String: Add support for a ""_s string literal syntax

Create a `static_string_view` type that binds only to the static storage
of string literals.  Teach `cm::String` to borrow from these implicitly.

3 files changed, 56 insertions, 1 deletions

diff --git a/Source/CMakeLists.txt b/Source/CMakeLists.txt
index 0ce09d4..ab3e835 100644
--- a/Source/CMakeLists.txt
+++ b/Source/CMakeLists.txt
@@ -377,6 +377,7 @@ set(SRCS
   cmake.h
   cm_string_view.cxx
   cm_string_view.hxx
+  cm_static_string_view.hxx
 
   cmCommand.cxx
   cmCommand.h
diff --git a/Source/cmString.hxx b/Source/cmString.hxx
index 3b59353..52e891c 100644
--- a/Source/cmString.hxx
+++ b/Source/cmString.hxx
@@ -5,6 +5,7 @@
 
 #include "cmConfigure.h" // IWYU pragma: keep
 
+#include "cm_static_string_view.hxx"
 #include "cm_string_view.hxx"
 
 #include <algorithm>
@@ -90,6 +91,12 @@ struct IntoString<string_view> : std::true_type
 };
 
 template <>
+struct IntoString<static_string_view> : std::true_type
+{
+  static string_view into_string(static_string_view s) { return s; }
+};
+
+template <>
 struct IntoString<char> : std::true_type
 {
   static std::string into_string(char const& c) { return std::string(1, c); }
@@ -157,6 +164,12 @@ struct AsStringView<string_view> : std::true_type
 };
 
 template <>
+struct AsStringView<static_string_view> : std::true_type
+{
+  static string_view view(static_string_view const& s) { return s; }
+};
+
+template <>
 struct AsStringView<String> : std::true_type
 {
   static string_view view(String const& s);
@@ -370,7 +383,7 @@ public:
   }
 
   /** Assign to an empty string.  */
-  void clear() { *this = String(string_view("", 0), Private()); }
+  void clear() { *this = ""_s; }
 
   /** Insert 'count' copies of 'ch' at position 'index'.  */
   String& insert(size_type index, size_type count, char ch);
diff --git a/Source/cm_static_string_view.hxx b/Source/cm_static_string_view.hxx
new file mode 100644
index 0000000..1bef0c6
--- /dev/null
+++ b/Source/cm_static_string_view.hxx
@@ -0,0 +1,41 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_static_string_view_hxx
+#define cm_static_string_view_hxx
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#include "cm_string_view.hxx"
+
+#include <cstddef>
+
+namespace cm {
+
+/** A string_view that only binds to static storage.
+ *
+ * This is used together with the `""_s` user-defined literal operator
+ * to construct a type-safe abstraction of a string_view that only views
+ * statically allocated strings.  These strings are const and available
+ * for the entire lifetime of the program.
+ */
+class static_string_view : public string_view
+{
+  static_string_view(string_view v)
+    : string_view(v)
+  {
+  }
+
+  friend static_string_view operator"" _s(const char* data, size_t size);
+};
+
+/** Create a static_string_view using `""_s` literal syntax.  */
+inline static_string_view operator"" _s(const char* data, size_t size)
+{
+  return string_view(data, size);
+}
+
+} // namespace cm
+
+using cm::operator"" _s;
+
+#endif