String: Add a custom string type

Create a `cm::String` type that holds a view of a string buffer and
optionally shares ownership of the buffer.  Instances can either
borrow longer-lived storage (e.g. static storage of string literals)
or internally own a `std::string` instance.  In the latter case,
share ownership with copies and substrings.  Allocate a new internal
string only on operations that require mutation.

This will allow us to recover string sharing semantics that we
used to get from C++98 std::string copy-on-write implementations.
Such implementations are not allowed by C++11 so code our own in
a custom string type instead.

1 files changed, 131 insertions, 0 deletions

diff --git a/Source/cmString.cxx b/Source/cmString.cxx
new file mode 100644
index 0000000..e965bfb
--- /dev/null
+++ b/Source/cmString.cxx
@@ -0,0 +1,131 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#define _SCL_SECURE_NO_WARNINGS
+
+#include "cmString.hxx"
+
+#include <memory>
+#include <ostream>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+
+namespace cm {
+
+static std::string const empty_string_;
+
+void String::internally_mutate_to_stable_string()
+{
+  // We assume that only one thread mutates this instance at
+  // a time even if we point to a shared string buffer refernced
+  // by other threads.
+  *this = String(data(), size());
+}
+
+std::string const& String::str()
+{
+  if (!data()) {
+    // We view no string.
+    // This is stable for the lifetime of our current value.
+    return empty_string_;
+  }
+
+  if (string_ && data() == string_->data() && size() == string_->size()) {
+    // We view an entire string.
+    // This is stable for the lifetime of our current value.
+    return *string_;
+  }
+
+  // Mutate to hold a std::string that is stable for the lifetime
+  // of our current value.
+  this->internally_mutate_to_stable_string();
+  return *string_;
+}
+
+const char* String::c_str()
+{
+  const char* c = data();
+  if (c == nullptr) {
+    return c;
+  }
+
+  // We always point into a null-terminated string so it is safe to
+  // access one past the end.  If it is a null byte then we can use
+  // the pointer directly.
+  if (c[size()] == '\0') {
+    return c;
+  }
+
+  // Mutate to hold a std::string so we can get a null terminator.
+  this->internally_mutate_to_stable_string();
+  c = string_->c_str();
+  return c;
+}
+
+String& String::insert(size_type index, size_type count, char ch)
+{
+  std::string s;
+  s.reserve(size() + count);
+  s.assign(data(), size());
+  s.insert(index, count, ch);
+  return *this = std::move(s);
+}
+
+String& String::erase(size_type index, size_type count)
+{
+  if (index > size()) {
+    throw std::out_of_range("Index out of range in String::erase");
+  }
+  size_type const rcount = std::min(count, size() - index);
+  size_type const rindex = index + rcount;
+  std::string s;
+  s.reserve(size() - rcount);
+  s.assign(data(), index);
+  s.append(data() + rindex, size() - rindex);
+  return *this = std::move(s);
+}
+
+String String::substr(size_type pos, size_type count) const
+{
+  if (pos > size()) {
+    throw std::out_of_range("Index out of range in String::substr");
+  }
+  return String(*this, pos, count);
+}
+
+String::String(std::string&& s, Private)
+  : string_(std::make_shared<std::string>(std::move(s)))
+  , view_(string_->data(), string_->size())
+{
+}
+
+String::size_type String::copy(char* dest, size_type count,
+                               size_type pos) const
+{
+  return view_.copy(dest, count, pos);
+}
+
+std::ostream& operator<<(std::ostream& os, String const& s)
+{
+  return os.write(s.data(), s.size());
+}
+
+std::string& operator+=(std::string& self, String const& s)
+{
+  return self += s.view();
+}
+
+String IntoString<char*>::into_string(const char* s)
+{
+  if (!s) {
+    return String();
+  }
+  return std::string(s);
+}
+
+string_view AsStringView<String>::view(String const& s)
+{
+  return s.view();
+}
+
+} // namespace cm