added keys() function in set

Author: jkalias

README.md

@@ -62,14 +62,14 @@ cmake -S . -B build
 Then open the generated ```functional_cpp.sln``` in the ```build``` folder.
 
 ## Functional vector usage (fcpp::vector)
-### extract unique (distinct) elements
+### extract unique (distinct) elements in a set
 ```c++
 #include "vector.h" // instead of <vector>
 
 const fcpp::vector<int> numbers({1, 4, 2, 5, 8, 3, 1, 7, 1});
 
 // contains only 1, 2, 3, 4, 5, 7, 8
-const auto& unique_numbers = numbers.distinct();
+const fcpp::set unique_numbers = numbers.distinct();
 ```
 
 ### zip, map, filter, sort
@@ -316,6 +316,9 @@ const fcpp::set<person, person_comparator> family({
 // all of our friends and family for the next party invitation
 // contains person(51, "George"), person(41, "Jackie"), person(42, "Crystal"), person(51, "Paul"), person(81, "Barbara") 
 const auto friends_and_family = friends.union_with(family);
+
+// all set keys in a vetor
+const fcpp::vector<person> = friends_and_family.keys();
 ```
 
 ### zip, map, filter

include/set.h

@@ -437,6 +437,15 @@ class set
         return *this;
     }
 
+    vector<TKey> keys() const {
+        vector<TKey> vec;
+        vec.reserve(size());
+        for_each([&vec](const TKey& key) {
+            vec.insert_back(key);
+        });
+        return std::move(vec);
+    }
+    
     // Removes an element from the set, if it exists, potentially changing the set's contents (mutating)
     //
     // example:

tests/set_test.cc

@@ -436,6 +436,13 @@ TEST(SetTest, ZipWithStdVectorDifferentSizes)
     EXPECT_DEATH(ages.zip(persons), "");
 }
 
+TEST(SetTest, Keys)
+{
+    const set<int> numbers({ 25, 45, 30, 63 });
+    const auto keys = numbers.keys();
+    EXPECT_EQ(vector<int>({25, 30, 45, 63}), keys);
+}
+
 TEST(SetTest, RemoveExistingElement)
 {
     set<int> numbers({1, 4, 2});