Macro for nested maps in C-CodePudding

Is it possible to create simple interface to create nested std::maps in C ? If this is possbile, can I go advanced and make it with different nested maps/vectors

CreateMaps(4); returns std::map<int,std::map<int,std::map<int,std::map<int,int>>>>>

CreateMaps(3); returns std::map<int,std::map<int,std::map<int,int>>>>

I am not really sure if this counts as macro or no.

My final target is to create maps at init and I want to divide into categories, type_0 has X subtypes which has Y subtypes and so on..., and I want to count how many times I reach certain scenario. Creating the map is defined after parsing a file, so I dont know the size and the number of nested maps at compile time.

CodePudding user response：

Yes you can, even without macros, but by using recursive templates. Here is what it could look like:

// Recursive definition
template<typename T, size_t N>
struct NestedMap {
    using type = std::map<T, typename NestedMap<T, N - 1>::type>;
};

// Termination condition for the recursion
template<typename T>
struct NestedMap<T, 0> {
    using type = T;
};

// Just a convenience
template<typename T, size_t N>
using NestedMap_t = NestedMap<T, N>::type;

And then you can use it like so:

NestedMap_t<int, 4> quadmap;
quadmap[1][2][3][4] = 42;

However, nesting containers is often not very efficient, and you might get better performance by flattening your data structure. If you want to have a map that is indexed by four integers, then you could also do:

std::map<std::array<int, 4>, int> quadmap;
quadmap[{1, 2, 3, 4}] = 42;

The above types are fixed at compile time. If you want something more flexible, you should make the map's key and value more dynamic. Consider:

std::map<std::vector<int>, std::any> anymap;
anymap[{1, 2, 3, 4}] = 42;