Use std::variant instead of std::any and std::array instead of std::unordered_map

[rsore]

Description

This branch introduces a new container for parsed values and a more efficient retrieval method, replacing the current implementation.

In the existing code, parsed values are stored in a std::unordered_map and type-erased using std::any. This approach has several drawbacks:

• Dynamic Allocation: Values are dynamically allocated on the heap, leading to potential performance overhead.
• Type Safety: Using std::any for type erasure is inherently unsafe.
• Lookup Overhead: Retrieving values from a std::unordered_map indexed by std::type_index can involve costly lookups and may require resolving hash conflicts.

This branch improves upon this by storing each parsed value in a std::optional with the actual ValueType specified in the option type. The values are stored in a contiguous std::array whose size matches the number of flags and options passed to the parser. To support multiple value types, the array’s underlying type is a std::variant capable of holding any permutation of std::optional<ValueType>.

Consider the following example:

using FooOption= CLArgs::Option<"--foo", "<hint>", "<description>", int>;
using BarOption= CLArgs::Option<"--bar", "<hint>", "<description>", float>;
using BazOption= CLArgs::Option<"--baz", "<hint>", "<description>", int>;
CLArgs::Parser<FooOption, BarOption, BazOption> parser;

In this example, the value types are int, float, and int. Duplicate types are filtered out, so the container type becomes:

std::array<std::variant<std::optional<int>, std::optional<float>>, 3>

Values are stored in the array in the order they were passed to the parser, ensuring type safety and eliminating hash lookups. Accessing values is as simple as indexing directly into the array, with the index determined at compile-time.

All logic related to this new container is encapsulated in a ValueContainer class, which is used by the Parser.

Related Issues

Closes #3.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"