io/BufferedOutputStream: add libfmt support

src/io/BufferedOutputStream.cxx

@@ -118,6 +118,20 @@ BufferedOutputStream::Format(const char *fmt, ...)
 	buffer.Append(size);
 }
 
+void
+BufferedOutputStream::VFmt(fmt::string_view format_str, fmt::format_args args)
+{
+	/* TODO format into this object's buffer instead of allocating
+	   a new one */
+	fmt::memory_buffer b;
+#if FMT_VERSION >= 80000
+	fmt::vformat_to(std::back_inserter(b), format_str, args);
+#else
+	fmt::vformat_to(b, format_str, args);
+#endif
+	return Write(b.data(), b.size());
+}
+
 #ifdef _UNICODE
 
 void

src/io/BufferedOutputStream.hxx

@@ -33,6 +33,11 @@
 #include "util/Compiler.h"
 #include "util/DynamicFifoBuffer.hxx"
 
+#include <fmt/core.h>
+#if FMT_VERSION < 70000 || FMT_VERSION >= 80000
+#include <fmt/format.h>
+#endif
+
 #include <cstddef>
 
 #ifdef _UNICODE
@@ -92,6 +97,24 @@ public:
 	gcc_printf(2,3)
 	void Format(const char *fmt, ...);
 
+	void VFmt(fmt::string_view format_str, fmt::format_args args);
+
+	template<typename S, typename... Args>
+	void Fmt(const S &format_str, Args&&... args) {
+#if FMT_VERSION >= 90000
+		VFmt(format_str,
+		     fmt::make_format_args(args...));
+#elif FMT_VERSION >= 70000
+		VFmt(fmt::to_string_view(format_str),
+		     fmt::make_args_checked<Args...>(format_str,
+						     args...));
+#else
+		/* expensive fallback for older libfmt versions */
+		const auto result = fmt::format(format_str, args...);
+		Write(result.data(), result.size());
+#endif
+	}
+
 #ifdef _UNICODE
 	/**
 	 * Write one narrow character.

src/io/meson.build

@@ -8,8 +8,14 @@ io = static_library(
   'FileOutputStream.cxx',
   'BufferedOutputStream.cxx',
   include_directories: inc,
+  dependencies: [
+    fmt_dep,
+  ],
 )
 
 io_dep = declare_dependency(
   link_with: io,
+  dependencies: [
+    fmt_dep,
+  ],
 )