Generalize Join to work for any container/element.

This is more scalable than explicitly instantiating templates for the
cross product of containers and element types.

Specifically I'm adding this so I can join an unordered_set in adb.

Change-Id: I0055f3390a0ff26a886a0d41bbf0d4fe3d210f9c

base/include/base/strings.h

@@ -17,6 +17,7 @@
 #ifndef BASE_STRINGS_H
 #define BASE_STRINGS_H
 
+#include <sstream>
 #include <string>
 #include <vector>
 
@@ -34,9 +35,24 @@ std::vector<std::string> Split(const std::string& s,
 // Trims whitespace off both ends of the given string.
 std::string Trim(const std::string& s);
 
-// Joins a vector of strings into a single string, using the given separator.
-template <typename StringT>
-std::string Join(const std::vector<StringT>& strings, char separator);
+// Joins a container of things into a single string, using the given separator.
+template <typename ContainerT>
+std::string Join(const ContainerT& things, char separator) {
+  if (things.empty()) {
+    return "";
+  }
+
+  std::ostringstream result;
+  result << *things.begin();
+  for (auto it = std::next(things.begin()); it != things.end(); ++it) {
+    result << separator << *it;
+  }
+  return result.str();
+}
+
+// We instantiate the common cases in strings.cpp.
+extern template std::string Join(const std::vector<std::string>&, char);
+extern template std::string Join(const std::vector<const char*>&, char);
 
 // Tests whether 's' starts with 'prefix'.
 bool StartsWith(const std::string& s, const char* prefix);

base/strings.cpp

@@ -79,25 +79,10 @@ std::string Trim(const std::string& s) {
   return s.substr(start_index, end_index - start_index + 1);
 }
 
-template <typename StringT>
-std::string Join(const std::vector<StringT>& strings, char separator) {
-  if (strings.empty()) {
-    return "";
-  }
-
-  std::string result(strings[0]);
-  for (size_t i = 1; i < strings.size(); ++i) {
-    result += separator;
-    result += strings[i];
-  }
-  return result;
-}
-
-// Explicit instantiations.
-template std::string Join<std::string>(const std::vector<std::string>& strings,
-                                       char separator);
-template std::string Join<const char*>(const std::vector<const char*>& strings,
-                                       char separator);
+// These cases are probably the norm, so we mark them extern in the header to
+// aid compile time and binary size.
+template std::string Join(const std::vector<std::string>&, char);
+template std::string Join(const std::vector<const char*>&, char);
 
 bool StartsWith(const std::string& s, const char* prefix) {
   return s.compare(0, strlen(prefix), prefix) == 0;

base/strings_test.cpp

@@ -20,6 +20,8 @@
 
 #include <string>
 #include <vector>
+#include <set>
+#include <unordered_set>
 
 TEST(strings, split_empty) {
   std::vector<std::string> parts = android::base::Split("", ",");
@@ -121,6 +123,17 @@ TEST(strings, join_separator_in_vector) {
   ASSERT_EQ(",,,", android::base::Join(list, ','));
 }
 
+TEST(strings, join_simple_ints) {
+  std::set<int> list = {1, 2, 3};
+  ASSERT_EQ("1,2,3", android::base::Join(list, ','));
+}
+
+TEST(strings, join_unordered_set) {
+  std::unordered_set<int> list = {1, 2};
+  ASSERT_TRUE("1,2" == android::base::Join(list, ',') ||
+              "2,1" == android::base::Join(list, ','));
+}
+
 TEST(strings, startswith_empty) {
   ASSERT_FALSE(android::base::StartsWith("", "foo"));
   ASSERT_TRUE(android::base::StartsWith("", ""));