Introduce string_printf

This introduces the string_printf function.  Like asprintf, but
returns a std::string.

gdb/ChangeLog:
2016-11-08  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMMON_OBS): Add utils-selftests.o.
	* common/common-utils.c (string_printf): New function.
	* common/common-utils.h: Include <string>.
	(string_printf): Declare.
	* utils-selftests.c: New file.

gdb/common/common-utils.c

@@ -150,6 +150,29 @@ xsnprintf (char *str, size_t size, const char *format, ...)
   return ret;
 }
 
+/* See documentation in common-utils.h.  */
+
+std::string
+string_printf (const char* fmt, ...)
+{
+  va_list vp;
+  int size;
+
+  va_start (vp, fmt);
+  size = vsnprintf (NULL, 0, fmt, vp);
+  va_end (vp);
+
+  std::string str (size, '\0');
+
+  /* C++11 and later guarantee std::string uses contiguous memory and
+     always includes the terminating '\0'.  */
+  va_start (vp, fmt);
+  vsprintf (&str[0], fmt, vp);
+  va_end (vp);
+
+  return str;
+}
+
 char *
 savestring (const char *ptr, size_t len)
 {