Add attributes field to the candidates

docs/encodings.md

@@ -83,6 +83,15 @@ recommendation is to use `std::variant` or a similar type for the gene
 type.
 
 
+### Extending the candidates
+
+The candidates contain an additional `attributes` field that may hold any
+additional data that is associated with the candidate. This field will always
+be empty by default, as the library does not use it for anything by default.
+The purpose of this field is to allow the user to extend the candidates with
+arbitrary data if neccessary, and it may be used freely to do so.
+
+
 ### Variable chromosome lengths
 
 The length of the chromosomes is specified as part of the fitness function.

src/core/candidate.hpp

@@ -11,6 +11,7 @@
 #include "../utility/iterators.hpp"
 #include <vector>
 #include <algorithm>
+#include <any>
 #include <utility>
 #include <concepts>
 #include <cstddef>
@@ -107,6 +108,9 @@ namespace gapp
         /** The solution's degree of constraint violation for each constraint. */
         CVVector constraint_violation;
 
+        /** Arbitrary data associated with the candidate. */
+        std::any attributes;
+
 
         /** 
         * @returns The number of objectives associated with the candidate.