new String8, String16 ctors to initialize empty static strings with static linkage

when libutils is statically linked, the ordering of the static
initializer is not guaranteed and therefore it's unsafe to use
empty static strings: e.g.:

static String8 sThisStaticStringIsNotSafe;

instead, this new constructor can be used:

static String8 sThisStaticStringIsSafe(kEmptyString);

Change-Id: Ia3daf1cab1c97d021c0ee9c2b394b5e27e8d6c0d

2 files changed, 18 insertions, 0 deletions

diff --git a/include/utils/String16.h b/include/utils/String16.h
index 6d4253d..d131bfc 100644
--- a/include/utils/String16.h
+++ b/include/utils/String16.h
@@ -41,7 +41,16 @@ class TextOutput;
 class String16
 {
 public:
+    /* use String16(StaticLinkage) if you're statically linking against
+     * libutils and declaring an empty static String16, e.g.:
+     *
+     *   static String16 sAStaticEmptyString(String16::kEmptyString);
+     *   static String16 sAnotherStaticEmptyString(sAStaticEmptyString);
+     */
+    enum StaticLinkage { kEmptyString };
+
                                 String16();
+    explicit                    String16(StaticLinkage);
                                 String16(const String16& o);
                                 String16(const String16& o,
                                          size_t len,
diff --git a/include/utils/String8.h b/include/utils/String8.h
index 9426fcf..ef59470 100644
--- a/include/utils/String8.h
+++ b/include/utils/String8.h
@@ -37,7 +37,16 @@ class TextOutput;
 class String8
 {
 public:
+    /* use String8(StaticLinkage) if you're statically linking against
+     * libutils and declaring an empty static String8, e.g.:
+     *
+     *   static String8 sAStaticEmptyString(String8::kEmptyString);
+     *   static String8 sAnotherStaticEmptyString(sAStaticEmptyString);
+     */
+    enum StaticLinkage { kEmptyString };
+
                                 String8();
+    explicit                    String8(StaticLinkage);
                                 String8(const String8& o);
     explicit                    String8(const char* o);
     explicit                    String8(const char* o, size_t numChars);