c++ - Compile-time map and inverse map values

Question

Can someone recommend a more elegant way to achieve these compile-time constants?

template <int> struct Map;
template <> struct Map<0> {static const int value = 4;};
template <> struct Map<1> {static const int value = 8;};
template <> struct Map<2> {static const int value = 15;};

template <int> struct MapInverse;
template <> struct MapInverse<4> {static const int value = 0;};
template <> struct MapInverse<8> {static const int value = 1;};
template <> struct MapInverse<15> {static const int value = 2;};

The values need to be constexpr in my program, yet the inverse mapped values are getting tedious to update (and easy to make mistakes or forget to do even).

Answer 1

In this C++11 solution all map items are kept in constexpr array and there are constexpr recursive functions to search by either key or value.

#include <utility>

using Item = std::pair<int, int>;
constexpr Item map_items[] = {
    { 6, 7 },
    { 10, 12 },
    { 300, 5000 },
};
constexpr auto map_size = sizeof map_items/sizeof map_items[0];

static constexpr int findValue(int key, int range = map_size) {
    return
            (range == 0) ? throw "Key not present":
            (map_items[range - 1].first == key) ? map_items[range - 1].second:
            findValue(key, range - 1);
};

static constexpr int findKey(int value, int range = map_size) {
    return
            (range == 0) ? throw "Value not present":
            (map_items[range - 1].second == value) ? map_items[range - 1].first:
            findKey(value, range - 1);
};

static_assert(findKey(findValue(10)) == 10, "should be inverse");