Implement formatting of objects with (s)printf.

format.cc

@@ -523,6 +523,12 @@ class PrintfArgFormatter :
     }
     *out = static_cast<Char>(value);
   }
+
+  void visit_custom(Arg::CustomValue c) {
+    BasicFormatter<Char> formatter(ArgList(), this->writer());
+    const char *format = "}";
+    c.format(&formatter, c.value, &format);
+  }
 };
 }  // namespace internal
 }  // namespace fmt

test/format-test.cc

@@ -148,19 +148,6 @@ TEST(CStringRefTest, Ctor) {
   EXPECT_STREQ("defg", CStringRef(std::string("defg")).c_str());
 }
 
-class TestString {
- private:
-  std::string value_;
-
- public:
-  explicit TestString(const char *value = "") : value_(value) {}
-
-  friend std::ostream &operator<<(std::ostream &os, const TestString &s) {
-    os << s.value_;
-    return os;
-  }
-};
-
 #if FMT_USE_TYPE_TRAITS
 TEST(WriterTest, NotCopyConstructible) {
   EXPECT_FALSE(std::is_copy_constructible<BasicWriter<char> >::value);

test/printf-test.cc

@@ -426,6 +426,10 @@ TEST(PrintfTest, Pointer) {
   EXPECT_PRINTF(fmt::format("{}", p), "%p", p);
 }
 
+TEST(PrintfTest, Custom) {
+  EXPECT_PRINTF("abc", "%s", TestString("abc"));
+}
+
 TEST(PrintfTest, Location) {
   // TODO: test %n
 }

test/util.h

@@ -67,3 +67,16 @@ inline FILE *safe_fopen(const char *filename, const char *mode) {
   return std::fopen(filename, mode);
 #endif
 }
+
+class TestString {
+ private:
+  std::string value_;
+
+ public:
+  explicit TestString(const char *value = "") : value_(value) {}
+
+  friend std::ostream &operator<<(std::ostream &os, const TestString &s) {
+    os << s.value_;
+    return os;
+  }
+};