[core] Counterpart of Pythons str.join function in C++

This complements the existing `ROOT::Split()` function.

core/foundation/inc/ROOT/StringUtils.hxx

@@ -18,11 +18,28 @@
 
 #include <string>
 #include <vector>
+#include <numeric>
 
 namespace ROOT {
 
 std::vector<std::string> Split(std::string_view str, std::string_view delims, bool skipEmpty = false);
 
+/**
+ * \brief Concatenate a list of strings with a separator
+ * \tparm Any container of strings (vector, initializer_list, ...)
+ * \param[in] sep Separator inbetween the strings.
+ * \param[in] strings container of strings
+ * \return the sep-delimited concatenation of strings
+ */
+template <class StringCollection_t>
+std::string Join(const std::string &sep, StringCollection_t &&strings)
+{
+ if (strings.empty())
+ return "";
+ return std::accumulate(std::next(std::begin(strings)), std::end(strings), strings[0],
+ [&sep](auto const &a, auto const &b) { return a + sep + b; });
+}
+
 } // namespace ROOT
 
 #endif

core/foundation/test/testStringUtils.cxx

@@ -21,3 +21,23 @@ TEST(StringUtils, Split)
  test(",,a", {"", "", "a"}, false);
  test("", {""}, false);
 }
+
+TEST(StringUtils, Join)
+{
+ // Test that ROOT::Join behaves like str.join from Python.
+
+ auto test = [](const std::string &ref, const std::string &sep, const std::vector<std::string> &strings) {
+ auto out = ROOT::Join(sep, strings);
+ EXPECT_EQ(out, ref) << "ROOT::Join gave wrong result.";
+ };
+ test("apple,orange,banana", ",", {"apple", "orange", "banana"});
+ test("apple.orange.banana", ".", {"apple", "orange", "banana"});
+ test("apple::orange::banana", "::", {"apple", "orange", "banana"});
+ test("appleorangebanana", "", {"apple", "orange", "banana"});
+ test("apple,,banana", ",", {"apple", "", "banana"});
+ test("apple", ",", {"apple"});
+ test("", ",", {""});
+ test("", "", {""});
+ test("", "", {});
+ test("", ";;", {""});
+}