Restructure printf_arg_formatter to make it customizable

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));
+}