BasicArgFormatter -> ArgFormatter. FullFormat -> ActiveFormatter. Use ActiveFormatter to implement Format and Print.

README.rst

@@ -46,6 +46,24 @@ An object of any user-defined type for which there is an overloaded
     std::string s = str(fmt::Format("The date is {0}") << Date(2012, 12, 9));
     // s == "The date is 2012-12-9"
 
+You can use ``fmt::ActiveFormatter`` to create your own functions
+similar to ``fmt::Format`` and ``fmt::Print`` with an arbitrary action
+performed when formatting is complete:
+
+    struct PrintError {
+      void operator()(const fmt::Formatter &f) const {
+        std::cerr << "Error: " << f.str() << std::endl;
+      }
+    };
+
+    // Formats an error message and prints it to std::cerr.
+    fmt::ActiveFormatter<PrintError> ReportError(const char *format) {
+      fmt::ActiveFormatter<PrintError> af(format);
+      return af;
+    }
+
+    ReportError("File not found: {0}") << path;
+
 Format string syntax
 --------------------
 

format.cc

@@ -366,8 +366,3 @@ void fmt::Formatter::DoFormat() {
   buffer_.append(start, s + 1);
   buffer_.resize(buffer_.size() - 1);  // Don't count the terminating zero.
 }
-
-fmt::BasicArgFormatter::~BasicArgFormatter() {
-  if (!formatter_) return;
-  FinishFormatting();
-}

format.h

@@ -94,7 +94,7 @@ class FormatError : public std::runtime_error {
   FormatError(const std::string &message) : std::runtime_error(message) {}
 };
 
-class BasicArgFormatter;
+class ArgFormatter;
 
 // Formatter provides string formatting functionality similar to Python's
 // str.format. The output is stored in a memory buffer that grows dynamically.
@@ -183,17 +183,17 @@ class Formatter {
       format(&Formatter::FormatCustomArg<T>) {}
 
     ~Arg() {
-      // Format is called here to make sure that the argument object
-      // referred to is still alive, for example:
+      // Format is called here to make sure that a referred object is
+      // still alive, for example:
       //
       //   Print("{0}") << std::string("test");
       //
       // Here an Arg object refers to a temporary std::string which is
       // destroyed at the end of the statement. Since the string object is
       // constructed before the Arg object, it will be destroyed after,
-      // so it will be alive in the Arg's destructor when Format is called.
+      // so it will be alive in the Arg's destructor where Format is called.
       // Note that the string object will not necessarily be alive when
-      // the destructor of BasicArgFormatter is called.
+      // the destructor of ArgFormatter is called.
       formatter->Format();
     }
   };
@@ -203,7 +203,7 @@ class Formatter {
 
   const char *format_;  // Format string.
 
-  friend class BasicArgFormatter;
+  friend class ArgFormatter;
 
   void Add(const Arg &arg) {
     args_.push_back(&arg);
@@ -238,36 +238,38 @@ class Formatter {
   }
 
  public:
-  Formatter() : format_(0) {}
+  Formatter() : format_(0) { buffer_[0] = 0; }
 
   // Formats a string appending the output to the internal buffer.
-  // Arguments are accepted through the returned BasicArgFormatter object
+  // Arguments are accepted through the returned ArgFormatter object
   // using inserter operator<<.
-  BasicArgFormatter operator()(const char *format);
+  ArgFormatter operator()(const char *format);
 
   std::size_t size() const { return buffer_.size(); }
 
   const char *data() const { return &buffer_[0]; }
   const char *c_str() const { return &buffer_[0]; }
+
+  std::string str() const { return std::string(&buffer_[0], buffer_.size()); }
 };
 
 // Argument formatter. This is a transient object that normally exists
 // only as a temporary returned by one of the formatting functions.
-// It stores a reference to a formatter and provides operators <<
-// that feed arguments to the formatter.
-class BasicArgFormatter {
+// It stores a reference to a formatter and provides operator<<
+// that feeds arguments to the formatter.
+class ArgFormatter {
  private:
   friend class Formatter;
 
  protected:
   mutable Formatter *formatter_;
 
-  BasicArgFormatter(BasicArgFormatter& other)
+  ArgFormatter(ArgFormatter& other)
   : formatter_(other.formatter_) {
     other.formatter_ = 0;
   }
 
-  BasicArgFormatter& operator=(const BasicArgFormatter& other) {
+  ArgFormatter& operator=(const ArgFormatter& other) {
     formatter_ = other.formatter_;
     other.formatter_ = 0;
     return *this;
@@ -283,33 +285,24 @@ class BasicArgFormatter {
   }
 
  public:
-  explicit BasicArgFormatter(Formatter &f) : formatter_(&f) {}
-  ~BasicArgFormatter();
+  explicit ArgFormatter(Formatter &f) : formatter_(&f) {}
+  ~ArgFormatter() { FinishFormatting(); }
 
-  BasicArgFormatter &operator<<(const Formatter::Arg &arg) {
+  // Feeds an argument to a formatter.
+  ArgFormatter &operator<<(const Formatter::Arg &arg) {
     arg.formatter = formatter_;
     formatter_->Add(arg);
     return *this;
   }
 
-  friend const char *c_str(const BasicArgFormatter &af) {
+  // Performs formatting and returns a C string with the output.
+  friend const char *c_str(const ArgFormatter &af) {
     return af.FinishFormatting()->c_str();
   }
 
-  friend std::string str(const BasicArgFormatter &af) {
-    return af.FinishFormatting()->c_str();
-  }
-};
-
-template <typename Callback>
-class ArgFormatter : public BasicArgFormatter {
- public:
-  explicit ArgFormatter(Formatter &f) : BasicArgFormatter(f) {}
-
-  ~ArgFormatter() {
-    if (!formatter_) return;
-    Callback callback;
-    callback(*formatter_);
+  // Performs formatting and returns a std::string with the output.
+  friend std::string str(const ArgFormatter &af) {
+    return af.FinishFormatting()->str();
   }
 };
 
@@ -325,55 +318,53 @@ void Formatter::FormatCustomArg(const void *arg, int width) {
     std::fill_n(out + str.size(), width - str.size(), ' ');
 }
 
-inline BasicArgFormatter Formatter::operator()(const char *format) {
-  BasicArgFormatter formatter(*this);
+inline ArgFormatter Formatter::operator()(const char *format) {
+  ArgFormatter formatter(*this);
   format_ = format;
   args_.clear();
   return formatter;
 }
 
-class FullFormat : public BasicArgFormatter {
+// A formatter with an action performed when formatting is complete.
+template <typename Action>
+class ActiveFormatter : public ArgFormatter {
  private:
   mutable Formatter formatter_;
 
   // Do not implement.
-  FullFormat& operator=(const FullFormat&);
+  ActiveFormatter& operator=(const ActiveFormatter&);
 
  public:
-  explicit FullFormat(const char *format) : BasicArgFormatter(formatter_) {
-    BasicArgFormatter::operator=(formatter_(format));
+  explicit ActiveFormatter(const char *format) : ArgFormatter(formatter_) {
+    ArgFormatter::operator=(formatter_(format));
   }
 
-  FullFormat(FullFormat& other) : BasicArgFormatter(other) {}
+  ActiveFormatter(ActiveFormatter& other) : ArgFormatter(other) {}
 
-  ~FullFormat() {
-    FinishFormatting();
+  ~ActiveFormatter() {
+    Action()(*FinishFormatting());
   }
 };
 
-inline FullFormat Format(const char *format) {
-  FullFormat ff(format);
-  return ff;
+struct Ignore {
+  void operator()(Formatter &) const {}
+};
+
+inline ActiveFormatter<Ignore> Format(const char *format) {
+  ActiveFormatter<Ignore> af(format);
+  return af;
 }
 
-class Print : public BasicArgFormatter {
- private:
-  Formatter formatter_;
-
-  // Do not implement.
-  Print(const Print&);
-  Print& operator=(const Print&);
-
- public:
-  explicit Print(const char *format) : BasicArgFormatter(formatter_) {
-    BasicArgFormatter::operator=(formatter_(format));
-  }
-
-  ~Print() {
-    FinishFormatting();
-    std::fwrite(formatter_.data(), 1, formatter_.size(), stdout);
+struct Write {
+  void operator()(Formatter &f) const {
+    std::fwrite(f.data(), 1, f.size(), stdout);
   }
 };
+
+inline ActiveFormatter<Write> Print(const char *format) {
+  ActiveFormatter<Write> af(format);
+  return af;
+}
 }
 
 namespace fmt = format;

format_test.cc

@@ -525,18 +525,58 @@ TEST(FormatterTest, FormatStringFromSpeedTest) {
 TEST(FormatterTest, ArgLifetime) {
   // The following code is for testing purposes only. It is a definite abuse
   // of the API and shouldn't be used in real applications.
-  const fmt::BasicArgFormatter &af = fmt::Format("{0}");
-  const_cast<fmt::BasicArgFormatter&>(af) << std::string("test");
+  const fmt::ArgFormatter &af = fmt::Format("{0}");
+  const_cast<fmt::ArgFormatter&>(af) << std::string("test");
   // String object passed as an argument to Print has been destroyed,
   // but BasicArgFormatter dtor hasn't been called yet.
   EXPECT_EQ("test", str(af));
 }
 
-TEST(FormatterTest, Formatter) {
+TEST(FormatterTest, FormatterCtor) {
+  Formatter format;
+  EXPECT_EQ(0, format.size());
+  EXPECT_STREQ("", format.data());
+  EXPECT_STREQ("", format.c_str());
+  EXPECT_EQ("", format.str());
+  format("part{0}") << 1;
+  format("part{0}") << 2;
+  EXPECT_EQ("part1part2", format.str());
+}
+
+TEST(FormatterTest, FormatterAppend) {
+  Formatter format;
+  format("part{0}") << 1;
+  EXPECT_EQ(strlen("part1"), format.size());
+  EXPECT_STREQ("part1", format.c_str());
+  EXPECT_STREQ("part1", format.data());
+  EXPECT_EQ("part1", format.str());
+  format("part{0}") << 2;
+  EXPECT_EQ(strlen("part1part2"), format.size());
+  EXPECT_STREQ("part1part2", format.c_str());
+  EXPECT_STREQ("part1part2", format.data());
+  EXPECT_EQ("part1part2", format.str());
+}
+
+TEST(FormatterTest, FormatterExample) {
   Formatter format;
   format("Current point:\n");
   format("({0:+f}, {1:+f})\n") << -3.14 << 3.14;
-  EXPECT_STREQ("Current point:\n(-3.140000, +3.140000)\n", format.c_str());
+  EXPECT_EQ("Current point:\n(-3.140000, +3.140000)\n", format.str());
 }
 
-// TODO: test API
+struct PrintError {
+  void operator()(const fmt::Formatter &f) const {
+    std::cerr << "Error: " << f.str() << std::endl;
+  }
+};
+
+fmt::ActiveFormatter<PrintError> ReportError(const char *format) {
+  fmt::ActiveFormatter<PrintError> af(format);
+  return af;
+}
+
+TEST(FormatterTest, ArgFormatter) {
+  std::string path = "somefile";
+  ReportError("File not found: {0}") << path;
+}
+// TODO: test ArgFormatter