free_indices_.pop_back();
            pointers_[i].swap(p);
            return i;
        } else {
            pointers_.push_back(std::make_unique<T>(std::move(x)));
            return static_cast<index>(pointers_.size() - 1);
        }
    }
    bool has(index i) {
        return i < pointers_.size() && pointers_[i];
    }
    T get(index i) {
        if (has(i)) {
            return *pointers_[i];
        } else {
            assert(false);
            return {};
        }
    }
    void remove(index i) { // invalidates index, but not other indices
        if (has(i)) {
            pointers_[i] = nullptr;
            free_indices_.push_back(i);
        } else {
            assert(false);
        }
    }
};