Add StringPrintf() wrapper utility.

Adds an efficient wrapper around std::snprintf() that returns the
formatted result in a std::string.

Background: The issue of either 1) wanting to use absl::StrFormat()
without bloating our binary sizes, or 2) manually writing char[] buffer
code around std::snprintf() has come up several times over the past year
or so. So, this new utility is about reducing friction and improving
future developer velocity.
Change-Id: Ie898f91575852b06d59844dd564a0da03df1f7ed
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/2487768
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Brandon Tolsch <btolsch@chromium.org>

3 files changed, 45 insertions, 0 deletions

diff --git a/util/stringprintf.cc b/util/stringprintf.cc
index 452c3282..2d9bba22 100644
--- a/util/stringprintf.cc
+++ b/util/stringprintf.cc
@@ -4,11 +4,34 @@
 
 #include "util/stringprintf.h"
 
+#include <cstdarg>
+#include <cstdio>
 #include <iomanip>
 #include <sstream>
 
+#include "util/osp_logging.h"
+
 namespace openscreen {
 
+std::string StringPrintf(const char* format, ...) {
+  va_list vlist;
+  va_start(vlist, format);
+  const int length = std::vsnprintf(nullptr, 0, format, vlist);
+  OSP_CHECK_GE(length, 0) << "Invalid format string: " << format;
+  va_end(vlist);
+
+  std::string result(length, '\0');
+  // Note: There's no need to add one for the extra terminating NUL char since
+  // the standard, since C++11, requires that "data() + size() points to [the
+  // NUL terminator]". Thus, std::vsnprintf() will write the NUL to a valid
+  // memory location.
+  va_start(vlist, format);
+  std::vsnprintf(&result[0], length + 1, format, vlist);
+  va_end(vlist);
+
+  return result;
+}
+
 std::string HexEncode(absl::Span<const uint8_t> bytes) {
   std::ostringstream hex_dump;
   hex_dump << std::setfill('0') << std::hex;
diff --git a/util/stringprintf.h b/util/stringprintf.h
index df24ac04..0e7984e1 100644
--- a/util/stringprintf.h
+++ b/util/stringprintf.h
@@ -14,6 +14,19 @@
 
 namespace openscreen {
 
+// Enable compile-time checking of the printf format argument, if available.
+#if defined(__GNUC__) || defined(__clang__)
+#define OSP_CHECK_PRINTF_ARGS(format_param, dots_param) \
+  __attribute__((format(printf, format_param, dots_param)))
+#else
+#define OSP_CHECK_PRINTF_ARGS(format_param, dots_param)
+#endif
+
+// Returns a std::string containing the results of a std::sprintf() call. This
+// is an efficient, zero-copy wrapper.
+[[nodiscard]] std::string StringPrintf(const char* format, ...)
+    OSP_CHECK_PRINTF_ARGS(1, 2);
+
 template <typename It>
 void PrettyPrintAsciiHex(std::ostream& os, It first, It last) {
   auto it = first;
diff --git a/util/stringprintf_unittest.cc b/util/stringprintf_unittest.cc
index df0d60f8..e37e7cb6 100644
--- a/util/stringprintf_unittest.cc
+++ b/util/stringprintf_unittest.cc
@@ -9,6 +9,15 @@
 namespace openscreen {
 namespace {
 
+TEST(StringPrintf, ProducesFormattedStrings) {
+  EXPECT_EQ("no args", StringPrintf("no args"));
+  EXPECT_EQ("", StringPrintf("%s", ""));
+  EXPECT_EQ("42", StringPrintf("%d", 42));
+  EXPECT_EQ(
+      "The result of foo(1, 2) looks good!",
+      StringPrintf("The result of foo(%d, %d) looks %s%c", 1, 2, "good", '!'));
+}
+
 TEST(HexEncode, ProducesEmptyStringFromEmptyByteArray) {
   const uint8_t kSomeMemoryLocation = 0;
   EXPECT_EQ("", HexEncode(absl::Span<const uint8_t>(&kSomeMemoryLocation, 0)));