[glibmm] Add Glib::c_str_or_nullptr()

[Kjell Ahlstedt <kjellahl src gnome org>]

commit d00ee0394d76251113e00a86fa40fc7071e0c1e1
Author: Kjell Ahlstedt <kjell ahlstedt bredband net>
Date:   Thu Jan 21 18:11:18 2016 +0100

    Add Glib::c_str_or_nullptr()
    
    * glib/glibmm/utility.h: Add Glib::c_str_or_nullptr().
    Code such as "s.empty() ? nullptr : s.c_str()" can be replaced by
    Glib::c_str_or_nullptr(s) when C functions are called, where s is
    a std::string or a Glib::ustring.
    A very small part of a fix of bug #755245.

 glib/glibmm/utility.h |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/glib/glibmm/utility.h b/glib/glibmm/utility.h
index 4b773d7..cae4452 100644
--- a/glib/glibmm/utility.h
+++ b/glib/glibmm/utility.h
@@ -130,6 +130,15 @@ std::string convert_return_gchar_ptr_to_stdstring(char* str)
                : std::string();
 }
 
+/** Get a pointer to the C style string in a std::string or Glib::ustring.
+ * If the string is empty, a nullptr is returned.
+ */
+template <typename T>
+inline const char* c_str_or_nullptr(const T& str)
+{
+  return str.empty() ? nullptr : str.c_str();
+}
+
 // Append type_name to dest, while replacing special characters with '+'.
 void append_canonical_typename(std::string& dest, const char* type_name);