Restructure printf_arg_formatter to make it customizable

include/fmt/printf.h

@@ -197,9 +197,7 @@ using internal::printf;  // For printing into memory_buffer.
 
 template <typename Range> class printf_arg_formatter;
 
-template <typename OutputIt, typename Char,
-          typename ArgFormatter =
-              printf_arg_formatter<back_insert_range<internal::buffer<Char>>>>
+template <typename OutputIt, typename Char>
 class basic_printf_context;
 
 /**
@@ -212,12 +210,12 @@ class printf_arg_formatter
     : public internal::function<
           typename internal::arg_formatter_base<Range>::iterator>,
       public internal::arg_formatter_base<Range> {
+ public:
+  typedef decltype(internal::declval<Range>().begin()) iterator;
  private:
   typedef typename Range::value_type char_type;
-  typedef decltype(internal::declval<Range>().begin()) iterator;
   typedef internal::arg_formatter_base<Range> base;
-  typedef basic_printf_context<iterator, char_type, printf_arg_formatter>
-      context_type;
+  typedef basic_printf_context<iterator, char_type> context_type;
 
   context_type& context_;
 
@@ -328,7 +326,7 @@ template <typename T> struct printf_formatter {
 };
 
 /** This template formats data and writes the output to a writer. */
-template <typename OutputIt, typename Char, typename ArgFormatter>
+template <typename OutputIt, typename Char>
 class basic_printf_context {
  public:
   /** The character type for the output. */
@@ -379,11 +377,12 @@ class basic_printf_context {
   }
 
   /** Formats stored arguments and writes the output to the range. */
+  template<typename ArgFormatter = printf_arg_formatter<back_insert_range<internal::buffer<Char>>>>
   OutputIt format();
 };
 
-template <typename OutputIt, typename Char, typename AF>
-void basic_printf_context<OutputIt, Char, AF>::parse_flags(format_specs& spec,
+template <typename OutputIt, typename Char>
+void basic_printf_context<OutputIt, Char>::parse_flags(format_specs& spec,
                                                            const Char*& it,
                                                            const Char* end) {
   for (; it != end; ++it) {
@@ -409,9 +408,9 @@ void basic_printf_context<OutputIt, Char, AF>::parse_flags(format_specs& spec,
   }
 }
 
-template <typename OutputIt, typename Char, typename AF>
-typename basic_printf_context<OutputIt, Char, AF>::format_arg
-basic_printf_context<OutputIt, Char, AF>::get_arg(unsigned arg_index) {
+template <typename OutputIt, typename Char>
+typename basic_printf_context<OutputIt, Char>::format_arg
+basic_printf_context<OutputIt, Char>::get_arg(unsigned arg_index) {
   if (arg_index == std::numeric_limits<unsigned>::max())
     arg_index = parse_ctx_.next_arg_id();
   else
@@ -419,8 +418,8 @@ basic_printf_context<OutputIt, Char, AF>::get_arg(unsigned arg_index) {
   return internal::get_arg(*this, arg_index);
 }
 
-template <typename OutputIt, typename Char, typename AF>
-unsigned basic_printf_context<OutputIt, Char, AF>::parse_header(
+template <typename OutputIt, typename Char>
+unsigned basic_printf_context<OutputIt, Char>::parse_header(
     const Char*& it, const Char* end, format_specs& spec) {
   unsigned arg_index = std::numeric_limits<unsigned>::max();
   char_type c = *it;
@@ -457,8 +456,9 @@ unsigned basic_printf_context<OutputIt, Char, AF>::parse_header(
   return arg_index;
 }
 
-template <typename OutputIt, typename Char, typename AF>
-OutputIt basic_printf_context<OutputIt, Char, AF>::format() {
+template <typename OutputIt, typename Char>
+template <typename ArgFormatter>
+OutputIt basic_printf_context<OutputIt, Char>::format() {
   auto out = this->out();
   const Char* start = parse_ctx_.begin();
   const Char* end = parse_ctx_.end();
@@ -567,7 +567,7 @@ OutputIt basic_printf_context<OutputIt, Char, AF>::format() {
     start = it;
 
     // Format argument.
-    visit_format_arg(AF(out, spec, *this), arg);
+    visit_format_arg(ArgFormatter(out, spec, *this), arg);
   }
   return std::copy(start, it, out);
 }
@@ -712,6 +712,19 @@ inline int vfprintf(
   return static_cast<int>(buffer.size());
 }
 
+/** Formats arguments and writes the output to the range. */
+template <typename ArgFormatter, typename Char,
+    typename Context = basic_printf_context<
+        typename ArgFormatter::iterator, Char>>
+typename ArgFormatter::iterator vprintf(internal::buffer<Char>& out,
+                                        basic_string_view<Char> format_str,
+                                        basic_format_args<Context> args) {
+  typename ArgFormatter::iterator iter(out);
+  Context(iter, format_str, args).template format<ArgFormatter>();
+  return iter;
+}
+
+
 /**
   \rst
   Prints formatted data to the stream *os*.

test/printf-test.cc

@@ -554,3 +554,51 @@ TEST(PrintfTest, VSPrintfMakeWArgsExample) {
                           fmt::make_wprintf_args(42, L"something")));
 #endif
 }
+
+typedef fmt::printf_arg_formatter<
+    fmt::back_insert_range<fmt::internal::buffer<char>>> formatter_t;
+typedef fmt::basic_printf_context<formatter_t::iterator, char> context_t;
+
+// A custom printf argument formatter that doesn't print `-` for floating-point
+// values rounded to 0.
+class custom_printf_arg_formatter : public formatter_t {
+ public:
+    using formatter_t::iterator;
+
+  custom_printf_arg_formatter(formatter_t::iterator iter, formatter_t::format_specs& spec, context_t& ctx)
+      : formatter_t(iter, spec, ctx) {}
+
+  using formatter_t::operator();
+
+#if FMT_MSC_VER > 0 && FMT_MSC_VER <= 1804
+      template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value)>
+      iterator operator()(T value) {
+#else
+      iterator operator()(double value) {
+#endif
+     // Comparing a float to 0.0 is safe.
+     if (round(value * pow(10, spec()->precision)) == 0.0) value = 0;
+     return formatter_t::operator()(value);
+  }
+};
+
+typedef fmt::basic_format_args<context_t> format_args_t;
+
+std::string custom_vformat(fmt::string_view format_str, format_args_t args) {
+  fmt::memory_buffer buffer;
+  fmt::vprintf<custom_printf_arg_formatter>(buffer, format_str, args);
+  return std::string(buffer.data(), buffer.size());
+}
+
+template <typename... Args>
+std::string custom_format(const char* format_str, const Args&... args) {
+  auto va = fmt::make_printf_args (args...);
+  return custom_vformat(format_str, va);
+}
+
+TEST(CustomFormatterTest, Format) {
+  EXPECT_EQ("0.00", custom_format("%.2f", -.00001));
+  EXPECT_EQ("0.00", custom_format("%.2f", .00001));
+  EXPECT_EQ("1.00", custom_format("%.2f", 1.00001));
+  EXPECT_EQ("-1.00", custom_format("%.2f", -1.00001));
+}