cmDefinitions: Add Unset and cm::string_view based Set methods

- The `cmDefinitions::Def` struct is simplified by removing an unused
  constructor and the `std_string` type definition.  The std::string value
  becomes a member variable instead of being derived from.

- The `cmDefinitions::Unset` method is added that unsets a definition.
  It has the same functionality as calling `cmDefinitions::Set` with a
  `nullptr` value argument,

- The `cmDefinitions::Set` method gets an overloaded version that takes a
  `cm::string_view` as value argument.

- The originl `cmDefinitions::Set` method with `const char*` argument is
  refactored to either call the `cm::string_view` based `cmDefinitions::Set`
  overload or `cmDefinitions::Unset`, depending on whether the value
  `const char*` is a nullptr (`Unset`) or not (`Set`).

1 files changed, 7 insertions, 2 deletions

diff --git a/Source/cmDefinitions.cxx b/Source/cmDefinitions.cxx
index 35cdc5a..42e70d6 100644
--- a/Source/cmDefinitions.cxx
+++ b/Source/cmDefinitions.cxx
@@ -36,7 +36,7 @@ const std::string* cmDefinitions::Get(const std::string& key, StackIter begin,
                                       StackIter end)
 {
   Def const& def = cmDefinitions::GetInternal(key, begin, end, false);
-  return def.Exists ? &def : nullptr;
+  return def.Exists ? &def.Value : nullptr;
 }
 
 void cmDefinitions::Raise(const std::string& key, StackIter begin,
@@ -56,11 +56,16 @@ bool cmDefinitions::HasKey(const std::string& key, StackIter begin,
   return false;
 }
 
-void cmDefinitions::Set(const std::string& key, const char* value)
+void cmDefinitions::Set(const std::string& key, cm::string_view value)
 {
   this->Map[key] = Def(value);
 }
 
+void cmDefinitions::Unset(const std::string& key)
+{
+  this->Map[key] = Def();
+}
+
 std::vector<std::string> cmDefinitions::UnusedKeys() const
 {
   std::vector<std::string> keys;