Add join argument allowing formating list of values separated by a (#466)

Add join argument allowing formating list of values separated by a
string. Each value is formated according the format specifier.

fmt/format.h

@@ -3865,6 +3865,66 @@ void BasicFormatter<Char, AF>::format(BasicCStringRef<Char> format_str) {
   }
   write(writer_, start, s);
 }
+
+template <typename Char, typename It>
+struct ArgJoin {
+  It first;
+  It last;
+  BasicCStringRef<Char> sep;
+
+  ArgJoin(It first, It last, const BasicCStringRef<Char>& sep) :
+    first(first),
+    last(last),
+    sep(sep) {}
+};
+
+template <typename It>
+ArgJoin<char, It> join(It first, It last, const BasicCStringRef<char>& sep) {
+  return ArgJoin<char, It>(first, last, sep);
+}
+
+template <typename It>
+ArgJoin<wchar_t, It> join(It first, It last, const BasicCStringRef<wchar_t>& sep) {
+  return ArgJoin<wchar_t, It>(first, last, sep);
+}
+
+#if FMT_HAS_GXX_CXX11
+template <typename Range>
+auto join(const Range& range, const BasicCStringRef<char>& sep)
+    -> ArgJoin<char, decltype(std::begin(range))> {
+  return join(std::begin(range), std::end(range), sep);
+}
+
+template <typename Range>
+auto join(const Range& range, const BasicCStringRef<wchar_t>& sep)
+    -> ArgJoin<wchar_t, decltype(std::begin(range))> {
+  return join(std::begin(range), std::end(range), sep);
+}
+#endif
+
+template <typename ArgFormatter, typename Char, typename It>
+void format_arg(fmt::BasicFormatter<Char, ArgFormatter> &f,
+    const Char *&format_str, const ArgJoin<Char, It>& e) {
+  const Char* end = format_str;
+  if (*end == ':')
+    ++end;
+  while (*end && *end != '}')
+    ++end;
+  if (*end != '}')
+    FMT_THROW(FormatError("missing '}' in format string"));
+
+  It it = e.first;
+  if (it != e.last) {
+    const Char* save = format_str;
+    f.format(format_str, internal::MakeArg<fmt::BasicFormatter<Char, ArgFormatter> >(*it++));
+    while (it != e.last) {
+      f.writer().write(e.sep);
+      format_str = save;
+      f.format(format_str, internal::MakeArg<fmt::BasicFormatter<Char, ArgFormatter> >(*it++));
+    }
+  }
+  format_str = end + 1;
+}
 }  // namespace fmt
 
 #if FMT_USE_USER_DEFINED_LITERALS

test/format-test.cc

@@ -1552,6 +1552,27 @@ TEST(FormatTest, Variadic) {
   EXPECT_EQ(L"abc1", format(L"{}c{}", L"ab", 1));
 }
 
+TEST(FormatTest, JoinArg) {
+  using fmt::join;
+  int v1[3] = { 1, 2, 3 };
+  std::vector<float> v2;
+  v2.push_back(1.2);
+  v2.push_back(3.4);
+
+  EXPECT_EQ("(1, 2, 3)", format("({})", join(v1 + 0, v1 + 3, ", ")));
+  EXPECT_EQ("(1)", format("({})", join(v1 + 0, v1 + 1, ", ")));
+  EXPECT_EQ("()", format("({})", join(v1 + 0, v1 + 0, ", ")));
+  EXPECT_EQ("(001, 002, 003)", format("({:03})", join(v1 + 0, v1 + 3, ", ")));
+  EXPECT_EQ("(+01.20, +03.40)", format("({:+06.2f})", join(v2.begin(), v2.end(), ", ")));
+
+  EXPECT_EQ(L"(1, 2, 3)", format(L"({})", join(v1 + 0, v1 + 3, L", ")));
+
+#if FMT_HAS_GXX_CXX11
+  EXPECT_EQ("(1, 2, 3)", format("({})", join(v1, ", ")));
+  EXPECT_EQ("(+01.20, +03.40)", format("({:+06.2f})", join(v2, ", ")));
+#endif
+}
+
 template <typename T>
 std::string str(const T &value) {
   return fmt::format("{}", value);