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.

2 files changed, 29 insertions, 0 deletions

diff --git a/gdb/common/common-utils.c b/gdb/common/common-utils.c
index 5a346ec..e112b62 100644
--- a/gdb/common/common-utils.c
+++ b/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)
 {
diff --git a/gdb/common/common-utils.h b/gdb/common/common-utils.h
index 47def11..a9053ff 100644
--- a/gdb/common/common-utils.h
+++ b/gdb/common/common-utils.h
@@ -20,6 +20,8 @@
 #ifndef COMMON_UTILS_H
 #define COMMON_UTILS_H
 
+#include <string>
+
 /* If possible, define FUNCTION_NAME, a macro containing the name of
    the function being defined.  Since this macro may not always be
    defined, all uses must be protected by appropriate macro definition
@@ -56,6 +58,10 @@ char *xstrvprintf (const char *format, va_list ap)
 int xsnprintf (char *str, size_t size, const char *format, ...)
      ATTRIBUTE_PRINTF (3, 4);
 
+/* Returns a std::string built from a printf-style format string.  */
+std::string string_printf (const char* fmt, ...)
+  ATTRIBUTE_PRINTF (1, 2);
+
 /* Make a copy of the string at PTR with LEN characters
    (and add a null character at the end in the copy).
    Uses malloc to get the space.  Returns the address of the copy.  */